Kuliang fisiologi lingkungan ing 2014 2

Importance of photosynthetic capacity inImportance of photosynthetic capacity in
yield determination of teayield determination of tea
 There have been some conflictingThere have been some conflicting
opinions on how important theopinions on how important the
photosynthetic rate is in determining thephotosynthetic rate is in determining the
productivity of tea.productivity of tea.
 Based on evidence compiled from severalBased on evidence compiled from several
studies, Squire and Callander (1981)studies, Squire and Callander (1981)
concluded that the current rate of Pn is notconcluded that the current rate of Pn is not
directly linked to leaf yield of tea.directly linked to leaf yield of tea.

 The basis of their argument was that leafThe basis of their argument was that leaf
yield of tea is controlled more by the ratesyield of tea is controlled more by the rates
of shoot initiation and extension ratherof shoot initiation and extension rather
than by the supply of assimilates fromthan by the supply of assimilates from
current Pn.current Pn.
 Rates of shoot initiation and extension arRates of shoot initiation and extension ar
primarily controlled by Ta and VPD andprimarily controlled by Ta and VPD and
shoot turgor (Squire, 1979) whereas Pn isshoot turgor (Squire, 1979) whereas Pn is
primarily controlled by light intensity.primarily controlled by light intensity.

 Moreover, the weight of harvested shootsMoreover, the weight of harvested shoots
of tea (i.e. 2-3 leaves and a bud) is only aof tea (i.e. 2-3 leaves and a bud) is only a
small fraction (0.05-0.15) of its totalsmall fraction (0.05-0.15) of its total
biomass production (Callander, 1978;biomass production (Callander, 1978;
Tanton, 1979).Tanton, 1979).
 Therefore, both Tanton (1979) and SquireTherefore, both Tanton (1979) and Squire
and Callander (1981) argued thatand Callander (1981) argued that
assimilate supply cannot be a limitingassimilate supply cannot be a limiting
factor in yield determination, whenfactor in yield determination, when
harvested yield is such a small fraction ofharvested yield is such a small fraction of
total biomass production.total biomass production.

 Instead, tea leaf yield per unit land area isInstead, tea leaf yield per unit land area is
strongly correlated withstrongly correlated with Nsh.Nsh.
 Hence, Tanton (1979) concluded that teaHence, Tanton (1979) concluded that tea
yield is sinklimited rather than source-yield is sinklimited rather than source-
limited.limited.
 Based on the results of a simulationBased on the results of a simulation
model, Matthews and Stephens (1998a)model, Matthews and Stephens (1998a)
also suggest that assimilate supply isalso suggest that assimilate supply is
unlikely to limit shoot growth under mostunlikely to limit shoot growth under most
conditions.conditions.

 However, it should be borne in mind thatHowever, it should be borne in mind that
photoinhibition could reduce sourcephotoinhibition could reduce source
capacity and thereby could impose acapacity and thereby could impose a
source-limitation as well on tea yieldsource-limitation as well on tea yield
(Mohotti et al., 2000; Mohotti and Lawlor,(Mohotti et al., 2000; Mohotti and Lawlor,
2002).2002).
 Squire and Callander (1981) also cite theSquire and Callander (1981) also cite the
frequent observation of higher Pn evenfrequent observation of higher Pn even
during periods of low shoot growth ratesduring periods of low shoot growth rates
due to higher VPD or cooler Ta (Squire,due to higher VPD or cooler Ta (Squire,
1979) or higher soil water deficits1979) or higher soil water deficits
(Stephens and Carr, 1991a) as evidence(Stephens and Carr, 1991a) as evidence
for the independence of tea yield from Pnfor the independence of tea yield from Pn

 However, Smith et al. (1993a) argue thatHowever, Smith et al. (1993a) argue that
assimilates produced during periods ofassimilates produced during periods of
slow shoot growth are subsequently usedslow shoot growth are subsequently used
during periods of higher shoot growth.during periods of higher shoot growth.
 Hence, although tea yield may not beHence, although tea yield may not be
correlated with current Pn, over a longer-correlated with current Pn, over a longer-
time period, time-integrated tea yield andtime period, time-integrated tea yield and
Pn should be positively correlated.Pn should be positively correlated.

Respiratory losses ofRespiratory losses of
photoassimilates:photoassimilates:
 Respiration is the process of generatingRespiration is the process of generating
metabolic energy for synthesis of newmetabolic energy for synthesis of new
biomass (‘growth respiration’) andbiomass (‘growth respiration’) and
maintenance of existing biomassmaintenance of existing biomass
(‘maintenance respiration’).(‘maintenance respiration’).
 Respiration rate also influences growthRespiration rate also influences growth
rates of a crop as respiration uses part ofrates of a crop as respiration uses part of
the photoassimilates as the substrate.the photoassimilates as the substrate.

 Tanton (1979) estimated that mature teaTanton (1979) estimated that mature tea
which accumulates biomass at a rate ofwhich accumulates biomass at a rate of
17.5 t ha-1 yr-1 uses 67% of its17.5 t ha-1 yr-1 uses 67% of its
photoassimilates for respiration.photoassimilates for respiration.
 Hadfield (1974) estimated the respiratoryHadfield (1974) estimated the respiratory
losses to be 85%.losses to be 85%.
 The estimate of Tanton (1979) agrees withThe estimate of Tanton (1979) agrees with
the measurements of Barbora and Baruathe measurements of Barbora and Barua
(1988) on six cultivars of mature tea under(1988) on six cultivars of mature tea under
plucking in North East India, where onlyplucking in North East India, where only
36% of photoassimilates remained in the36% of photoassimilates remained in the
plant, and the balance 64% were lost inplant, and the balance 64% were lost in
metabolic respiration.metabolic respiration.

 These respiratory losses are much greaterThese respiratory losses are much greater
than those of annual crops (i.e. 30-50%,than those of annual crops (i.e. 30-50%,
Charles-Edwards, 1982).Charles-Edwards, 1982).
 It was further observed that respiration inIt was further observed that respiration in
all plant organs increased linearly as theall plant organs increased linearly as the
temperature increased from 20oC totemperature increased from 20oC to
40oC.40oC.
 Respiratory losses of higher yieldingRespiratory losses of higher yielding
cultivars were higher than those of thecultivars were higher than those of the
lower yielding cultivars.lower yielding cultivars.

 Because of the increase in total biomassBecause of the increase in total biomass
with time during the period between twowith time during the period between two
successive prunings maintenancesuccessive prunings maintenance
respiration increases as the croprespiration increases as the crop
progresses through a pruning cycleprogresses through a pruning cycle
(Navaratne et al., unpublished results).(Navaratne et al., unpublished results).
 Using young tea (9-month-old) seedlingsUsing young tea (9-month-old) seedlings
growing in sand culture,growing in sand culture,
Anandacoomaraswamy et al. (2002)Anandacoomaraswamy et al. (2002)
estimated the respiratory cost of tea rootsestimated the respiratory cost of tea roots
for maintenance and nitrate uptake asfor maintenance and nitrate uptake as
0.324 μmol CO2 kg-1 (root dry matter) s-10.324 μmol CO2 kg-1 (root dry matter) s-1
and 0.64 mol CO2 (mol N)-1, respectively.and 0.64 mol CO2 (mol N)-1, respectively.

 Root respiration rates increased withRoot respiration rates increased with
increasing N supply, primarily because ofincreasing N supply, primarily because of
the increased respiratory cost for nitratethe increased respiratory cost for nitrate
uptake.uptake.
 De Costa et al. (unpublished results)De Costa et al. (unpublished results)
estimated daily growth respiration to varyestimated daily growth respiration to vary
from 4.42 to 6.96 g CO2 m-2 (land area)from 4.42 to 6.96 g CO2 m-2 (land area)
d-1 in two cultivars of mature, field-grownd-1 in two cultivars of mature, field-grown
tea at high altitude in Sri Lanka.tea at high altitude in Sri Lanka.

 These rates varied for different years ofThese rates varied for different years of
the pruning cycle.the pruning cycle.
 This variation was positively correlatedThis variation was positively correlated
with the variation of the rates of biomasswith the variation of the rates of biomass
production in different years.production in different years.

WATER RELATIONSWATER RELATIONS
 The water status of a tea plant isThe water status of a tea plant is
determined by the balance between waterdetermined by the balance between water
absorption by its root system and waterabsorption by its root system and water
loss through transpiration.loss through transpiration.
 The amount of water absorbed is mainlyThe amount of water absorbed is mainly
determined by the maximum rooting depthdetermined by the maximum rooting depth
and the water available within the rootand the water available within the root
zone.zone.

 Tea is generally considered to be aTea is generally considered to be a
shallow-rooting plant, sensitive to theshallow-rooting plant, sensitive to the
physical condition of the soil (Harler,physical condition of the soil (Harler,
1964).1964).
 Although seedling tea has a tap root whichAlthough seedling tea has a tap root which
has been reported to penetrate as deephas been reported to penetrate as deep
as 4.5 m (Carr, 1971a, 1972), a majoras 4.5 m (Carr, 1971a, 1972), a major
portion of the root system of clonal tea isportion of the root system of clonal tea is
located within the first 30 cm of the soillocated within the first 30 cm of the soil
profile (De Costa and Surenthran, 2005).profile (De Costa and Surenthran, 2005).

 Nixon and Sanga (1995) also observedNixon and Sanga (1995) also observed
that the weight of fine roots with diametersthat the weight of fine roots with diameters
less than 1 mm decreased exponentiallyless than 1 mm decreased exponentially
with increasing soil depth.with increasing soil depth.
 However, Carr (1977a) has observed rootHowever, Carr (1977a) has observed root
systems as deep as 3 m in clones, whilesystems as deep as 3 m in clones, while
Stephens and Carr (1991b) haveStephens and Carr (1991b) have
observed a maximum rooting depth of 5.5observed a maximum rooting depth of 5.5
m.m.
 Carr (1977b) showed that during theCarr (1977b) showed that during the
gradual development of a drought, clonalgradual development of a drought, clonal
tea develops water stress earlier thantea develops water stress earlier than
seedling tea.seedling tea.

 This was partly attributed to the variationThis was partly attributed to the variation
in their root systems. Earlier developmentin their root systems. Earlier development
of water stress was indicated by the fasterof water stress was indicated by the faster
decline of shoot xylem ψw and earlierdecline of shoot xylem ψw and earlier
stomatal closure.stomatal closure.
 However, the results of Carr (1977a)However, the results of Carr (1977a)
suggested that seedlings were moresuggested that seedlings were more
susceptible to extreme dry conditions (i.e.susceptible to extreme dry conditions (i.e.
a potential soil water deficit of 300 mm m-a potential soil water deficit of 300 mm m-
1of soil depth, representing a 50% loss of1of soil depth, representing a 50% loss of
available water) than clonal tea, which hadavailable water) than clonal tea, which had
deep root systems.deep root systems.

 In another study carried out over allIn another study carried out over all
different tea growing agroclimatic regionsdifferent tea growing agroclimatic regions
of Sri Lanka, the seedlings had aof Sri Lanka, the seedlings had a
significantly deeper root system thansignificantly deeper root system than
clones.clones.
 Moreover, clones exhibited a significantlyMoreover, clones exhibited a significantly
deeper active root zone comprising ofdeeper active root zone comprising of
more feeder roots than seedlings (Mohotti,more feeder roots than seedlings (Mohotti,
unpublished results).unpublished results).

 However, in a similar study carried outHowever, in a similar study carried out
using five-year-old, field-grownusing five-year-old, field-grown
comparison trial of different seedlingcomparison trial of different seedling
progenies and clones (TRI 2023 and aprogenies and clones (TRI 2023 and a
known drought tolerant clone, DN), theknown drought tolerant clone, DN), the
overall mean root depth was higher inoverall mean root depth was higher in
clones than in seedlings (Liyanapatabendiclones than in seedlings (Liyanapatabendi
et al., 2007).et al., 2007).
 The overall root growth of the clone DNThe overall root growth of the clone DN
was superior to all other seedlingwas superior to all other seedling
progenies and the clone TRI 2023.progenies and the clone TRI 2023.

 The clones had a significantly deeperThe clones had a significantly deeper
active root zone comprising of moreactive root zone comprising of more
feeder roots.feeder roots.
 Therefore, depth of the root systemTherefore, depth of the root system
probably has a greater influence inprobably has a greater influence in
determining the drought tolerance of adetermining the drought tolerance of a
given genotype than its method ofgiven genotype than its method of
propagation (i.e. whether seedling orpropagation (i.e. whether seedling or
clonal).clonal).

 Mohotti et al. (2003b) have shown thatMohotti et al. (2003b) have shown that
organicallymanaged tea has a greaterorganicallymanaged tea has a greater
proportion of its root system in deeperproportion of its root system in deeper
layers of the soil profile as compared tolayers of the soil profile as compared to
tea managed conventionally wheretea managed conventionally where
inorganic fertilizer applied to the soilinorganic fertilizer applied to the soil
surface along with other recommendedsurface along with other recommended
management practices.management practices.

 There were associated changes in theThere were associated changes in the
anatomy of roots of 1-2 mm diameter withanatomy of roots of 1-2 mm diameter with
organicallygrown tea having significantlyorganicallygrown tea having significantly
thicker cork layers, smaller xylem vesselthicker cork layers, smaller xylem vessel
diameters and xylem wall diametersdiameters and xylem wall diameters
(Mohotti et al., 2003c).(Mohotti et al., 2003c).
 Sap flow studies by Mohotti et al.Sap flow studies by Mohotti et al.
(unpublished results) have shown that(unpublished results) have shown that
organicallygrown tea also had higher WUEorganicallygrown tea also had higher WUE
compared to conventionally-grown tea.compared to conventionally-grown tea.

Transpiration:Transpiration:
 Water use of tea and its controlling factorsWater use of tea and its controlling factors
have been studied extensivelyhave been studied extensively
 However, water use (orHowever, water use (or
evapotranspiration) include bothevapotranspiration) include both
transpiration from the foliage canopy andtranspiration from the foliage canopy and
soil evaporation.soil evaporation.

 A well-maintained tea canopy covers theA well-maintained tea canopy covers the
ground almost completely allowing veryground almost completely allowing very
little solar radiation to penetrate down tolittle solar radiation to penetrate down to
the soil surface.the soil surface.
 In such situations, evapotranspiration isIn such situations, evapotranspiration is
almost equal to transpiration.almost equal to transpiration.
 There are only a few studies where directThere are only a few studies where direct
measurements of transpiration have beenmeasurements of transpiration have been
done in tea over prolonged periods.done in tea over prolonged periods.

 In one such study, AnandacoomaraswamyIn one such study, Anandacoomaraswamy
et al. (2000) showed that both hourly andet al. (2000) showed that both hourly and
daily transpiration rates were highlydaily transpiration rates were highly
sensitive to soil water availability.sensitive to soil water availability.
 Daily transpiration rate was maintained atDaily transpiration rate was maintained at
a maximum of 1.6 L plant-1 d-1 when thea maximum of 1.6 L plant-1 d-1 when the
soil water content (SWC)soil water content (SWC) decreased fromdecreased from
field capacity (44% v/v) down to 33%.field capacity (44% v/v) down to 33%.

Control of transpiration by gs andControl of transpiration by gs and
shootshoot øøw:w:
 Tea has highly sensitive stomata, whichTea has highly sensitive stomata, which
show partial closure during midday evenshow partial closure during midday even
when the plants are growing on a wet soilwhen the plants are growing on a wet soil
(Williams, 1971; Carr, 1977a).(Williams, 1971; Carr, 1977a).
 Stomatal closure was slightly preceded byStomatal closure was slightly preceded by
reducedreduced shoot xylem øwshoot xylem øw, indicating that, indicating that
stomatal closure occurred as a responsestomatal closure occurred as a response
to anto an internal water deficit in the shoot.internal water deficit in the shoot.

 This indicates that the rate of root waterThis indicates that the rate of root water
absorption and its subsequent transferabsorption and its subsequent transfer
through the xylem is not very efficient inthrough the xylem is not very efficient in
tea even under conditions of moderatetea even under conditions of moderate
atmospheric demand (i.e. > 5 mm d-1).atmospheric demand (i.e. > 5 mm d-1).
 This could be due to specificThis could be due to specific
characteristics in the absorbing region ofcharacteristics in the absorbing region of
the root system and/or the xylem vessels.the root system and/or the xylem vessels.

 On a relatively wet soil, shootOn a relatively wet soil, shoot øwøw
recovered during late afternoon along withrecovered during late afternoon along with
increasedincreased ggs.s.
 In contrast, whenIn contrast, when SWSWCC was significant,was significant,
shoot øw did not recover even by lateshoot øw did not recover even by late
afternoon.afternoon.
 However, even when shoot øw was still asHowever, even when shoot øw was still as
low as -1.8 MPa, stomata showed somelow as -1.8 MPa, stomata showed some
re-opening under cloudy conditions,re-opening under cloudy conditions,
indicating a stomatal response whichindicating a stomatal response which waswas
independent from shootindependent from shoot øw.øw.

 This stomatal reopening was probably aThis stomatal reopening was probably a
response to decreasing VPD in theresponse to decreasing VPD in the
surrounding air in late afternoon undersurrounding air in late afternoon under
cloudy conditions.cloudy conditions.
 Stomatal movement is one of severalStomatal movement is one of several
important physiological processes, whichimportant physiological processes, which
respond to VPD.respond to VPD.
 In fact, Carr (1977a) showed that bothIn fact, Carr (1977a) showed that both ggss
(measured indirectly as liquid infiltration(measured indirectly as liquid infiltration
score) and shoot øw werescore) and shoot øw were negativelynegatively
correlated with VPD,correlated with VPD, Ta and incident solarTa and incident solar
radiation intensity.radiation intensity.

 Interestingly, Carr (1977a) found thatInterestingly, Carr (1977a) found that
shoot øw (and therebyshoot øw (and thereby ggs) of tea wass) of tea was
more sensitive to Ta and VPD when themore sensitive to Ta and VPD when the
soil was wet than when it was dry.soil was wet than when it was dry.
 This was probably because higherThis was probably because higher TaTa andand
VPD caused greater transpiration rates,VPD caused greater transpiration rates,
which in turn, would have loweredwhich in turn, would have lowered shootshoot
øw even when tea is grown on a wet soil.øw even when tea is grown on a wet soil.

 In contrast, on a dry soil, early stomatalIn contrast, on a dry soil, early stomatal
closure would have preventedclosure would have prevented
transpiration from responding to higher Tatranspiration from responding to higher Ta
and VPD, making shoot xylem øw lessand VPD, making shoot xylem øw less
sensitive to atmospheric water stress.sensitive to atmospheric water stress.
 Squire (1978) also observed thatSquire (1978) also observed that ggs of teas of tea
growing in Malawi was principallygrowing in Malawi was principally
determined by irradiance, except duringdetermined by irradiance, except during
the dry months.the dry months.
 During the wet period,During the wet period, ggs wass was
independent of shoot øw, VPD and Pn. Inindependent of shoot øw, VPD and Pn. In
contrast, during the dry season,contrast, during the dry season, ggss
remained unaffected by øw and VPD, butremained unaffected by øw and VPD, but
was more closely related to Pn.was more closely related to Pn.

 The observation that shoot øw andThe observation that shoot øw and ggs ofs of
tea is less sensitive to Ta and VPD in atea is less sensitive to Ta and VPD in a
dry soil provides indirect evidence thatdry soil provides indirect evidence that
stomatal opening of tea may bestomatal opening of tea may be
controlledsensitive to Ta and VPD in a drycontrolledsensitive to Ta and VPD in a dry
soil provides indirect evidence thatsoil provides indirect evidence that
stomatal opening of tea may be controlledstomatal opening of tea may be controlled
by hormonal signals originating from rootsby hormonal signals originating from roots
(Zhang et al.,(Zhang et al., 1987; Davies and Zhang,1987; Davies and Zhang,
1991).1991).

 Callander and Woodhead (1981) observedCallander and Woodhead (1981) observed
that the canopy conductance per unit leafthat the canopy conductance per unit leaf
area of tea was little affected by soil waterarea of tea was little affected by soil water
deficits as large as 370 mm. This wasdeficits as large as 370 mm. This was
attributed to the deep root system of tea.attributed to the deep root system of tea.
 The wet season canopy conductance wasThe wet season canopy conductance was
12% higher than for a dry season canopy,12% higher than for a dry season canopy,
receiving the same net irradiance andreceiving the same net irradiance and
VPD.VPD.
 This observation also indicates greaterThis observation also indicates greater
stomatal opening when the soil is wet andstomatal opening when the soil is wet and
provides further evidence for hormonalprovides further evidence for hormonal
signals from roots controlling the stomatasignals from roots controlling the stomata
of tea.of tea.

Stomatal dynamics and droughtStomatal dynamics and drought
resistance of tea:resistance of tea:
 Carr (1977a) observed a clear linearCarr (1977a) observed a clear linear
relationship between shoot øw and gs,relationship between shoot øw and gs,
showing the close mechanistic linkshowing the close mechanistic link
between shoot water status and stomatalbetween shoot water status and stomatal
opening.opening.
 Importantly, the slope of this relationship,Importantly, the slope of this relationship,
which is a measurement of the sensitivitywhich is a measurement of the sensitivity
of stomata to water stress, differed forof stomata to water stress, differed for
different genotypes (including bothdifferent genotypes (including both
seedlings and clones).seedlings and clones).

 Furthermore, x-axis intercept of the aboveFurthermore, x-axis intercept of the above
relationship (i.e. shoot øw at zerorelationship (i.e. shoot øw at zero ggs)s)
showed significant variation betweenshowed significant variation between
genotypes. Carr (1977a) showed that bothgenotypes. Carr (1977a) showed that both
the slope and x-axis intercept were relatedthe slope and x-axis intercept were related
to drought resistance of a given genotypeto drought resistance of a given genotype
of tea, with relatively resistant genotypesof tea, with relatively resistant genotypes
showing a lower slope (i.e. indicatingshowing a lower slope (i.e. indicating
stomata which are less sensitive to waterstomata which are less sensitive to water
stress) and a lower x-axis intercept (i.e.stress) and a lower x-axis intercept (i.e.
complete stomatalcomplete stomatal closure occurring at aclosure occurring at a
lower shoot øw).lower shoot øw).

 In addition, Carr (1977a) also showed thatIn addition, Carr (1977a) also showed that
different tea genotypes can be screeneddifferent tea genotypes can be screened
for their drought resistance on the basis offor their drought resistance on the basis of
the y-axis intercept of the abovethe y-axis intercept of the above
relationship, which indicates the maximumrelationship, which indicates the maximum
ggs at shoot øx = 0 (i.e. shoot tissue at fulls at shoot øx = 0 (i.e. shoot tissue at full
saturation).saturation).
 The ratio between the respectiveThe ratio between the respective ggs at as at a
specified level of water stress (e.g. øw =specified level of water stress (e.g. øw =
-1.5 MPa) and at øw = 0, was higher in-1.5 MPa) and at øw = 0, was higher in
relatively drought resistant genotypes andrelatively drought resistant genotypes and
decreased in genotypes with increasingdecreased in genotypes with increasing
drought susceptibility.drought susceptibility.

 In a separate study, Carr (1977b)In a separate study, Carr (1977b)
identified two possibleidentified two possible
mechanisms/pathways of stomatalmechanisms/pathways of stomatal
response to increasing drought.response to increasing drought.
 Some clones had highly sensitive stomata,Some clones had highly sensitive stomata,
which closed earlier during a drying cyclewhich closed earlier during a drying cycle
and thereby conserved water by reducingand thereby conserved water by reducing
transpiration. This mechanism wastranspiration. This mechanism was
demonstrated by Wijeratne et al. (1998a)demonstrated by Wijeratne et al. (1998a)
in a relatively drought-tolerant Sri Lankanin a relatively drought-tolerant Sri Lankan
genotype TRIgenotype TRI 2025.2025.

 Moreover, the critical leaf øw forMoreover, the critical leaf øw for
significantly reducingsignificantly reducing ggs and transpirations and transpiration
rate was higher for TRI 2025 (-0.65 MPa)rate was higher for TRI 2025 (-0.65 MPa)
than for a drought-susceptible genotypethan for a drought-susceptible genotype
TRITRI 2023 (-0.85 MPa) (Wijeratne et al.,2023 (-0.85 MPa) (Wijeratne et al.,
1998b).1998b).
 Over a diurnal drying cycle, the drought-Over a diurnal drying cycle, the drought-
susceptible genotype transpired at asusceptible genotype transpired at a
higher rate over a longer period thanhigher rate over a longer period than
thethedrought-tolerant genotype.drought-tolerant genotype.

 Consequently, the droughttolerantConsequently, the droughttolerant
genotype maintained a higher leaf øw andgenotype maintained a higher leaf øw and
leaf relative water content (RWC).leaf relative water content (RWC).
 The rate of transpiration and theThe rate of transpiration and the
consequent rate of reduction of leaf øwconsequent rate of reduction of leaf øw
with increasing VPD were greater in thewith increasing VPD were greater in the
drought-susceptible genotype.drought-susceptible genotype.
 Greater stomatal densities of theGreater stomatal densities of the
droughtsusceptible genotypedroughtsusceptible genotype
(Wadasinghe and Wijeratne, 1989)(Wadasinghe and Wijeratne, 1989)
contributed to its higher transpirationcontributed to its higher transpiration
rates.rates.

 In a study comparing the water relationsIn a study comparing the water relations
of three genotypes of tea, Sandanam et al.of three genotypes of tea, Sandanam et al.
(1981) also showed that the most drought(1981) also showed that the most drought
tolerant genotype (DN) had the highesttolerant genotype (DN) had the highest
leaf diffusive resistance under waterleaf diffusive resistance under water
stress.stress.

 In contrast, some clones were able toIn contrast, some clones were able to
keep their stomata open for a longerkeep their stomata open for a longer
period during a drought and therebyperiod during a drought and thereby
maintain higher transpiration rates.maintain higher transpiration rates.
 However, there was no appreciableHowever, there was no appreciable
reduction in shoot øw. Hence, it is likelyreduction in shoot øw. Hence, it is likely
that such clones probably had a deeperthat such clones probably had a deeper
root system and thereby had a greaterroot system and thereby had a greater
water absorptionwater absorption capacity.capacity.

 This strategy of keeping the stomata openThis strategy of keeping the stomata open
for a longer period during a drought wouldfor a longer period during a drought would
probably allow greater uptake of CO2 andprobably allow greater uptake of CO2 and
higher Pn and probably greaterhigher Pn and probably greater Wsh.Wsh.
 However, such a strategy would only beHowever, such a strategy would only be
possible in deep soils with a higher waterpossible in deep soils with a higher water
storage capacity. It could be a high riskstorage capacity. It could be a high risk
strategy on shallow soils, on which astrategy on shallow soils, on which a
considerable portion of tea is grown inconsiderable portion of tea is grown in
some countries (e.g. Sri Lanka).some countries (e.g. Sri Lanka).

 Osmotic adjustment also enables a givenOsmotic adjustment also enables a given
genotype of tea to keep its stomata opengenotype of tea to keep its stomata open
for a longer period during a drought.for a longer period during a drought.
 Wijeratne (1994) and Karunaratne et al.Wijeratne (1994) and Karunaratne et al.
(1999) have shown that a relatively(1999) have shown that a relatively
drought resistant genotype had a greaterdrought resistant genotype had a greater
capability for osmotic adjustment than acapability for osmotic adjustment than a
relatively drought susceptible genotype.relatively drought susceptible genotype.

 Analysis of measured pressure-volumeAnalysis of measured pressure-volume
curves (Wijeratne et al., 1998b) showedcurves (Wijeratne et al., 1998b) showed
that the drought-tolerant genotype had athat the drought-tolerant genotype had a
lower osmotic potential (øs) at full turgorlower osmotic potential (øs) at full turgor
and a higherand a higher apoplasticapoplastic water content thanwater content than
the drought-susceptiblethe drought-susceptible genotype.genotype.

 The lower øs at full turgor providesThe lower øs at full turgor provides
evidence for osmotic adjustment throughevidence for osmotic adjustment through
active accumulation of solutes in theactive accumulation of solutes in the
leaves during drought stress.leaves during drought stress.
 The lowerThe lower øsøs also allows the drought-also allows the drought-
tolerant genotype to absorb water fromtolerant genotype to absorb water from
drier soils because of the greaterdrier soils because of the greater øwøw
gradientgradient between soil and plant.between soil and plant.

 This was confirmed by the lower soil waterThis was confirmed by the lower soil water
content and soil øw at permanent wiltingcontent and soil øw at permanent wilting
point of the drought-tolerant genotype.point of the drought-tolerant genotype.
 Furthermore, the higher apoplastic waterFurthermore, the higher apoplastic water
content also allows a plant to bettercontent also allows a plant to better
tolerate drought by transferring apoplastictolerate drought by transferring apoplastic
water to the cytoplasm during periods ofwater to the cytoplasm during periods of
water deficits.water deficits.

 Sandanam et al. (1981) also showedSandanam et al. (1981) also showed
significant genotypic variation in thesignificant genotypic variation in the
pressure-volume curves of both youngpressure-volume curves of both young
and old leaves of tea.and old leaves of tea.
 At a given leaf øw, the leaf RWC wasAt a given leaf øw, the leaf RWC was
higher in drought-tolerant genotypes thanhigher in drought-tolerant genotypes than
in droughtsusceptible genotypes.in droughtsusceptible genotypes.
 These results show that both stomatalThese results show that both stomatal
control and osmotic adjustment contributecontrol and osmotic adjustment contribute
to drought tolerance of tea.to drought tolerance of tea.

Environmental factorsEnvironmental factors
controlling transpiration of teacontrolling transpiration of tea

Incident radiation and shade:Incident radiation and shade:
 When the soil is at or near saturation,When the soil is at or near saturation,
solar radiation intensity is the mainsolar radiation intensity is the main
determinant of transpiration as it providesdeterminant of transpiration as it provides
the latent heat energy required forthe latent heat energy required for
evaporation of water.evaporation of water.
 Anandacoomaraswamy et al. (2000)Anandacoomaraswamy et al. (2000)
observed that transpiration rate of field-observed that transpiration rate of field-
grown mature tea decreased linearly withgrown mature tea decreased linearly with
decreasing irradiance from full sunlightdecreasing irradiance from full sunlight
until 15% of full sunlight at the rate ofuntil 15% of full sunlight at the rate of
0.031 L plant-1 d-1 for0.031 L plant-1 d-1 for each 1% reductioneach 1% reduction
of solar irradiance.of solar irradiance.

 Shade also reduces transpiration rateShade also reduces transpiration rate
primarily by decreasing the irradianceprimarily by decreasing the irradiance
incident on the tea canopy and byincident on the tea canopy and by
reducing canopy temperature (Tc).reducing canopy temperature (Tc).
 Tea growing under the shade ofTea growing under the shade of GrevilleaGrevillea
robustarobusta had substantially lowerhad substantially lower
transpiration rates (0.42-1.071 L plant-1 d-transpiration rates (0.42-1.071 L plant-1 d-
1) than unshaded tea (3.511 L1) than unshaded tea (3.511 L plant-1 d-1)plant-1 d-1)
(Anadacoomaraswamy et al., 2000).(Anadacoomaraswamy et al., 2000).

 Some of the antitranspirants also reducedSome of the antitranspirants also reduced
transpiration by reducing the energy loadtranspiration by reducing the energy load
on the foliage canopy of tea.on the foliage canopy of tea.
 For example, Anandacoomaraswamy etFor example, Anandacoomaraswamy et
al. (2000) showed that the antitranspirant,al. (2000) showed that the antitranspirant,
Kaolin, increased the reflectance ofKaolin, increased the reflectance of
incident radiation from leaves and reducedincident radiation from leaves and reduced
Tc by 2-4oC, thereby reducingTc by 2-4oC, thereby reducing
transpiration rate, especially during thetranspiration rate, especially during the
midday.midday.

 Wind and shelter:Wind and shelter: Some of the tea-Some of the tea-
growing regions, especially at highgrowing regions, especially at high
altitudes, experience periods of high windaltitudes, experience periods of high wind
speeds during certain times of the year.speeds during certain times of the year.
 High wind speeds generally tend toHigh wind speeds generally tend to
increase transpiration rates from extensiveincrease transpiration rates from extensive
tea canopies and thereby accelerate thetea canopies and thereby accelerate the
development of soil water deficits duringdevelopment of soil water deficits during
dry periods.dry periods.

 To counter these adverse effects of highTo counter these adverse effects of high
wind speeds, wind breaks and shelterwind speeds, wind breaks and shelter
belts of different tree species are grown.belts of different tree species are grown.
 However, contrary to these expectations,However, contrary to these expectations,
Carr(1971b, 1985) showed that teaCarr(1971b, 1985) showed that tea
sheltered from winds by shelter belts (4 msheltered from winds by shelter belts (4 m
talltall Hakea salignaHakea saligna) can have higher) can have higher
transpiration rates than unsheltered teatranspiration rates than unsheltered tea
and thereby experience water stress asand thereby experience water stress as
indicated by lower leaf øw,indicated by lower leaf øw, ggss and higherand higher
SWD during periods of dry weather.SWD during periods of dry weather.

 This can be explained by basic physicalThis can be explained by basic physical
principles (Carr, 1985).principles (Carr, 1985).
 Reduction of wind speed by shelterReduction of wind speed by shelter
increases the aerodynamic resistance (increases the aerodynamic resistance (rra)a)
of the tea canopy.of the tea canopy.
 This increases day-time Tc (by 1-2oC)This increases day-time Tc (by 1-2oC)
because of the greater resistance to heatbecause of the greater resistance to heat
transfer from the canopy to thetransfer from the canopy to the
surrounding air.surrounding air.

 Although Carr (1985) observed a higherAlthough Carr (1985) observed a higher
canopy resistance (canopy resistance (rrc) in sheltered tea asc) in sheltered tea as
compared to the unsheltered, the greatercompared to the unsheltered, the greater
Tc under shelter can induce higherTc under shelter can induce higher
transpiration rates (because of greatertranspiration rates (because of greater
leaf-air VPD) and reduce leaf øw,leaf-air VPD) and reduce leaf øw,
especially during dry periods.especially during dry periods.
 On the other hand, during rainy periods,On the other hand, during rainy periods,
greater soil water availability allowedgreater soil water availability allowed
sheltered tea to maintain highersheltered tea to maintain higher
transpiration rates without reducing leaftranspiration rates without reducing leaf
øw.øw.

 Moreover, during the rainy periods, theMoreover, during the rainy periods, the
higher Tc and leaf øw values of shelteredhigher Tc and leaf øw values of sheltered
tea allowed it to produce a greater yieldtea allowed it to produce a greater yield
than unsheltered tea by increasing thethan unsheltered tea by increasing the
rates of shoot initiation and extension.rates of shoot initiation and extension.
 Therefore, Carr (1985) concluded thatTherefore, Carr (1985) concluded that
shelter is beneficial for tea growing undershelter is beneficial for tea growing under
the following conditions: (a) in coolerthe following conditions: (a) in cooler
regions where Ta is closer to theTb forregions where Ta is closer to theTb for
shoot growth; (b) under irrigation; and (c)shoot growth; (b) under irrigation; and (c)
in areas with significant advection of hotin areas with significant advection of hot
dry air on to tea fields from thedry air on to tea fields from the
surrounding areas.surrounding areas.

Transpiration efficiency and waterTranspiration efficiency and water
use efficiency:use efficiency:
 Transpiration efficiency (TE) of tea can beTranspiration efficiency (TE) of tea can be
computed on the basis of the weight ofcomputed on the basis of the weight of
made tea per unit of water transpired.made tea per unit of water transpired.
 Anandacoomaraswamy et al. (2000)Anandacoomaraswamy et al. (2000)
obtained a TE of 9.637 kg ha-1 (made tea)obtained a TE of 9.637 kg ha-1 (made tea)
mm-1 of water transpired for field-grownmm-1 of water transpired for field-grown
clonal tea in Sri Lanka.clonal tea in Sri Lanka.

 Stephens and Carr (1991b) obtained WUEStephens and Carr (1991b) obtained WUE
values, estimated as the slope of thevalues, estimated as the slope of the
linear regression between made tea yieldlinear regression between made tea yield
and evapotranspiration, ranging from 1.5and evapotranspiration, ranging from 1.5
to 5.2 kg ha-1 mm-1to 5.2 kg ha-1 mm-1 for clonal tea infor clonal tea in
Tanzania.Tanzania.

 These are lower than the TE valueThese are lower than the TE value
obtained by Anandacoomaraswamy et al.obtained by Anandacoomaraswamy et al.
(2000), probably because the water use(2000), probably because the water use
estimate of Stephens and Carr (1991b)estimate of Stephens and Carr (1991b)
incorporated both soil evaporation andincorporated both soil evaporation and
transpiration.transpiration.
 Although Ta of both sites was similar, theAlthough Ta of both sites was similar, the
VPD in the Sri Lankan site (daily meanVPD in the Sri Lankan site (daily mean
ranging from 0.52- 0.88 kPa) was probablyranging from 0.52- 0.88 kPa) was probably
lower than that in the Tanzanianlower than that in the Tanzanian site (notsite (not
exceeding 2 kPa).exceeding 2 kPa).

 Although Stephens and Carr (1991a)Although Stephens and Carr (1991a)
contend that the VPD was unlikely to havecontend that the VPD was unlikely to have
restricted shoot growth, it is possible thatrestricted shoot growth, it is possible that
the slightly greater VPD in their study maythe slightly greater VPD in their study may
have increased evapotranspiration andhave increased evapotranspiration and
thereby reduced TE and WUE.thereby reduced TE and WUE.
 This reduction of TE with increasing VPDThis reduction of TE with increasing VPD
is in agreement with the theoreticalis in agreement with the theoretical
analyses of Bierhuizen and Slatyer (1965)analyses of Bierhuizen and Slatyer (1965)
and Monteith (1986).and Monteith (1986).

 Stephens and Carr (1991b) furtherStephens and Carr (1991b) further
showed that WUE of tea is influenced byshowed that WUE of tea is influenced by
water availability, nitrogen application andwater availability, nitrogen application and
season.season.
 During a ‘warm’ (daily mean Ta increasingDuring a ‘warm’ (daily mean Ta increasing
from 16-18oC up to 22-24oC) dry season,from 16-18oC up to 22-24oC) dry season,
WUE increased from 0.6-1.0 kg ha-1 mm-WUE increased from 0.6-1.0 kg ha-1 mm-
1 under rainfed conditions up to 2.2-4.1 kg1 under rainfed conditions up to 2.2-4.1 kg
ha-1 mm-1 under irrigatedha-1 mm-1 under irrigated conditions.conditions.

 The response of WUE to irrigationThe response of WUE to irrigation
increased with increasing nitrogen fertilizerincreased with increasing nitrogen fertilizer
application up to 225 kg N ha-1. Inapplication up to 225 kg N ha-1. In
contrast, during a ‘cool’ (daily mean Tacontrast, during a ‘cool’ (daily mean Ta
around 14oC) dry season, WUE did notaround 14oC) dry season, WUE did not
respond significantly to irrigation,respond significantly to irrigation,
irrespective of the level of fertilizeirrespective of the level of fertilize
application.application.

 However, as compared to the warm dryHowever, as compared to the warm dry
season, in the cool dry season, WUEseason, in the cool dry season, WUE
showed a greater response to nitrogenshowed a greater response to nitrogen
fertilizer application up to 375 kg N ha-1.fertilizer application up to 375 kg N ha-1.
At any givenAt any given irrigation x fertilizer treatmentirrigation x fertilizer treatment
combination,combination,

 WUE in the warm dry season was greaterWUE in the warm dry season was greater
than that in the cool dry season.than that in the cool dry season.
 This could be a positive response to theThis could be a positive response to the
higher Ta in the warm dry season as Ta inhigher Ta in the warm dry season as Ta in
the cool dry season was closer to the Tbthe cool dry season was closer to the Tb
for shoot extension, i.e. 12-15oC (Tanton,for shoot extension, i.e. 12-15oC (Tanton,
1982b; Stephens and Carr, 1990).1982b; Stephens and Carr, 1990).
 When averaged acrossWhen averaged across seasons andseasons and
irrigation treatments,irrigation treatments,

 WUE of tea increased with increasingWUE of tea increased with increasing
nitrogen fertilizer application, from 1.5-2.6nitrogen fertilizer application, from 1.5-2.6
kg ha-1 mm-1 in unfertilized tea to 3.3- 5.2kg ha-1 mm-1 in unfertilized tea to 3.3- 5.2
kg ha-1 mm-1 in tea fertilized at 225 kg Nkg ha-1 mm-1 in tea fertilized at 225 kg N
ha-1.ha-1.

 The above-mentioned results of StephensThe above-mentioned results of Stephens
and Carr (1990) show interesting insightsand Carr (1990) show interesting insights
into the physiological basis of theinto the physiological basis of the
determination of WUE in tea.determination of WUE in tea.
 As WUE is the amount of yield producedAs WUE is the amount of yield produced
per unit of water used throughper unit of water used through
evapotranspsiration, it could be affectedevapotranspsiration, it could be affected
by factorsby factors influencing shoot growth andinfluencing shoot growth and
water use.water use.

 At lower Ta (e.g. cool dry season), WUEAt lower Ta (e.g. cool dry season), WUE
responds more to nitrogen fertilizer than toresponds more to nitrogen fertilizer than to
irrigation.irrigation.
 This could probably be because underThis could probably be because under
cooler conditions, nitrogen availabilitycooler conditions, nitrogen availability
through absorption, which would be slowerthrough absorption, which would be slower
at low Ta, would be a greater limitation toat low Ta, would be a greater limitation to
shoot growth than water availability.shoot growth than water availability.
 Under cooler conditions, theUnder cooler conditions, the
evapotranspiration rates would be lowerevapotranspiration rates would be lower
and the likelihood of development ofand the likelihood of development of
internal water deficits, which wouldinternal water deficits, which would restrictrestrict
shoot growth, would be lower.shoot growth, would be lower.

 In contrast, under warm dry conditions,In contrast, under warm dry conditions,
water availability would be a greaterwater availability would be a greater
limitation than nitrogen availability andlimitation than nitrogen availability and
consequently WUE responds more toconsequently WUE responds more to
irrigation than fertilizer application.irrigation than fertilizer application.

 The TE values measured in terms of kgThe TE values measured in terms of kg
ha-1 (made tea) mm-1 of water transpiredha-1 (made tea) mm-1 of water transpired
can be directly converted to gram ofcan be directly converted to gram of
biomass produced per kilogram of waterbiomass produced per kilogram of water
transpired when a harvest index (i.e. ratiotranspired when a harvest index (i.e. ratio
of made tea yield to total bush dry weight)of made tea yield to total bush dry weight)
of 0.10 (Carr and Stephens, 1992) isof 0.10 (Carr and Stephens, 1992) is
assumed.assumed.

 The TE and WUE values obtained by bothThe TE and WUE values obtained by both
Anandacoomaraswamy et al. (2000) andAnandacoomaraswamy et al. (2000) and
Stephens and Carr (1991b) are higherStephens and Carr (1991b) are higher
than the compiled ranges of TE for C3than the compiled ranges of TE for C3
plant species, i.e. 0.68-1.44 g kg-1 (Jones,plant species, i.e. 0.68-1.44 g kg-1 (Jones,
1992) and 0.88- 2.65 g kg-1 (Shantz and1992) and 0.88- 2.65 g kg-1 (Shantz and
Piemeisel, 1927 as compiled byPiemeisel, 1927 as compiled by Jones,Jones,
1992).1992).

 Whether this indicates a greater intrinsicWhether this indicates a greater intrinsic
capacity of tea to use water morecapacity of tea to use water more
efficiently needs to be ascertained throughefficiently needs to be ascertained through
more accurate estimates of TE and WUE.more accurate estimates of TE and WUE.
 The inverse relationship between TE andThe inverse relationship between TE and
VPD of the growing environment meansVPD of the growing environment means
that their product TE x VPD isthat their product TE x VPD is
approximately constant (Tanner andapproximately constant (Tanner and
Sinclair, 1983;Sinclair, 1983; Monteith, 1986).Monteith, 1986).

 Compiling data from several works, SquireCompiling data from several works, Squire
(1990) showed that the product TE x VPD(1990) showed that the product TE x VPD
was around 4.0- 5.0 g kg-1 kPa for awas around 4.0- 5.0 g kg-1 kPa for a
range of C3 crops.range of C3 crops.
 In comparison, the corresponding valueIn comparison, the corresponding value
obtained by Anandacoomaraswamyobtained by Anandacoomaraswamy et al.et al.
(2000) for tea was 6.9 g kg-1 kPa.(2000) for tea was 6.9 g kg-1 kPa.

 In addition to the environmental factorsIn addition to the environmental factors
described above, Wijeratne et al. (2007a)described above, Wijeratne et al. (2007a)
reported that WUE of tea is influenced byreported that WUE of tea is influenced by
CCa as well.a as well.
 Accordingly, higherAccordingly, higher CCa (600 ìmol mol-1)a (600 ìmol mol-1)
enhanced Pn and reduced transpiration,enhanced Pn and reduced transpiration,
and thereby increased WUE of tea atand thereby increased WUE of tea at
higherhigher CCa.a.

Effects of water deficits on tea yieldEffects of water deficits on tea yield
and yield componentsand yield components
 :: Most of the tea growing in differentMost of the tea growing in different
regions of the world experiences SWD ofregions of the world experiences SWD of
varying magnitudes and durations.varying magnitudes and durations.
 Periods of SWD often coincide with higherPeriods of SWD often coincide with higher
VPD and higher Ta.VPD and higher Ta.

 The effects of water deficits on tea yieldThe effects of water deficits on tea yield
can be predicted by examining the effectscan be predicted by examining the effects
on the two principal yield components,on the two principal yield components,
Nsh and Wsh.Nsh and Wsh.
 Carr et al. (1987) observed that the rate ofCarr et al. (1987) observed that the rate of
shoot production, which primarilyshoot production, which primarily
determines Nsh, decreased when thedetermines Nsh, decreased when the
averageaverage midday shoot øw fell below -0.6midday shoot øw fell below -0.6
to -0.7 MPa.to -0.7 MPa.

 Squire and Callander (1981) observed thisSquire and Callander (1981) observed this
limiting shoot øw to be -0.8 MPa. Shootlimiting shoot øw to be -0.8 MPa. Shoot
øw could fall below the limiting value dueøw could fall below the limiting value due
to an increase of SWD during prolongedto an increase of SWD during prolonged
rainless periods.rainless periods.
 Interestingly, the limiting shoot øw couldInterestingly, the limiting shoot øw could
be reached even when tea is growing on abe reached even when tea is growing on a
wet soil, if the VPD increaseswet soil, if the VPD increases beyond abeyond a
threshold (Williams, 1971).threshold (Williams, 1971).

 Carr et al. (1987) identified this thresholdCarr et al. (1987) identified this threshold
VPD to be around 2 kPa.VPD to be around 2 kPa.
 Wijeratne and Fordham (1996) found thatWijeratne and Fordham (1996) found that
shoot øw of tea decreased linearly withshoot øw of tea decreased linearly with
rising SWD above 30-40 mm at lowrising SWD above 30-40 mm at low
altitudes in Sri Lanka.altitudes in Sri Lanka.

Influence of vapour pressure deficitInfluence of vapour pressure deficit
 Tea is one of the plant species which hasTea is one of the plant species which has
been shown to be highly sensitive tobeen shown to be highly sensitive to
atmospheric VPD of the growingatmospheric VPD of the growing
environment.environment.
 During the dry periods of many teaDuring the dry periods of many tea
growing regions of the world, VPD couldgrowing regions of the world, VPD could
rise to levels which would influencerise to levels which would influence ggs,s,
shoot øw and the rates of shootshoot øw and the rates of shoot initiationinitiation
and extension (Squire and Callander,and extension (Squire and Callander,
1981).1981).

 In addition, VPD influences these keyIn addition, VPD influences these key
processes of yield formation of tea evenprocesses of yield formation of tea even
during periods when the soil is wet.during periods when the soil is wet.
 Furthermore, the linear relationshipFurthermore, the linear relationship
between shoot extension rate andbetween shoot extension rate and
temperature breaks down at highertemperature breaks down at higher VPDVPD
(Squire and Callander, 1981).(Squire and Callander, 1981).

 During wet periods with frequent rain,During wet periods with frequent rain,
shoot øw of tea has an inverse, linearshoot øw of tea has an inverse, linear
relationship with VPD (Williams, 1971;relationship with VPD (Williams, 1971;
Squire, 1976, 1979).Squire, 1976, 1979).
 This probably operates through theThis probably operates through the
influence of VPD on transpiration, whichinfluence of VPD on transpiration, which
increases with increasingincreases with increasing VPD causing aVPD causing a
decrease in shoot øw.decrease in shoot øw.

 During these wet periods, VPD did notDuring these wet periods, VPD did not
exceed 2 kPa and shoot øw did not fallexceed 2 kPa and shoot øw did not fall
below –1 MPa. Furthermore, during wetbelow –1 MPa. Furthermore, during wet
periods, this relationship did not showperiods, this relationship did not show
hysteresis.hysteresis.
 However, it broke down during dryHowever, it broke down during dry
periods, with shoot øw quickly falling toperiods, with shoot øw quickly falling to
around -1.5 to -2.0 MPa during the earlyaround -1.5 to -2.0 MPa during the early
part of the day around 0900 h and thenpart of the day around 0900 h and then
remaining at this minimum level while theremaining at this minimum level while the
VPD continued to increase up to 4.0 kPa.VPD continued to increase up to 4.0 kPa.

 Moreover, even if the VPD decreasedMoreover, even if the VPD decreased
during the latter part of the day, shoot øwduring the latter part of the day, shoot øw
remained at its minimum until the end ofremained at its minimum until the end of
the day. Even when shoot øw began tothe day. Even when shoot øw began to
rise again during late afternoon, it showedrise again during late afternoon, it showed
hysteresis and lagged behind thehysteresis and lagged behind the
decrease of VPD.decrease of VPD.
 Irrigated tea bushes during the dry periodIrrigated tea bushes during the dry period
showed a similar diurnal pattern but theirshowed a similar diurnal pattern but their
minimum shoot øw was about 0.8 MPaminimum shoot øw was about 0.8 MPa
higher.higher.

 Interestingly, when the soil was re-wettedInterestingly, when the soil was re-wetted
by rains at the end of the dry season, theby rains at the end of the dry season, the
linear relationship between short øw andlinear relationship between short øw and
VPD was re-established and shoot øwVPD was re-established and shoot øw
quickly returned to its higher values (i.e. >quickly returned to its higher values (i.e. >
-1 MPa).-1 MPa).
 This indicated that the roots in the top soilThis indicated that the roots in the top soil
(0-15 cm) had(0-15 cm) had remained alive during theremained alive during the
dry period.dry period.

 Despite the absence of a clear relationshipDespite the absence of a clear relationship
between shoot øw and VPD during drybetween shoot øw and VPD during dry
periods on a diurnal basis, Squire (1979)periods on a diurnal basis, Squire (1979)
found a close inverse relationship betweenfound a close inverse relationship between
the weekly rate of shoot extension andthe weekly rate of shoot extension and
mean VPD measured at 1400 h during themean VPD measured at 1400 h during the
dry season in Malawi.dry season in Malawi.

Influence of increasing VPD and decreasingInfluence of increasing VPD and decreasing
shootshoot øøw on shoot growthw on shoot growth
 :: In addition to decreasing the rate ofIn addition to decreasing the rate of
shoot initiation, decreasing shoot øw,shoot initiation, decreasing shoot øw,
induced by either increasing VPD or SWD,induced by either increasing VPD or SWD,
decreases SER.decreases SER.
 Shoot extension rates have been found toShoot extension rates have been found to
be inversely related to VPDbe inversely related to VPD (Squire,(Squire,
1979).1979).

 Tanton (1982a) found that theTanton (1982a) found that the relativerelative
shoot extension rate (RSER)shoot extension rate (RSER) was notwas not
affected by VPD when it was below 2.3affected by VPD when it was below 2.3
kPa.kPa.
 However, when VPD rose above 2.3 kPa,However, when VPD rose above 2.3 kPa,
RSER decreased linearly with increasingRSER decreased linearly with increasing
VPD so that RSER was reduced by 75%VPD so that RSER was reduced by 75%
at 4.0 kPa.at 4.0 kPa.

 This reduction of RSER occurs because ofThis reduction of RSER occurs because of
the reduction in shoot øw caused bythe reduction in shoot øw caused by
increasing VPD.increasing VPD.
 Odhiambo et al. (1993) showed for teaOdhiambo et al. (1993) showed for tea
growing at high altitude (2710 m a.s.l.) ingrowing at high altitude (2710 m a.s.l.) in
Kenya that prolonged SWD (> 80 mm)Kenya that prolonged SWD (> 80 mm)
and high VPD s (> 2.2 kPa) reduced shootand high VPD s (> 2.2 kPa) reduced shoot
øw, Nsh, SER and leaføw, Nsh, SER and leaf yield.yield.

 Furthermore, a relatively droughtFurthermore, a relatively drought
susceptible cultivar had a lower shoot øw,susceptible cultivar had a lower shoot øw,
SER, Nsh, rate of shoot regeneration andSER, Nsh, rate of shoot regeneration and
yield than more drought tolerant cultivarsyield than more drought tolerant cultivars
during periods of high SWD and highduring periods of high SWD and high
VPD.VPD.
 Smith et al. (1993c) also found aSmith et al. (1993c) also found a
significant decrease in exponential RSERsignificant decrease in exponential RSER
with increasing VPD for tea growing inwith increasing VPD for tea growing in
Malawi.Malawi.

 During periods of rainfall, when VPD isDuring periods of rainfall, when VPD is
below 2 kPa, Ta is the main determinantbelow 2 kPa, Ta is the main determinant
of SER.of SER.
 Tanton (1982b) observed a linearTanton (1982b) observed a linear
relationship between RSER and Ta aboverelationship between RSER and Ta above
a Tb of 12.5oC.a Tb of 12.5oC.
 However, during dry periods, shoot øwHowever, during dry periods, shoot øw
becomesbecomes the main determinant of shootthe main determinant of shoot
growth rate.growth rate.

 According to the relationships betweenAccording to the relationships between
VPD and shoot øw developed by WilliamsVPD and shoot øw developed by Williams
(1971) and Squire (1976, 1979), the(1971) and Squire (1976, 1979), the
limiting VPD of 2.3 kPa observed bylimiting VPD of 2.3 kPa observed by
Tanton (1982a) is equivalent to a shootTanton (1982a) is equivalent to a shoot
øw of -0.8 to -1.0 MPa, which is close toøw of -0.8 to -1.0 MPa, which is close to
thethe limiting shoot øw for shoot initiation aslimiting shoot øw for shoot initiation as
well.well.

 Therefore, both shoot initiation rate andTherefore, both shoot initiation rate and
SER of tea are reduced when shoot øwSER of tea are reduced when shoot øw
decreases below a threshold of around -decreases below a threshold of around -
0.7 to -1.0 MPa.0.7 to -1.0 MPa.
 Accordingly, tea yield would also reduceAccordingly, tea yield would also reduce
below this threshold as observed, forbelow this threshold as observed, for
example by Carrexample by Carr (1971b).(1971b).

 Reduction of tea yields during prolongedReduction of tea yields during prolonged
dry periods have been observed by manydry periods have been observed by many
workers in several tea-growing regions ofworkers in several tea-growing regions of
the world (Carr, 1974; Carr et al., 1987;the world (Carr, 1974; Carr et al., 1987;
Stephens and Carr, 1991a; Othieno et al.,Stephens and Carr, 1991a; Othieno et al.,
1992; Burgess and Carr, 1993; Nixon et1992; Burgess and Carr, 1993; Nixon et
al., 2001; De Costa and Surenthran,al., 2001; De Costa and Surenthran,
2005).2005).

 A characteristic phenomenon of tea is theA characteristic phenomenon of tea is the
rapid increases of yield on re-wetting ofrapid increases of yield on re-wetting of
the soil following a dry period (Fordhamthe soil following a dry period (Fordham
and Palmer-Jones, 1977).and Palmer-Jones, 1977).
 This is caused by the rapid extension ofThis is caused by the rapid extension of
the large number of shoots that hadthe large number of shoots that had
initiated but had remained dormant on theinitiated but had remained dormant on the
plucking table because of the lower shootplucking table because of the lower shoot
øw during the dryøw during the dry period.period.

 Re-wetting of the soil coupled with theRe-wetting of the soil coupled with the
lowering of VPD by rains quickly increaseslowering of VPD by rains quickly increases
shoot øw above the threshold of -1 MPashoot øw above the threshold of -1 MPa
and allows rapid extension of the initiatedand allows rapid extension of the initiated
shoots.shoots.

 While the leaf appearance rateWhile the leaf appearance rate
(1/phyllochron) is primarily related to Ta, it(1/phyllochron) is primarily related to Ta, it
is also influenced by increasing wateris also influenced by increasing water
deficits, both soil and atmospheric, duringdeficits, both soil and atmospheric, during
a drought.a drought.
 Burgess and Carr (1998) showed that theBurgess and Carr (1998) showed that the
phyollochron increased from 5.8-7.9 dphyollochron increased from 5.8-7.9 d
during the warm, wet season to 11-19 dduring the warm, wet season to 11-19 d
during the cool, dry season probablyduring the cool, dry season probably
because of the influence of lowerbecause of the influence of lower
temperaturestemperatures and lower shoot øw.and lower shoot øw.

 Wijeratne (1994) also showed that theWijeratne (1994) also showed that the
phyllochron of tea seedlings grown inphyllochron of tea seedlings grown in
controlled environmental growth chamberscontrolled environmental growth chambers
increased from 5.70 d under well-wateredincreased from 5.70 d under well-watered
conditions to 7.46 d under waterconditions to 7.46 d under water stressedstressed
conditions.conditions.

 Similarly, water stress reduced SER (fromSimilarly, water stress reduced SER (from
10.78 to 6.65 mm wk-1) and leaf10.78 to 6.65 mm wk-1) and leaf
expansion rate (from 5.18 to 1.99 cm2 wk-expansion rate (from 5.18 to 1.99 cm2 wk-
1) in the same plants.1) in the same plants.
 All these results show that water stressAll these results show that water stress
reduces both the rates of leaf production,reduces both the rates of leaf production,
leaf expansion and shoot extension in tealeaf expansion and shoot extension in tea
and thereby reduces the number of shootsand thereby reduces the number of shoots
that develop up to the pluckable stagethat develop up to the pluckable stage
within a given time period.within a given time period.
 Consequently, yield is reduced as a resultConsequently, yield is reduced as a result
o reductions in both Nsh and Wsh.o reductions in both Nsh and Wsh.

Critical soil water deficit andCritical soil water deficit and
response to irrigationresponse to irrigation
 The level of SWD at which plantThe level of SWD at which plant
processes or yield (which is the integrationprocesses or yield (which is the integration
of all processes) begin to be affectedof all processes) begin to be affected
significantly is termed the ‘critical soilsignificantly is termed the ‘critical soil
water deficit (CSWD)’.water deficit (CSWD)’.
 The CSWD for reducing tea yield hasThe CSWD for reducing tea yield has
been found to vary from 40-200 mmbeen found to vary from 40-200 mm
(Stephens and Carr,(Stephens and Carr, 1989; Carr and1989; Carr and
Stephens, 1992; Burgess, 1993).Stephens, 1992; Burgess, 1993).

 Wijeratne and Fordham (1996) reportedWijeratne and Fordham (1996) reported
that Nsh, Wsh, shoot extension rate andthat Nsh, Wsh, shoot extension rate and
shoot øw of tea were significantly reducedshoot øw of tea were significantly reduced
when SWD exceeded 30-40 mm.when SWD exceeded 30-40 mm.
 In contrast, Stephens and Carr (1994)In contrast, Stephens and Carr (1994)
found that the reduction in tea yield due tofound that the reduction in tea yield due to
SWD was more due to restricted shootSWD was more due to restricted shoot
extension than due to reduced number ofextension than due to reduced number of
activelygrowingactivelygrowing shoots.shoots.

 Hence, they concluded that the criticalHence, they concluded that the critical
SWD for shoot extension should also beSWD for shoot extension should also be
close to 30-40 mm.close to 30-40 mm.
 Because of the close functional linkBecause of the close functional link
between shoot øw and SER, the CSWDbetween shoot øw and SER, the CSWD
should be related to a critical shoot øw.should be related to a critical shoot øw.
 Carr (1971b) showed that the critical shootCarr (1971b) showed that the critical shoot
øw for reduction and shoot extension andøw for reduction and shoot extension and
thereby reductionthereby reduction of tea yield was aroundof tea yield was around
-0.7 to -0.8 MPa.-0.7 to -0.8 MPa.

 In a study which showed significantIn a study which showed significant
cultivar variation in drought sensitivity, thecultivar variation in drought sensitivity, the
cultivars with the lowest relative reductioncultivars with the lowest relative reduction
in yield in response to drought had thein yield in response to drought had the
highest øw, highesthighest øw, highest ggs and the highests and the highest
ratios of CO2 fixed to H2Oratios of CO2 fixed to H2O transpiredtranspired
(Smith et al., 1993c).(Smith et al., 1993c).

 Furthermore, Wijeratne (1994) found thatFurthermore, Wijeratne (1994) found that
the plastochron of tea increased withthe plastochron of tea increased with
increasing SWD. Wijeratne (2004) alsoincreasing SWD. Wijeratne (2004) also
observed that the CSWD for dry matterobserved that the CSWD for dry matter
accumulation in harvested shoots wasaccumulation in harvested shoots was
slightly higher for a relatively drought-slightly higher for a relatively drought-
tolerant genotype than for a drought-tolerant genotype than for a drought-
susceptible genotype growing at lowsusceptible genotype growing at low
altitudealtitude (60 m a.s.l.) in Sri Lanka.(60 m a.s.l.) in Sri Lanka.

 In contrast,In contrast, Othieno et al.Othieno et al. (1992) observed(1992) observed
the CSWD to be 85-100 mm for teathe CSWD to be 85-100 mm for tea
growing at high altitude (2178 m a.s.l.,growing at high altitude (2178 m a.s.l.,
which is close to the upper limit forwhich is close to the upper limit for
economically-viable production of tea) ineconomically-viable production of tea) in
Kenya.Kenya.
 The lower VPD, which rarely exceeds theThe lower VPD, which rarely exceeds the
critical value of 2.3 kPa at high altitudecritical value of 2.3 kPa at high altitude
(Tanton, 1982a) was mainly responsible(Tanton, 1982a) was mainly responsible
for the higher CSWD in this environment.for the higher CSWD in this environment.

 The feasibility of irrigation has beenThe feasibility of irrigation has been
explored as a means of countering theexplored as a means of countering the
adverse effects of SWD.adverse effects of SWD.
 Stephens and Carr (1991a) showed that inStephens and Carr (1991a) showed that in
a situation where VPD is not high enougha situation where VPD is not high enough
to restrict shoot extension, yield of well-to restrict shoot extension, yield of well-
fertilized tea increased by 2.9 kg ha-1 forfertilized tea increased by 2.9 kg ha-1 for
eacheach mm of SWD reduced throughmm of SWD reduced through
irrigation.irrigation.

 Unfertilized tea showed a lower responseUnfertilized tea showed a lower response
to irrigation with 1.4 kg ha-1 mm-1.to irrigation with 1.4 kg ha-1 mm-1.
 This experiment was done in an area withThis experiment was done in an area with
a dry period of up to six months per yeara dry period of up to six months per year
during which the SWDduring which the SWD increases up toincreases up to
600-700 mm.600-700 mm.
 Therefore,Therefore, Therefore, Stephens and CarrTherefore, Stephens and Carr
(1991a) showed that the yield gain(1991a) showed that the yield gain
through irrigation substantially outweighedthrough irrigation substantially outweighed
the additional cost involved.the additional cost involved.

 Field observations in Sri Lanka haveField observations in Sri Lanka have
shown that drip irrigation increased teashown that drip irrigation increased tea
yield by 50-100%.yield by 50-100%.
 Furthermore, yield response to irrigationFurthermore, yield response to irrigation
could be as high as 300% during drycould be as high as 300% during dry
weather (Anonymous 2001, 2002).weather (Anonymous 2001, 2002).
 In contrast, the yield loss due to increasingIn contrast, the yield loss due to increasing
SWD is lower at higherSWD is lower at higher altitudes wherealtitudes where
VPD is also low.VPD is also low.

 For example, Othieno et al. (1992)For example, Othieno et al. (1992)
observed a yield loss of 1.3 kg ha-1 mm-observed a yield loss of 1.3 kg ha-1 mm-
1of SWD at high altitude in Kenya during1of SWD at high altitude in Kenya during
the dry season where the potential SWDthe dry season where the potential SWD
could increase up to 400 mm.could increase up to 400 mm.
 In a detailed study, Nixon et al. (2001)In a detailed study, Nixon et al. (2001)
showed a clear difference between youngshowed a clear difference between young
(i.e. five-year-old) and mature tea (22-(i.e. five-year-old) and mature tea (22-
year-old) in their susceptibility to droughtyear-old) in their susceptibility to drought
andand response to irrigation.response to irrigation.

 As expected, young tea was moreAs expected, young tea was more
susceptible to drought than mature tea.susceptible to drought than mature tea.
 The critical SWD required for causing aThe critical SWD required for causing a
significant yield reduction on ansignificant yield reduction on an annualannual
basis was 200-250 mm for young tea asbasis was 200-250 mm for young tea as
compared to 400-500 mm for mature tea.compared to 400-500 mm for mature tea.
 Furthermore, the rate of reduction ofFurthermore, the rate of reduction of
annual yield with increasing SWD was 22annual yield with increasing SWD was 22
kg ha-1 mm-1 (SWD) in young tea askg ha-1 mm-1 (SWD) in young tea as
against 6.5 kg ha-1 mm-1against 6.5 kg ha-1 mm-1 in mature tea.in mature tea.

 The lower sensitivity ofThe lower sensitivity of annualannual yields ofyields of
mature tea was primarily because it wasmature tea was primarily because it was
able to compensate for part of the yieldable to compensate for part of the yield
loss that occurred during the dry period byloss that occurred during the dry period by
having a substantially higher yieldhaving a substantially higher yield duringduring
the wet period of the year.the wet period of the year.
 Nixon et al. (2001) related the differentialNixon et al. (2001) related the differential
responses of young and mature tea toresponses of young and mature tea to
variation in their root systems and shoot-variation in their root systems and shoot-
root dry matter partitioning.root dry matter partitioning.

 Interestingly, mature tea had four timesInterestingly, mature tea had four times
more structural roots (> 1 mm diameter)more structural roots (> 1 mm diameter)
and eight times more fine roots (< 1 mmand eight times more fine roots (< 1 mm
diameter) than young tea.diameter) than young tea.
 More importantly, the weight of fine rootMore importantly, the weight of fine root
per unit of canopy area in mature tea wasper unit of canopy area in mature tea was
six times greater than in young tea, whichsix times greater than in young tea, which
enabled mature tea a greater capacity toenabled mature tea a greater capacity to
extract and supply water to the foliageextract and supply water to the foliage
canopy during a dry period.canopy during a dry period.

 Burgess and Carr (1993, 1996b) showedBurgess and Carr (1993, 1996b) showed
significant variation in drought sensitivitysignificant variation in drought sensitivity
(measured as the reduction in yield per(measured as the reduction in yield per
unit increase in time-integrated SWDunit increase in time-integrated SWD
above a CSWD) among differentabove a CSWD) among different
genotypes of tea growing in Tanzania.genotypes of tea growing in Tanzania.
 The genotype which showed the highestThe genotype which showed the highest
drought sensitivity had the highest yielddrought sensitivity had the highest yield
under irrigation, but showed the greatestunder irrigation, but showed the greatest
yield reduction withyield reduction with increasing SWD.increasing SWD.

 The drought resistant genotypes wereThe drought resistant genotypes were
able to maintain a greater crop canopyable to maintain a greater crop canopy
cover than the susceptible genotypescover than the susceptible genotypes
during the dry period.during the dry period.
 Burgess and Carr (1996b) also showedBurgess and Carr (1996b) also showed
that the CSWD for yield reduction onthat the CSWD for yield reduction on
annual basis (70-90 mm) was greater thanannual basis (70-90 mm) was greater than
that for yield reduction during the drythat for yield reduction during the dry
season (40-50 mm).season (40-50 mm).

 This is because of the contribution to wetThis is because of the contribution to wet
season yield from shoots, which areseason yield from shoots, which are
initiated during the droughtinitiated during the drought period.period.
 These grow up to the harvestable stageThese grow up to the harvestable stage
only after the relief of drought during theonly after the relief of drought during the
wet season.wet season.

PARTITIONING OF BIOMASS, HARVESTPARTITIONING OF BIOMASS, HARVEST
INDEX AND YIELD 323INDEX AND YIELD 323
 Biomass partitioning:Biomass partitioning: PhotoassimilatesPhotoassimilates
originating in the foliage canopy of teaoriginating in the foliage canopy of tea
have to be partitioned to new shoots forhave to be partitioned to new shoots for
yield formation and to other parts of theyield formation and to other parts of the
bush (i.e. maintenance foliage, branches,bush (i.e. maintenance foliage, branches,
stem and root system) for further growth ofstem and root system) for further growth of
the bush.the bush.

 Particularly, the fraction of assimilatesParticularly, the fraction of assimilates
partitioned to new shoots determines Wshpartitioned to new shoots determines Wsh
and thereby partly determines tea yield.and thereby partly determines tea yield.
 Wijeratne (2004) estimated the annual dryWijeratne (2004) estimated the annual dry
matter accumulation in young tea shootsmatter accumulation in young tea shoots
to be around 10-13 t ha-1 yr-1. Out of thisto be around 10-13 t ha-1 yr-1. Out of this
48-58% was harvested and the rest was48-58% was harvested and the rest was
added to the canopy.added to the canopy.

 Growing shoots of tea act as a strong sink forGrowing shoots of tea act as a strong sink for
photosynthates (Rahman, 1988).photosynthates (Rahman, 1988).
 The respective relative sink capacities of a bud, one budThe respective relative sink capacities of a bud, one bud
and a young leaf, and two leaves and a bud were 100%,and a young leaf, and two leaves and a bud were 100%,
75% and 35%, respectively.75% and 35%, respectively.
 The mature leaves (i.e. maintenance leaves) produceThe mature leaves (i.e. maintenance leaves) produce
photosynthates to meet the demand of the growingphotosynthates to meet the demand of the growing
shoots and other organs. The direction of movement ofshoots and other organs. The direction of movement of
photosynthesis depends on the presence or absence ofphotosynthesis depends on the presence or absence of
shoots on the plucking table.shoots on the plucking table.
 The presence of young, growing shoots results inThe presence of young, growing shoots results in
upward movement of a relatively greater proportion ofupward movement of a relatively greater proportion of
photosynthates.photosynthates.

 In intact bushes, the carbon assimilated byIn intact bushes, the carbon assimilated by
mature leaves readily move into themature leaves readily move into the
developing shoot tips, and becomedeveloping shoot tips, and become
generally distributed in the shoots ingenerally distributed in the shoots in ca.ca.
24 h (Sanderson and Sivapalan, 1966b).24 h (Sanderson and Sivapalan, 1966b).
 Movement of photosynthates into dormantMovement of photosynthates into dormant
buds is much less than into actively-buds is much less than into actively-
growing buds. Photosynthates produced ingrowing buds. Photosynthates produced in
immature tea leaves do not move out ofimmature tea leaves do not move out of
the same leaves nor move out into otherthe same leaves nor move out into other
mature leaves.mature leaves.

 Older leaves do not become parasitic onOlder leaves do not become parasitic on
other leaves when they becomeother leaves when they become
unproductive.unproductive.
 The assimilated carbon tends to divideThe assimilated carbon tends to divide
itself between upward movement towardsitself between upward movement towards
the shoot tips and downwards towards thethe shoot tips and downwards towards the
stems and roots.stems and roots.
 In a study carried out by using 14C inIn a study carried out by using 14C in
unpruned tea, at monthly intervals in aunpruned tea, at monthly intervals in a
year, only 11% of photosynthatesyear, only 11% of photosynthates
produced by the maintenance leaves wereproduced by the maintenance leaves were
allocated to the commercially usefulallocated to the commercially useful
shoots (Barman and Saikia, 2005).shoots (Barman and Saikia, 2005).

 The photosynthetically active maintenanceThe photosynthetically active maintenance
leaves retained 19% and 25% wasleaves retained 19% and 25% was
distributed to the branches. The rootsdistributed to the branches. The roots
utilized 31% of net photosynthates.utilized 31% of net photosynthates.
 At the crop level, biomass partitioning ofAt the crop level, biomass partitioning of
tea has been observed to vary withtea has been observed to vary with
genotype (Burgess and Carr, 1996a, b;genotype (Burgess and Carr, 1996a, b;
Navaratne et al., unpublished results),Navaratne et al., unpublished results),
season (Burgess and Carr, 1996a),season (Burgess and Carr, 1996a),

 water availability (Burgess and Carr,water availability (Burgess and Carr,
1996a), stage of the pruning cycle1996a), stage of the pruning cycle
(Navaratne et al., unpublished results) and(Navaratne et al., unpublished results) and
mineral nutrition (Anandacoomaraswamymineral nutrition (Anandacoomaraswamy
et al., 2002).et al., 2002).
 Less biomass is partitioned to harvestedLess biomass is partitioned to harvested
shoots at cooler temperatures, at highershoots at cooler temperatures, at higher
SWDs, during the latter part of the pruningSWDs, during the latter part of the pruning
cycle and under lower nitrogen supply.cycle and under lower nitrogen supply.
 Conversely, greater biomass is partitionedConversely, greater biomass is partitioned
to roots at cooler temperatures and underto roots at cooler temperatures and under
lower N supply.lower N supply.

 However, contrary to expectations, thereHowever, contrary to expectations, there
was no observable shift in biomasswas no observable shift in biomass
partitioning towards roots during drypartitioning towards roots during dry
periods (Burgess and Carr, 1996a).periods (Burgess and Carr, 1996a).
 Nevertheless, root growth of tea wasNevertheless, root growth of tea was
much less sensitive than shoot and leafmuch less sensitive than shoot and leaf
growth to low soil øw (Burgess and Carr,growth to low soil øw (Burgess and Carr,
1996a).1996a).

Harvest index:Harvest index:
 Harvest index (HI) plays a critically importantHarvest index (HI) plays a critically important
role in yield determination of tea.role in yield determination of tea.
 For example, a greater partitioning of biomass toFor example, a greater partitioning of biomass to
harvested leaves was identified as the principalharvested leaves was identified as the principal
reason for the superior yield potential of thereason for the superior yield potential of the
higher yielding genotypes examined by Burgesshigher yielding genotypes examined by Burgess
and Carr (1996a).and Carr (1996a).
 Navaratne et al. (unpublished results) alsoNavaratne et al. (unpublished results) also
identified decreasing HI as the major factoridentified decreasing HI as the major factor
causing the yield decline of tea during the lattercausing the yield decline of tea during the latter
part of its pruning cycle.part of its pruning cycle.

 The low percentage of photosynthatesThe low percentage of photosynthates
partitioned to harvestable shoots ispartitioned to harvestable shoots is
commensurate with the very low HI valuescommensurate with the very low HI values
obtained for tea (Hadfield, 1974; Tanton,obtained for tea (Hadfield, 1974; Tanton,
1979; Magambo and Cannell, 1981).1979; Magambo and Cannell, 1981).
 The reported values of HI of tea rangeThe reported values of HI of tea range
from 0.10 to 0.20, compared to 0.30-0.70from 0.10 to 0.20, compared to 0.30-0.70
in tropical grasslands, forests and rootin tropical grasslands, forests and root
crops (Tanton, 1979; Magambo andcrops (Tanton, 1979; Magambo and
Cannel, 1981; Barbora and Barua, 1988).Cannel, 1981; Barbora and Barua, 1988).

 For a high yielding (i.e. 5600 kg ha-1 yr-1)For a high yielding (i.e. 5600 kg ha-1 yr-1)
genotype under irrigation, Burgess andgenotype under irrigation, Burgess and
Carr (1996a) reported a HI of 0.24, whichCarr (1996a) reported a HI of 0.24, which
was substantially greater than the otherwas substantially greater than the other
reported values for tea.reported values for tea.
 Wijeratne (2001) reported that harvestingWijeratne (2001) reported that harvesting
policies, particularly the severity ofpolicies, particularly the severity of
harvesting, have a significant influence onharvesting, have a significant influence on
HI.HI.

 Experimental observations have shownExperimental observations have shown
thatthat fish leaf pluckingfish leaf plucking (removing a shoot(removing a shoot
immediately above the fish leaf; Figure 4)immediately above the fish leaf; Figure 4)
harvests about 80% of the total biomass ofharvests about 80% of the total biomass of
a shoot whilea shoot while single leaf pluckingsingle leaf plucking
(removing a shoot immediately above the(removing a shoot immediately above the
most matured true leaf) harvests onlymost matured true leaf) harvests only
around 40% of the total biomass of thearound 40% of the total biomass of the
shoot.shoot.
 Tanton (1979) and Wijeratne (2001) haveTanton (1979) and Wijeratne (2001) have
shown that plucking of young shoots,shown that plucking of young shoots,
which are strong sinks for partitioning ofwhich are strong sinks for partitioning of
assimilates, is the main reason for theassimilates, is the main reason for the
lower HI of tea.lower HI of tea.

 Yield:Yield: The harvested leaf yield of tea (i.e.The harvested leaf yield of tea (i.e.
made tea) can generally reach 4-5 t ha-1made tea) can generally reach 4-5 t ha-1
yr-1 under favourable climatic and soilyr-1 under favourable climatic and soil
conditions with proper management.conditions with proper management.
 There are occasions where yields up toThere are occasions where yields up to
6.5 t ha-1 yr-1 have been reported (Carr6.5 t ha-1 yr-1 have been reported (Carr
and Stephens, 1992).and Stephens, 1992).

 Even at these upper limits, tea yields areEven at these upper limits, tea yields are
much lower than the 10-20 t ha-1 yr-1much lower than the 10-20 t ha-1 yr-1
range of yields for crops in which arange of yields for crops in which a
vegetative part is harvested.vegetative part is harvested.
 Reasons for this lower yield potential ofReasons for this lower yield potential of
tea were discussed earlier in this section.tea were discussed earlier in this section.
In the absence of soil constraints, teaIn the absence of soil constraints, tea
yields under proper management areyields under proper management are
higher at lower elevations than at higherhigher at lower elevations than at higher
elevations.elevations.

 Both climatic and soil constraints wouldBoth climatic and soil constraints would
reduce tea yields from their upper limitsreduce tea yields from their upper limits
under optimum conditions.under optimum conditions.
 Out of the two yield components of teaOut of the two yield components of tea
(i.e. Nsh and Wsh), it is the variation of(i.e. Nsh and Wsh), it is the variation of
Nsh that has the stronger correlation withNsh that has the stronger correlation with
yield variation.yield variation.
 occasions where yields up to 6.5 t ha-1 yr-occasions where yields up to 6.5 t ha-1 yr-
1 have been reported (Carr and Stephens,1 have been reported (Carr and Stephens,
1992).1992).

RESPONSE OF TEA TO CLIMATERESPONSE OF TEA TO CLIMATE
CHANGECHANGE
 Long-term gradual climate changeLong-term gradual climate change
involves increasinginvolves increasing CCa and thea and the
consequent warming of the atmosphere.consequent warming of the atmosphere.
 The rising Ta triggers a variety of changesThe rising Ta triggers a variety of changes
in the atmosphere leading to modifiedin the atmosphere leading to modified
rainfall patterns, evapotranspiration ratesrainfall patterns, evapotranspiration rates
and VPD.and VPD.

 Because of the close relationshipsBecause of the close relationships
between tea yield and these atmosphericbetween tea yield and these atmospheric
variables, long-term climate change isvariables, long-term climate change is
likely to cause significant impacts on thelikely to cause significant impacts on the
key physiological and developmentalkey physiological and developmental
processes that determine the yield andprocesses that determine the yield and
yield components of tea (Wijeratne, 1996).yield components of tea (Wijeratne, 1996).

 Responses to different aspects of climateResponses to different aspects of climate
change can be both positive and negative.change can be both positive and negative.
 A clear positive effect is the response toA clear positive effect is the response to
increasedincreased CCa through increaseda through increased
photosynthetic ratesphotosynthetic rates
(Anandacoomaraswamy et al., 1996) and(Anandacoomaraswamy et al., 1996) and
yields.yields.

 However, there can be substantialHowever, there can be substantial
genotypic variation in the response togenotypic variation in the response to
increasedincreased CCa.a.
 For example, Anandacoomaraswamy etFor example, Anandacoomaraswamy et
al. (unpublished results) showed that whileal. (unpublished results) showed that while
the total dry weights and root dry weightsthe total dry weights and root dry weights
of sand-cultured,of sand-cultured,

nine-month-old tea seedlings of onenine-month-old tea seedlings of one
genotype (TRI 3019) increasedgenotype (TRI 3019) increased
significantly at elevatedsignificantly at elevated CCa over a three-a over a three-
month period of CO2 enrichment at highmonth period of CO2 enrichment at high
altitude, those of another genotype (TRIaltitude, those of another genotype (TRI
3072) did not show a significant response3072) did not show a significant response
under the same conditions.under the same conditions.
 Such genotypic variation in the responseSuch genotypic variation in the response
to elevatedto elevated CCa has been shown for othera has been shown for other
crops (De Costa et al., 2007 for rice) andcrops (De Costa et al., 2007 for rice) and
natural plant species (Poorter and Navas,natural plant species (Poorter and Navas,
2003) as well.2003) as well.

 In two CO2 fertilization field experimentsIn two CO2 fertilization field experiments
carried out over a period of 18 months atcarried out over a period of 18 months at
low (60 m a.s.l.) and high (1380 m a.s.l.)low (60 m a.s.l.) and high (1380 m a.s.l.)
elevations in Sri Lanka, Wijeratne et al.elevations in Sri Lanka, Wijeratne et al.
(2007b) showed that an increase of(2007b) showed that an increase of CCaa
from the present ambient levelfrom the present ambient level

of 370 μmol mol-1 to 600 μmol mol-1 (whichof 370 μmol mol-1 to 600 μmol mol-1 (which
is predicted to occur during the middle ofis predicted to occur during the middle of
this century) increased tea yields in Srithis century) increased tea yields in Sri
Lanka by 33 and 37% at high and lowLanka by 33 and 37% at high and low
elevations, respectively.elevations, respectively.
 The long-term averages ofThe long-term averages of
maximum/minimum temperatures at themaximum/minimum temperatures at the
high and low elevations were 20.5o/high and low elevations were 20.5o/
11.5oC and 32.0o/22.9oC, respectively.11.5oC and 32.0o/22.9oC, respectively.

 Increases in both Nsh and WshIncreases in both Nsh and Wsh
contributed to these yield increases.contributed to these yield increases.
 Tea at elevatedTea at elevated CCa also showed higher Pna also showed higher Pn
and transpiration rates than atand transpiration rates than at ambientambient CCaa
(Wijeratne et al., 2007a).(Wijeratne et al., 2007a).

 The study of Wijeratne et al. (2007b) alsoThe study of Wijeratne et al. (2007b) also
identified several climate change-inducedidentified several climate change-induced
variables which would have negativevariables which would have negative
impacts on tea yields and thereby reduceimpacts on tea yields and thereby reduce
the potential yield gains due to increasingthe potential yield gains due to increasing
CCa.a.
 Particularly, a quadratic relationship, withParticularly, a quadratic relationship, with
the optimum around 22oC, was foundthe optimum around 22oC, was found
between monthly tea yield and monthlybetween monthly tea yield and monthly
mean Ta during ‘wet’ periods.mean Ta during ‘wet’ periods.

 Similarly, a quadratic relationship wasSimilarly, a quadratic relationship was
found between monthly tea yield andfound between monthly tea yield and
rainfall of the previous month.rainfall of the previous month.
 These data were used in a simulationThese data were used in a simulation
model to predict the impacts of increasingmodel to predict the impacts of increasing
CCa, increasing Ta and varying rainfall ona, increasing Ta and varying rainfall on
tea yields at different altitudes.tea yields at different altitudes.
 Results of the simulations showed that theResults of the simulations showed that the
yield increases due to increasingyield increases due to increasing CCa werea were
augmented by increasing Ta at highaugmented by increasing Ta at high
altitudes (Table 4).altitudes (Table 4).

 However, at low altitudes, yield gains ofHowever, at low altitudes, yield gains of
higherhigher CCa were pulled back because thea were pulled back because the
rising Ta pushed the already high Ta in torising Ta pushed the already high Ta in to
the supra-optimal range for most of thethe supra-optimal range for most of the
key physiological processes thatkey physiological processes that
determine yield.determine yield.
 Predicted tea yields by the year 2050Predicted tea yields by the year 2050
under the climate change scenariosunder the climate change scenarios
specified by different Global Circulationspecified by different Global Circulation
Models also showed increased yields atModels also showed increased yields at
higher altitudes, but reduced yields athigher altitudes, but reduced yields at
lower altitudes (Table 4).lower altitudes (Table 4).

 Table 4.Table 4. Projected tea yields at fourProjected tea yields at four
locations in Sri Lanka under differentlocations in Sri Lanka under different
scenarios of climate change (increase inscenarios of climate change (increase in
temperature by 1 and 2oC, increase intemperature by 1 and 2oC, increase in
rainfall by 10%, decrease in rainfall byrainfall by 10%, decrease in rainfall by
10% and increase in ambient CO210% and increase in ambient CO2
concentration up to 435 ìmol mol-1) for theconcentration up to 435 ìmol mol-1) for the
year 2050.year 2050.

 Location characteristics: altitude (m a.s.l.)Location characteristics: altitude (m a.s.l.)
maximum/minimum temperature, andmaximum/minimum temperature, and
annual rainfall: Ratnapura = 60 m,annual rainfall: Ratnapura = 60 m,
32.0/22.9oC, 3617 mm; Kandy = 472 m,32.0/22.9oC, 3617 mm; Kandy = 472 m,
29.0/20.2oC, 1863 mm; Nuwara Eliya =29.0/20.2oC, 1863 mm; Nuwara Eliya =
2013 m, 20.5/11.5oC, 907 mm; Passara =2013 m, 20.5/11.5oC, 907 mm; Passara =
1028 m, 28.7/18.5 oC, 1777 mm.1028 m, 28.7/18.5 oC, 1777 mm. Source:Source:
((Wijeratne et al., 2007b).Wijeratne et al., 2007b).

Kuliang fisiologi lingkungan ing 2014 2

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