ecofisiologia

1. Plant–Water Relations☆
C Giménez, University of Cordoba, Cordoba, Spain
M Gallardo and RB Thompson, University of Almeria, Almeria, Spain
ã 2013 Elsevier Inc. All rights reserved.
Introduction 1
Water in Plants 1
Plant Water Status Indicators 2
Direct Indicators of Plant Water Status 2
Relative water content 2
Water potential and components 2
Indirect Indicators of Plant Water Status 3
Stomatal conductance 3
Stem diameter variations 3
Sap flow 4
Reflectance indices 4
Leaf or canopy temperature 4
Other Methods 4
Water Deficits in Plants 5
Expansive Growth 5
Stomatal Responses 6
Root Signals 6
Applications of Plant–Water Relations to Irrigation Scheduling 7
Introduction
Water constitutes the largest single chemical component of plants, yet the volume within the plant is very small in relation to the
total volume transpired. Adequate plant water status requires that root uptake of soil water satisfies the atmospheric demand (i.e.,
transpiration requirements). When water uptake is insufficient, a plant water deficit can develop. Various parameters are used to
measure plant water status, the most common being water potential. Numerous plant processes are affected by declining plant
water status, the most sensitive being cell expansion, which affects new growth. Stomatal closure is another sensitive process. Plants’
responses to declining soil water content are mediated by lowered leaf water potential and/or by chemical agents produced in roots.
Some parameters of plant water status can be used for irrigation-scheduling purposes.
Water in Plants
In herbaceous plants, water normally constitutes more than 90% of fresh weight, although in rare cases it can be less than 70%.
In woody plants, over 50% of fresh weight consists of water. Of the total water content of plants, 60–90% is located within cells, the
rest (10–40% of total water) is mainly in cell walls. The water in cell walls forms a continuum with specialized transport cells
throughout the plant.
The total volume of water within a plant is very small in relation to the total volume of water transpired during its lifetime. Even
on a daily basis, the volume of water within plants is insufficient to buffer appreciably daily transpiration requirements on a warm,
sunny day. The very large amounts of water transpired by plants, in relation to that retained within plant tissue, can be viewed as the
‘cost’ that plants incur as a consequence of stomatal opening to allow CO2 absorption for photosynthesis.
A primary function of water contained in cells is the maintenance of cell and tissue turgor. Cell turgor is essential for cell
enlargement and therefore for optimal plant growth. Other primary functions are the transport of solutes and participation in
metabolic activities. Because of its high dielectric constant, water acts as a solvent for many mineral and organic solutes, enabling
their transport within cells and throughout the plant. Also, water is directly involved in chemical reactions in cells such as CO2
reduction in photosynthesis. Cooling is another primary function of water in plants. Because of the high energy requirement for
water vaporization (10.5 kJ mol1
at 25
C), water evaporating from leaf surfaces (during transpiration) cools the leaf, thereby
avoiding excessive daytime heating from incoming solar radiation.
☆
Change History: May 2013. C Gimenez updated the following sections: Indirect indicators of plant-water status, Other methods, Applications of plant-water
relations to irrigation scheduling, and Further readings.
Reference Module in Earth Systems and Environmental Sciences http://dx.doi.org/10.1016/B978-0-12-409548-9.05257-X 1

2. The water status of plants is a primary determinant of plant growth and development, and therefore of crop productivity in
agricultural systems, and of plant survival in natural systems. Almost every plant physiological process is directly or indirectly
affected by plant water content. For example, cell enlargement is dependent on the level of cell turgor, photosynthesis is directly
inhibited by insufficient water, and stomatal control of transpiration and CO2 absorption is dependent on the water status of
stomatal guard cells.
The water status of plants is the sum of the interaction of various atmospheric, plant, and soil factors. The availability of soil
water, the atmospheric demand (determined by radiation, humidity, temperature, wind), the capacities of the root system to
absorb water and of the plant to transport absorbed water to transpiring leaves, and stomatal responses for regulating transpiration,
can all appreciably influence plant water status.
Plant water status is commonly characterized by its water potential (C). Water potential is a measure of the free energy status of
water, which, because of its applicability to each component of the soil–plant–atmosphere system, enables water movement
between these components to be considered. It theoretically represents the work involved in moving one mole of water from a
selected point within the plant (or soil) to a reference point of pure water at the same temperature and at atmospheric pressure.
C varies from zero at the reference point to negative values within the plant and soil. It is normally measured in units of pressure,
with megapascals (MPa) being most commonly used.
Leaves are the plant organs where most of the exchange of CO2 and H2O between the plant and the atmosphere occurs. The
pathway for the inward diffusion of CO2 is much the same as that for the outward diffusion of H2O vapor. The outward diffusion of
H2O vapor from the saturated surfaces within the plant to the drier atmosphere follows a gradient of the partial pressure of H2O
vapor. To maximize CO2 fixation by photosynthesis, stomata must remain open for as long as possible during daylight periods.
This also maximizes the period of water loss by transpiration.
The water status of leaves (considered as C) is the balance between the water lost to the atmosphere by transpiration (T) and the
water absorbed by the plant from soil, which is a function of soil water potential (csoil) and the combined resistance to water
movement within the roots and shoots (r). These relationships are described by the equation:
c ¼ csoil Tr [1]
Even in saturated soils (where csoil ¼0), C is negative when transpiration occurs. During daylight periods, absorption of water
lags behind transpiration owing mainly to the high resistance to water flow from soil into root xylem tissue. As atmospheric
evaporative demand increases during the morning, transpiration increases, which lowers the water potential of cells from which
water is evaporating. Within the plant, water then moves from nonevaporating parenchyma cells of leaves, which have a higher C,
toward the evaporating cells, establishing a C gradient. This gradient is transmitted throughout the plant–soil system, enabling
continuous water movement. In the afternoon, transpiration decreases on account of reduced atmospheric evaporative demand.
However, water uptake by roots continues until parenchyma cells fully rehydrate, and their C equals soil C, which usually occurs
during the night. At this stage, plant and soil water are in equilibrium, and absorption by roots ceases. In some species under certain
climatic conditions (high nighttime vapor pressure deficit, wind), transpiration can occur at night; when it does, it is generally
relatively small compared to daytime transpiration. However, it can be sufficient to prevent nighttime equilibration of plant and
soil water potentials.
Plant Water Status Indicators
Direct Indicators of Plant Water Status
Relative water content
The water content of plant tissue is the fraction of the total fresh tissue weight that is water. It can be considered either on a fresh- or
dry-weight basis. As there can be considerable variation between different plant tissues and organs in the maximum water content,
the normalized parameter relative water content (RWC) is used. RWC is the water content of plant tissue or a plant organ relative to
its maximum water content when it is fully hydrated (i.e., saturated). RWC provides a measure of the degree of hydration of tissue,
which is considered to have an important controlling role on numerous plant functions.
Relative water content is expressed as:
RWC ¼ FW DW
ð Þ= SFW DW
ð Þ [2]
where FW is the fresh weight (grams), DW the dry weight (grams), and SFW is the saturated fresh weight (grams).
Water potential and components
The most commonly used parameter to characterize plant water status is water potential (C). Total water potential (C) has four
components: the osmotic potential (cs), pressure potential (cp), matric potential (cm), and gravitational potential (cg):
C ¼ cs þ cp þ cm þ cg [3]
Osmotic potential (cs) results from dissolved solutes in cell sap and is proportional to solute concentration and inversely
proportional to cell water volume. cs in plants is always negative and decreases as solutes concentrate during plant dehydration.
2 Plant–Water Relations

3. Pressure potential is a measure of tissue turgor produced by the diffusion of water into the protoplast of cells enclosed by largely
inelastic cell walls. Matric potential (cm) arises from the action, on water, of electrostatic forces of attraction associated with cell
wall and colloidal surfaces, and of capillary forces associated with narrow transport vessels. In plants it is considered to be
negligible. Gravitational potential (cg) results from gravitational forces acting on the water within plants. On account of the
gradient of cg with height of 0.01 MPa m1
, cg is normally negligible compared with the other components of plant C; it is only
considered to be significant in very tall trees. In most situations, total plant water potential is considered to be the sum of the
pressure potential (cp) and osmotic potential (cs).
As both pressure and osmotic potential are dependent on tissue water content, there are relationships between RWC and cs and
cp, and consequently between RWC and C. These relationships are schematically represented in Figure 1. In a fully hydrated plant,
RWC equals 1, cp is positive, cs is negative, and C (the sum of cp and cs) equals zero. As RWC progressively declines, both cp and
cs decline, and consequently C becomes more negative. When RWC has declined sufficiently for cp to equal zero, the plant loses
turgor and wilts.
Several methods are used to measure the total water potential of plant tissue. Thermocouple psychrometry and hygrometry are
the most accurate methods, but both require very stable environmental conditions. Consequently, whilst they can be used under
controlled conditions, they are very difficult to use in field studies. The pressure-chamber method is suitable for routine and rapid
field measurement.
Pressure potential (cp) can only be measured directly by the pressure microprobe, which is inserted into cell protoplasm. This
technique is limited to relatively large cells and is generally restricted to laboratory measurement. In practice, the most commonly
used procedure is to measure both C and cs, and then to calculate cp as the difference between C and cs. Osmotic potential (cs) is
commonly measured by either thermocouple psychrometry or hygrometry after previously freezing and thawing the sample to
break cell membranes and reduce cp to zero; normally a correction is made for the dilution by apoplastic water. Another method
for measuring cs is by determining a moisture-release curve (also called pressure–volume curve) with a pressure chamber.
Indirect Indicators of Plant Water Status
Stomatal conductance
Stomatal conductance (gl) is a measure of the degree of stomatal opening and can be used as an indicator of plant water status.
Stomatal conductance is related to leaf C by feedback processes. Reductions in gl prevent further decreases in C by reducing
transpiration; also, reductions in C can induce stomatal closure, resulting in lowered gl. Stomatal responses are discussed more
fully in the section Stomatal Responses, below. Stomatal conductance can be measured with both dynamic and steady-state
diffusion porometers.
Stem diameter variations
Transpiring plants and trees undergo diurnal variation in stem or trunk diameter. During the morning, when plants are most
vigorously transpiring, some water from stem tissue, mostly from phloem tissue, is incorporated into the transpiration stream,
producing a measurable reduction in stem diameter. During the afternoon, as transpiration slows, this tissue begins to rehydrate,
with a corresponding increase in stem diameter. Rehydration continues throughout the night as some water uptake continues from
soil, when transpiration has usually completely ceased. Commonly, stem diameter has a maximum daily value just before dawn,
Figure 1 Höffler–Thoday diagram illustrating the relationships between total water potential (C), pressure potential (cp), osmotic potential (cs), and
relative water content (RWC) as a cell or tissue loses water from a fully turgid state. Full and zero turgor, and wilting are indicated by arrows.
Adapted from Jones HG. (1992) Plants and Microclimate. Cambridge, UK: Cambridge University Press, with permission.
Plant–Water Relations 3

4. and a minimum daily value in the early afternoon. The magnitude of the diurnal contraction is dependent on atmospheric
evaporative demand and plant water status. Under given atmospheric conditions, the magnitude of daily stem contraction
increases with increasing plant water deficit. Consequently, stem contraction can be used as an indicator of plant water status.
In some fruit-tree and vegetable species, variations in stem diameter have been shown to be very sensitive measures of plant water
status.
In young herbaceous plants and young trees, well-watered healthy plants show a trend of increasing daily maximum stem-
diameter values, which is an indication of growth. In such plants, a reduction in the rate of growth is apparent in a change in the
slope of daily maximum stem-diameter values, which may be due to a lack of water or also to inadequate crop management or
changed environmental conditions. In more mature plants, daily maximum stem-diameter values tend to be more constant.
Sensors incorporating sensitive pressure transducers, known as linear variable differential transducers (LVDT), connected to data
loggers enable continuous monitoring of stem diameter.
Sap flow
Sap-flow sensors measure transpiration flow as the ascent of sap within xylem tissue; measurements can be made in stems, trunks,
branches, or tillers. Given that transpiration is sensitive to plant water status, with the effect being mediated by stomatal opening
(see Section Stomatal Responses, below), sap flow can be used as an indicator of plant water status.
Sap flow rates can be out of phase with transpiration because of capacitance effects in stems or branches arising from the storage
of water. Continuous data recording enables the time course of transpiration to be followed.
Two different techniques are used to measure sap flow; both use heat as a tracer. One is the stem (or trunk) sector heat-balance
method, in which a section of the entire stem circumference is electrically heated, and the axial and radial heat-loss measured. The
mass flow rate of sap is calculated as a function of the heat dissipated by the ascending sap. The other method is the heat-pulse
method in which the heater and temperature sensor probes are placed inside the trunk in a radial direction. The sap velocity is
calculated as a function of the time required by the flowing sap to transport heat to a particular location.
Reflectance indices
Electromagnetic radiation is strongly absorbed by water at specific wavelengths. Therefore, plant water status can be
assessed by measuring absorption of these wavelengths since their absorption changes in response to plant water content. Most
commonly, reflectance is measured and spectral vegetation indices are used to integrate measurements of different
wavelengths (commonly 2 or 3). Numerous indices have been developed. To assess crop water status, reflectance of different
wavelengths in the near infrared (NIR, 700–1300 nm) and the short infrared (SIR, 1300–2500 nm) are used. The normalized
difference water index (NDWI, [R860–R1240]/[R860 þR1240]), the water index (WI, R970/R900) and different normalized water indices
(NWI, [R970–Rx]/[R970 þRx]) with x being the wavelengths 850, 880 or 900 nm seem to be the most promising for estimating water
content in different species. These indices are based on the hypothesis that NIR wavelengths penetrate deeper into the canopy and
therefore most accurately estimate water content.
The use of these indices for estimating plant/canopy water status should be checked against other well-established, reference
indices, such as leaf water potential. Reflectance measurements can be made with hand-held instruments, ground-based equip-
ment, or using satellite and aircraft imagery. They are easy and quick measurements, and integrate the water status at the canopy
level. They also allow the simultaneous estimation of additional parameters (i.e. photosynthetic capacity, N status etc.) using other
spectral vegetation indices.
Leaf or canopy temperature
When plant water status is adequate, canopy temperature (i.e., average temperature of total leaf surface) is appreciably less than the
air temperature, through the cooling effect of evaporation. As stomata partially close in response to increasing plant water stress, the
energy balance of the plant is altered; less heat is dissipated through the evaporation of water, and consequently canopy
temperature increases. With increasing plant water stress, the difference between canopy and air temperature lessens and, under
conditions of severe water stress, may become positive. Canopy temperature is most commonly measured with infrared
thermometers.
Other Methods
Plants absorb approximately 75% of the visible irradiance, but only a very small fraction is used in producing carbon compounds.
Most of the absorbed energy is mainly dissipated as heat, but it can be also re-emitted as light by chlorophyll molecules. When leaf
chlorophyll absorbs energy of a given wavelength, part of it is dissipated by light emission at longer wavelengths within a very short
time. This process is known as chlorophyll fluorescence. Plant stresses modify the relative proportions of absorbed energy that are
used for chlorophyll fluorescence, photosynthetic quantum conversion and heat emission. Chlorophyll fluorescence derived
parameters are affected, by amongst other factors, water stress, and when used carefully chlorophyll fluorescence can provide
useful information about how water stress is affecting photosynthetic performance.
As plant water status affects overall plant growth, measurable parameters such as fruit growth and expansive growth of leaves or
stems have been used as indicators of plant water status. Visual characters such as leaf rolling, color change, and visible wilting have
also been used. However, visual characteristics are normally an indication that the water stress is severe.
4 Plant–Water Relations

5. Water Deficits in Plants
Traditionally, ‘plant water deficit’ or ‘plant water stress’ has been defined as being when plant water status is reduced sufficiently to
affect normal plant functioning (e.g., plant growth, stomatal conductance, rate of photosynthesis). It is not possible to define water
deficit in terms of absolute values on account of the complex interaction of atmospheric, plant, and soil factors involved. Also,
preconditioning effects from the prior history of the plant can modify plant response at a given water potential. Consequently, a
commonly used approach is to express the measured values of the stressed plants in relation to those of well-irrigated plants which
have experienced the same environmental conditions and previous general history.
Plant water deficits occur when any plant process is affected by:
1. Limited water absorption by roots because of dry, cool, or poorly aerated soil;
2. High evaporative demand, on account of low relative humidity, high air temperature, high wind speed, high radiation, or
combinations of the four;
3. A combination of limited water absorption and high evaporative demand.
Generally, plant water deficits can be considered as being induced by either insufficient available soil water, or a high atmospheric
evaporative demand. Plant water deficits induced by lack of soil water may continue for days, possibly weeks, until they are either
alleviated by rain or irrigation or the plant dies. Those that are solely due to high atmospheric evaporative demand (‘midday’ water
deficits) are much more transient, occurring for no more than several hours in the middle part of the day.
When not alleviated, water deficits induced by lack of soil water become progressively more intense. The time course of leaf and
soil C during a theoretical drying cycle of several days is shown in Figure 2. As soil continues to dry and soil C to decrease, there is
increasingly less available soil water for root uptake. This results in a general tendency for plant C to decrease over time. Eventually,
the plant is unable to absorb sufficient water for plant C to equilibrate with soil C at night. When plants are unable to recover cell
and tissue turgor, which occurs at a threshold soil-water potential value (usually approximately 1.5 MPa), permanent wilting of
the plant occurs.
Midday water deficits can even occur in plants growing in moist soil or nutrient solutions. They occur when high atmospheric
evaporative demand causes high midday transpiration rates that exceed the rate of water uptake by roots. Where these situations are
recurrent, daytime growth is inhibited by these water deficits, and most growth occurs at night, as long as nighttime temperatures
are not limiting. Midday water deficits are not restricted to hot and dry environments; they have also been observed in the humid
tropics and in greenhouses. Recurrent midday water deficits can reduce yield in cropping situations. In intensive greenhouse crop-
production systems, fog systems are commonly used to prevent midday water deficits.
Plant processes affected by water deficits, and an indication of their sensitivity, are shown in Table 1. Processes such as cell
enlargement, cell wall synthesis, and protein synthesis are very sensitive, being affected by relatively small reductions in C. Other
processes, such as stomatal opening and CO2 assimilation, generally require larger reductions in C. As plant water deficits intensify,
an increasing number of plant processes are affected (Table 1), and there can be complex interactions between them. Two of these
processes, expansive growth and stomatal responses, are discussed more fully below.
Expansive Growth
The plant process most sensitive to water deficit is cell enlargement, which affects expansive growth. The effects of plant water
deficits on expansive growth are very important, because they result in reduced expansion of the plant assimilation surface (i.e., leaf
Figure 2 Time course of soil water potential (csoil) and leaf water potential (cleaf) during a drying cycle. Dashed line represents soil water
potential at which permanent wilting of the plant occurs. Adapted from Kramer PJ. (1983) Water Relations of Plants. New York: Academic Press, with
permission.
Plant–Water Relations 5

6. area). The main factor controlling cell enlargement and therefore leaf expansion is turgor pressure (cp). The relationship between
cp and cell expansion can be represented by the Lockhart equation:
dV=Vdt ¼ Eg cp cp,th
[4]
where the relative change in cell volume (V), with time, is dependent on cp above a threshold value (cp,th) below which no cell
expansion occurs. Eg is the gross extensibility of the cell wall (i.e., its capacity to expand). Increases in cell wall extension during
growth are irreversible. Values of cp,th are normally only slightly less than cp values of nonstressed plants, so even small reductions
in cp can result in reduced growth. Neither Eg nor cp,th is constant: they can be influenced by previous exposure to water stress and
by growing conditions. With the progressive imposition of water stress, plants can adapt by either reducing cp,th, increasing Eg, or
doing both in order to maintain growth at lower cp.
Stomatal Responses
When leaf water status is adequate (i.e., well-watered plants) and atmospheric evaporative demand is low, stomatal opening is
determined primarily by light conditions and the low CO2 partial pressure of substomatal cavities. Under these conditions, the
stomata are fully open during daylight periods, maximizing assimilation of CO2, thereby ensuring optimal rates of photosynthesis.
When plant water deficits develop and leaf C decreases, partial stomatal closure occurs to reduce water loss from the plant, with
a consequent reduction in photosynthesis. In many species, C must decline to a threshold value before stomatal closure
commences. Threshold C values for stomatal closure vary with species, leaf age, previous exposure to radiation, the stress history
of the plant, and environmental conditions. If the plant has previously been subjected to a progressive, mild water stress, it can be
preconditioned so that the threshold C value is lowered, thereby enabling assimilation to proceed at lower C. Stomata can also
close in response to low air humidity, even when leaf water status is adequate.
When recovering from water stress, stomata respond relatively slowly. Unlike C, which rapidly recovers to nonstressed values
once water stress is alleviated, stomatal conductance can take several hours, even days, to recover to nonstressed values.
Stomatal closure in response to plant water deficits is not solely controlled by turgor pressure of leaf cells. The process is
complex, with the additional involvement of solute movement. When water deficits develop, stomatal guard cells simultaneously
lose both water and cell solutes, particularly Kþ
ions. It seems that water stress promotes the efflux of Kþ
ions from guard cells,
resulting in the loss of cell turgor, which induces stomatal closure. In the contrary situation, when water stress is relieved,
synchronized changes in Kþ
concentration and water content of guard cells have been detected prior to stomatal opening.
Root Signals
Traditionally it has been accepted that drying soil has a hydraulic effect on plant water relations and consequently on cell expansion
and leaf gas exchange. The basic theory is that a reduction in soil water potential causes a reduction in plant pressure potential,
inducing reduced cell expansion and stomatal closure. In recent decades, it has become clear that stomatal closure, and also leaf
Table 1 Plant process affected by water stress and their sensitivity. Length of the horizontal lines represent the range of stress levels within which
a process becomes first affected. Dashed lines signify deductions based on more tenuous data
a
With C of well-watered plants under mild evaporative demand as the reference point.
Source: Hsaio, T. C. (1973). Plant response to water stress. Annual Review of Plant Physiology 24, 519–570.
6 Plant–Water Relations

7. expansion, are controlled by a mechanism or mechanisms additional to changes in pressure potential of leaf cells. Reductions in
stomatal conductance (gl) of plants, in drying soil, have been related more strongly to changes in soil water status than to leaf water
status. These observations indicate that plants can ‘sense’ that soil in the root zone is drying and can communicate this information
to the leaves by a means other than reduced leaf water status. The evidence to date suggests that a chemical signal, abscisic acid
(ABA), is produced in roots in drying soil and is transported to leaves in the xylem sap, where it induces stomatal closure before
reductions in leaf water potential occur. Although ABA is produced in both roots and leaves, it is accepted that root tips are the
major source of ABA produced in response to drying soil. The sensitivity of stomata to ABA is mediated by the nutritional and water
status of the plant and can be genetically determined. The triggering of stomatal closure by chemical root signals has been observed
in many agricultural species.
Much of the research work to date on chemical root signals has been conducted on the effects of ABA on stomatal responses.
However, other plant growth processes sensitive to water stress, such as leaf expansion and leaf initiation, are also influenced by
chemical root signals. For example, in some species, even when leaf water potential is artificially maintained, soil drying induces
reductions in leaf elongation.
The relative degree of control of hydraulic and chemical root signals over stomatal conductance and leaf expansion, in field-
grown plants, is the subject of considerable research and debate. In some studies, changes in ABA concentration in xylem sap do not
correlate well with changes in stomatal conductance, suggesting that there may be chemical agents other than ABA that induce
stomatal closure. As yet, no other such agents have been identified. Additionally, there is controversy associated with some data
from studies conducted under controlled conditions, such as the induction of ABA production by mechanical impedance to root
growth in potted plants, where the nature of the experimental conditions may contribute to ABA production.
It is now accepted that plant processes such as stomatal conductance and leaf expansion that are sensitive to water stress are
affected by both chemical root signals and changes in leaf water status. Currently, research is being conducted to investigate
possible interactions between hydraulic and chemical signals.
Applications of Plant–Water Relations to Irrigation Scheduling
The most common practical application of plant–water relations is to assist in irrigation management. Different plant water status
indicators have been proposed for use in irrigation scheduling (IS). The advantage of plant-based measurements over alternative IS
methods based on soil-moisture monitoring or estimation of evapotranspiration (ET) requirements is that the actual unit of
production (the plant) is being assessed, rather than using an environmental parameter (e.g., the soil) or a mathematical estimate
(e.g., of ET).
Generally, indicators of plant water status are suitable for determining the timing of irrigation (i.e., when to irrigate), but not the
amount of water to apply. An additional method, for example that of the Food and Agriculture Organization of the UN (FAO) for
estimating crop evapotranspiration requirements, can be used to estimate the amounts to apply. Interpretation of data of plant
water status indicators requires comparison with previously established threshold values that define the need for irrigation (e.g., a
maximum allowable daily stem-diameter contraction). Plant water status data are normalized to take into account variations in
water status due to evaporative demand, i.e., to distinguish effects mostly due to insufficient soil water from those mostly due to
atmospheric demand. Ideally, measurements of plant water status are considered in relation to those obtained from fully irrigated
plants.
In some crops such as cotton, the timing of irrigation has been determined by predawn or midday measurements of leaf water
potential. To use leaf C for this purpose, the relationship between leaf C and growth is established previously, for the individual
species. In some fruit-tree species, stem C has been used in preference to leaf C, because variations in leaf C, due to rapid changes in
evaporative demand, are avoided. Stem C is measured in leaves that have been previously covered to prevent transpiration.
Recently, crop simulation models have been used with the objective of predicting midday stem C.
Until recently, plant water status measurements were generally based on discrete manual measurements (e.g., leaf water
potential, stomatal conductance), which limited the amount of data that could be obtained on account of the time and labor
required for each measurement. Recent developments in sensor and data technology have enabled continuous monitoring of plant
water status. Data from infrared thermometers, LVDT (stem-diameter) sensors and sap-flow gauges can be recorded every few
minutes, providing detailed information on the dynamics of plant water status. Automatic programmers for irrigation management
are being increasingly used; these systems can be integrated with continuously measured parameters of plant water status so that
crops can be automatically irrigated when the selected indicator of plant water status reaches a defined limit. Discrete measure-
ments of plant water status cannot be used in this way.
Different methods based on temperature difference between air and canopy have been developed to schedule irrigation. The
most widely used is the crop water stress index, which relates canopy (Tc) and air (Ta) temperature to the vapor pressure deficit
(VPD), and compares this with Tc Ta for a well-watered crop at the same VPD.
Trunk-diameter fluctuations (TDF) measurements with LVDT sensors have been proposed for IS in a number of fruit-tree
species. Absolute TDF values, without consideration of evaporative demand, can be difficult to interpret. For that reason, values are
normalized with respect to those in non-limiting soil water conditions at the same evaporative demand. The normalized value is
called signal intensity (SI). Applying trunk- or stem-diameter measurements for IS requires firstly the definition of parameters
derived from the data, and then the development of criteria for applying the derived parameters, as discussed previously. Among
Plant–Water Relations 7

8. the TDF-derived parameters to be used in irrigation scheduling the most sensitive in trees with slow trunk growth is the signal
intensity of the maximum daily shrinkage (MDS-SI). In young trees, the trunk growth rate (TGR), expressed as the difference of the
maximum trunk diameter over two consecutive days (Figure 3), has been proposed as the most sensitive parameter because
decreases in trunk growth occur rapidly in response to water stress. Irrigation protocols have been successfully developed for some
mature fruit trees and vegetable species that involve: (1) selecting the derived parameter most suitable for an individual species,
particular growth stage and crop load, and (2) relating the derived parameters to reference values of well-watered crops and
normalizing them for VPD. In the case of MDS, equations to predict reference values from meteorological data are available for
several woody crops. Considerations when using this method are the number of replicate measurements required to account for
between-plant variability, and that other biotic and abiotic stresses, as pests, diseases and flooding, can affect TDF.
Sap-flow sensors have also been used for irrigation scheduling in fruit trees. A method for the automatic control of irrigation has
been developed, in which sap-flow sensors determine the amount of water consumed by the plant, at short time intervals.
An irrigation system can be programmed to replenish this water at specified times or time intervals. Variability between plants is
an issue when scaling from single plants to crops.
Further Reading
Baker NR and Rosenqvist E (2004) Applications of chlorophyll fluorescence can improve crop production strategies: An examination of future possibilities. Journal of Experimental
Botany 55: 1607–1621.
Davies WJ and Zhang J (1991) Root signals and the regulations of growth and development of plants in drying soil. Annual Review of Plant Physiology 42: 55–76.
Fernández JE and Cuevas MV (2010) Irrigation scheduling from stem diameter variations: A review. Agricultural and Forest Meteorology 150: 135–151.
Goldhamer DA, Fereres E, Mata M, Girona J, and Cohen M (1999) Sensitivity of continuous and discrete plant and soil water stress monitoring in peach trees subjected to deficit
irrigation. Journal of the American Society for Horticultural Science 124: 437–444.
Govender M, Dye PJ, Weiersbye IM, Witkowski ETF, and Ahmed F (2009) Review of commonly used remote sensing and ground-based technologies to measure plant water stress.
Water South Africa 35: 741–752.
Hsiao TC (1973) Plant responses to water stress. Annual Review of Plant Physiology 24: 519–570.
Hsiao TC (1990) Measurements of plant water status. In: Stewart BA and Nielsen DR (eds.) Irrigation of Agricultural Crops, pp. 243–279. Madison, WI: ASA–CSSA–SSSA.
Hsiao TC and Bradford KJ (1983) Physiological consequences of cellular water deficits. In: Jordan WR, Sinclair T, and Taylor HM (eds.) Limitations to Efficient Water Use in Crop
Production, pp. 227–265. Madison/WI: ASA–CSSA–SSSA.
Jones HG and Tardieu F (1998) Modelling water relations of horticultural crops. Scientia Horticulturae 74: 21–46.
Kirkham MB (2005) Principles of Soil and Plant Water Relations. San Diego, CA: Elsevier, Academic Press.
Marsal J and Stöckle CO (2012) Use of CropSyst as a decision support system for scheduling regulated deficit irrigation in a pear orchard. Irrigation Science 30: 139–147.
Ortuño MF, Conejero W, Moreno F, Moriana A, Intrigliolo DS, Biel C, Mellisho CD, Pérez-Pastor A, Domingo R, Ruiz-Sánchez MC, Casadesús J, Bonany J, and Torrecillas A (2010)
Could trunk diameter sensors be used in woody crops for irrigation scheduling? A review of current knowledge and future perspectives. Agricultural Water Management 97: 1–11.
Sebastiani L, Tognetti R, and Motisi A (eds.) (2012) Proceedings of the VIII International Symposium on Sap Flow, Acta Horticulturae 951, International Society for Horticultural
Science, Leuven, Belgium.
Smith DM and Allen SJ (1996) Measurement of sap flow in plant stems. Journal of Experimental Botany 47: 1833–1844.
Turner NC (1997) Further progress in crop water relations. Advances in Agronomy 58: 293–338.
Usha K and Singh B (2013) Potential applications of remote sensing in horticulture – A review. Scientia Horticulturae 153: 71–83.
Figure 3 Parameters that can be derived from trunk-diameter measurements, including maximum daily trunk contraction, and trunk growth expressed
as daily differences in maximum and minimum daily trunk diameters (MXTD and MNTD, respectively). Adapted from Goldhamer DA and Fereres E
(2001) Irrigation scheduling protocols using continuously recorded trunk diameter measurements. Irrigation Science 20; 115–125, with permission.)
8 Plant–Water Relations