review article

1. Effect of Ethylene in Wheat
Summary
Ethylene is one of the most important compound in horticulture and its effects on plants
have studied for nearly 100 years (Crocker, W. et al. 1932, Burg and Burg 1965). It is the
lightest of the alkene series of hydrocarbons. It occurs as a gas under Earth ambient
conditions e.g., 300K and 100kPa pressure; hence measuring ethylene for plant research
typically uses direct gas analysis, e.g., gas chromatography (Abeles, Morgan et al. 2012).
These measurements are often carried out by placing plant tissues e.g., leaves, fruits etc. in
a sealed vessel or chamber and then analyzing the headspace of the vessel (Abeles, Morgan
et al. 2012) . The tissues can be degassed using vacuum techniques (Beyer, M. et al. 1970)
or allowed to incubate permitting ethylene to accumulate. These procedures are relatively
convenient and can target different organs or tissues, but they are limited by the size and
types of tissues than can be enclosed for sampling. In addition, this approach generally
requires excision or manipulations resulting in wounding, which can generate ethylene, and
often exposes the tissues to different CO2 and O2 concentrations, which can also effect
ethylene production (Bassi and Spencer 1979). An alternative approach is to place whole
plants or intact organs e.g., fruits in a controlled environment that is atmospherically sealed
(Greef and Profitte 1978, Bassi and Spencer 1979). This allows observation of ethylene
production with minimal perturbation and provides the integrated measure of whole plants
or communities of plants. Unfortunately this latter technique requires controlled
environment chambers that are sufficiently large to accommodate whole plants, and the
atmosphere must either be closed to allow ethylene accumulationor ventilated such that the
air stream passes through a sorbent material, which can later be degassed and sampled
(Bassi and Spencer 1979, Saito, T. et al. 1996).
Excised leaf segments and intact plants of wheat (Triticum aestivum) used to study effect of
water stress on ethylene evolution. By comparing, ethylene production rates by head space
analysis via gas chromatography, it was inferred that ethylene production by observed
segments decreased with the increase in severity of water stress. Its production was
increased in salinity treatments when wheat plants treated with gibberellic acid but reverse
effect with indole-3-acetic acid. It also effect other parameters like decrease in leaf area,
length, water content, fresh, dry matter but Increase in osmatically active solutes, soluble
sugars, soluble protein, amino acids. Various stresses enhanced the ethylene production
include flooding, mechanical wounding, insect infestation herbicides, metals, ozone, fungal
exudates, chilling, freezing and high temperature.
Measurements of ethylene production by plants are especially pertinent to spaceflights
applications, where research on gravity effects and the use of plants for human life support
are being studied (Leather, R. et al. 1972, Wheeler, M. et al. 2001). Because spacecraft must
be protected from the vacuum and harsh environments of space, the atmosphere must be

2. tightly sealed and controlled. Thus any ethylene produced in these environments can
accumulate and cause problems; moreover, these problems can be exacerbated if ethylene
production is increased by the low gravity environment of space leading to adverse plant
growth and sterility (Crocker, W. et al. 1932, Leather, R. et al. 1972, Wheeler, M. et al.
2001). Wheat plants were direct seeded, germinated, and grown in chambers under
Hypobaric conditions. It increased plant growth and did not alter germination rate but the
rate of production was reduced more than 65% under 30kPa compared with ambient
pressure 101kPa. There is direct relation between ethylene production and pressure but
direct with oxygen. But there was a negative correlation between increasing ethylene
concentration and decreasing chlorophyll content. Ethylene levels declined during seed fill
and maturation for wheat. Low concentration of Al rapidly inhibits growth and
subsequently leads to poor nutrient acquisition and reduced crop production. Its toxic effect
is the inhibition of root elongation. The protective role of Put on Al-induced root inhibition
of wheat plants. Most importantly, Put application reduced ACS activity, and thus ethylene
production, which may explain how Put alleviated root inhibition under Al tress.
0.2% to 1.0% concentration of ethylene, in air causes inhibition of the elongation of shoots
and roots of germinating wheat without any inhibition of the rate of carbon dioxide
emission. There are different categories to observe ethylene production e.g., production
during vegetative growth, production during climacteric fruit ripening, production from
environmental stress.
Discussion
Ethylene is known as gaseous plant hormone and its role in regulating aspects of plant
responses to the environment and plant development is well established (Morgan PW 1997;
Campbell WF, DT et al. 2001). In coordination with other plant hormones, it controls
important physiological processes e.g. leaf and flower senescence, abscission and fruit
ripening. During growth and development, it is produced and usually released into the
atmosphere without accumulating (Klassen 2002). Ethylene produced by plants, slowly
accumulated to reach biologically significant Levels because of very low volume changes
per unit time (Abeles, Morgan et al. 2012). More ethylene accumulated, longer the plants
were in the sealed chambers.
Plant species vary in their sensitivity and response to decrease in water potential caused by
drought, temperature, or high salinity. All plants encoded capability for stress perception
signaling and response. Most cultivated species exhibit excellent tolerance to abiotic stress.
Biochemical studies include metabolites as nitrogen containing compounds proline or other
aminoacids, quaternaryamino compounds and polymine and hydroxyl compounds (McCue
and Hanson 1990, Bohnert, Nelson et al. 1995). Molecular studies have revealed a variety
of species a common set of genes and similar proteins, play active roles in the response to
stress (Skriver and Mundy 1990, Vilardell, Martínez-Zapater et al. 1994). Plants encounter
a variety of external and environmental changes. External factors which are critical for
survival of plants are water, temperature, light and other organisms. Internal environmental

3. factors include plant hormones such as ABA, auxin, cytokinins, ethylene, jibberellic acid,
Jasmonic acid, and brassino-steriods (Hong, Jon et al. 1997). Ethylene evolution is
associated with stress and is involved in modulating a broad spectrum of physiological
processes such as senescence, flowering, fruit ripening (Goeschl, Rappaport et al. 1966,
Morgan, He et al. 1990, Narayana, Lalonde et al. 1991). Fluhr 1992 reported that water
stress induced ethylene production in wheat plant. There are also some evidence that salt
stress alters plant growth which could be due to a decrease in natural growth hormones in
plant tissues (Shah and Loomis 1965, Itai, Meidner et al. 1978, Shaddad and El Tayeb
1990).
Ethylene at concentrations of 0.2% to 1.0% in air causes inhibition of the elongation of the
shoots and roots of germinating wheat without any Inhibition of the rate of carbon dioxide
emission. Gassing does not cause any major disturbances in gross protein hydrolysis in the
endosperm, protein synthesis in the embryo, or translocation of nitrogenous substances as
judged by soluble and protein nitrogen determinations. The dry weight of treated embryos
is similar to that of untreated embryos of similar age during the first five days of
germination. Treatment inhibits the water uptake of the embryos during the second, third,
and fourth days of germination but increases the water uptake during the fifth day.
Wheat plants were grown in plastic pots in the soil without NaCl and saline solutions began
when seedlings were two week old corresponded to osmotic potential of NaCl of various
concentrations. The salinized and non-salinized plants were irrigated every other day with
1/10 Pfeffer’s nutrient solution for two weeks. Then Gibbberellic acid and Indole-3-acetic
acid solutions were sprayed three times. The control plants were sprayed with distilled
water a week after the plants were used for analysis. Dry matter was determined after drying
plants in an aerated oven to constant mass. Leaf area was measured by the disk method
(Watson and Watson 1953). Ethylene production was determined by gas liquid
chromatography according to Morgan et al. (1990). Growth parameters (fresh, dry matter,
length, and water contents) of shoots and roots of wheat plant tended to decrease with the
increase of NaCl concentration in the culture media. This reduction was more pronounced
at the higher salinity levels as compared with non-treated plants. Spraying the vegetative
parts of wheat plant with Gibbberellic acid and Indole-3-acetic acid resulted in increase in
the values of fresh and dry matter and water content. Ethylene production increased with
increasing the osmotic pressure in the soil, but it also increased by applying indole-3-acetic
acid and decreased by GA3 (El-Samad 2013).
Ethylene production is increased by a number of biotic and abiotic stress (Abeles, Morgan
et al. 2012). The phenomena is common, known as stress ethylene production. Plant water
deficit is one stress which has been extensively associated with elevated release of ethylene
(El‐Beltagy and Hall 1974, Guinn 1976, Hoffman, Liu et al. 1983). The impact of water
stress on ethylene synthesis is of interest because ethylene could be responsible for
senescence and abscission induced by water stress (McMichael, Jordan et al. 1972). To lost
8% of their fresh weight, 8days or 6 days week old excised leaf segments of plants were
dried. Glass tubes were used to seal, dried and non-dried control leaf segments. 8 days,

4. dried leaves produced considerably more ethylene than the non-dried controls. The results
were qualitatively in accord with the previous reports that drying of detached wheat leaves
caused a short-lived burst of ethylene production. Results were also obtained with leaves
from 6 weeks old plants, except that these leaves produced nearly four times less ethylene
per unit fresh weight than the younger leaves. The burst of ethylene production from the
older leaves lasted about 3 hours longer than that from the younger ones. The water
potential of dried about-2.2MPa and control about-1.0MPa samples were comparable
between the younger and the older leaves. With 6-week old plants, despite this substantial
drop in the plant water status, no obvious difference in ethylene evolution between stressed
and well-watered plants were observed in the continuous flow system. The analysis of data
for individual plants did not reveal any diurnal pattern of ethylene evolution (Narayana,
Lalonde et al. 1991).
Water-stressed intact plants did not produce any more ethylene then the well watered plants
but comparatively lower rates, indeed appeared to diminish the capacity of ethylene
production was observed in the sealed tubes by the leaves excised from the stressed plants.
Excised tissues might represent a shock reaction to sudden desiccation, probably
superimposed on the effects of wounding, senescence, and confinement (Wright 1979,
Wright 1980).This could also explain why wright observed increased ethylene evolution
from rapidly dried intact wheat seedlings enclosed in a sealed container for 6 h,45min
(Wright 1979, Wright 1980). Several studies had confined excised leaves to study
interactions among hormones and environmental factors, as well as the regulation of
ethylene biosynthesis under water stress (Crocker, W. et al. 1932, Hoffman, Liu et al. 1983).
However drying induced ethylene production by excised leaves presented an artifact, with
little direct relevance to the whole plant hormone relations under water stress conditions.
The use of excised leaves was convenient had led to some interesting findings, it probably
couldn’t serve as a valid model the very existence of water stress induced ethyl production
in the whole plants was in question (Wheeler, Petersonet al. 2004). The extrapolation to the
whole plants of the results obtained with excised leaves (Bassi and Spencer 1979) should,
therefore be viewed with caution. Production of ethylene and
1-(malonylamino)cyclopropane-1-carboxylic acid (MACC) had been suggested as
indicators of stress (Beyer Jr and Morgan 1970, Wheeler, M. et al. 2001).
(MACC)
These findings with wheat didn’t imply that water stress did not enhance ethylene
production in other species. Monocots and dicots might had fundamental differences in this
regard, particularly if increase in ethylene production was related to processes such as
abscission that are specific to dicots (Jordan, Morgan et al. 1972, Lipe and Morgan 1973).
However, the rehydration of the plants at full turgor after desiccation caused a high
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5. emission of ethylene. The desiccation would not irreversibly inactivate the enzymes of the
ethylene pathway, since rehydration made the synthesis recommence almost immediately.
Water deficit also increased the free radical levels and the antioxidant scavengers, such as
superoxide dismutase. Free radicals promoted the conversion of
1-amino-cyclopropane-1-carboxylic acid to ethylene and then it was logical to think that
both chemical species were involved in the phenomenon of the acceleration of the grain
maturity before the plant collapses.
Atmospheric measurements made before planting showed low or undetectable levels of
ethylene, indicating that construction materials and internal components of the chamber
were not a significant source of ethylene. Ethylene production varied withspecies, reaching
peak levels 120ppb with wheat. Studies showed a distinctive rise early in growth followed
by a decline during head maturation and senescence. Peak ethylene production occurred
during rapid vegetative growth, about 30d close to when the highest rate of photosynthesis,
respiration and nutrient uptake of the stand were occurring (Wheeler, Corey et al. 1993). it
was also possible that the plant metabolized some ethylene, but the rates would likely be
low and overall concentration not affected (Raskin and Beyer 1989). These observations
indicated that the plant was the primarysource of ethylene and the chamber leakage was the
primary factors in decreasing any elevated ethylene concentrations. If ethylene
concentrations were normalized for standing biomass, wheat showed a trend of high
production when plant was young followed by a gradual decrease with age. Peak rates of
production during rapid growth varied because of slow leakage from the chamber. Because
wheat plant was still alive at harvest although shoot was partially dry, ethylene production
during final stages of senescence was not monitored and could have increased somewhat
(Aharoni, Lieberman et al. 1979). When ripe fruit was harvested, ethylene concentrations
reduced. But concentrations increased again as additional fruit ripened. Estimates of
ethylene production rates are most likely conservatives due to chamber leakage. Riemann
Philipp is determinate and largely parthenocarpic for fruit set and thus it was not clear
whether high ethylene concentrations would have adverse effect on less determinate
cultivars that continue to flower and produce fruit through pollination/fertilization (Klassen
and Bugbee 2002). With the exception of some studies with the wheat, where ethylene
concentrations were not monitored closely, no attempts were made to scrub or reduce
ethylene levels. Flag leaves of wheat showed longitudinal, epinastic rolling soon after peak
ethylene levels were measured.
Epinasty is a classic symptom of ethylene exposure (Crocker, W. et al. 1932, Abeles, B. et
al. 1992). This finding was consistent with elevated levels of ethylene in spaceflight
experiments at microgravity leading to adverse changes in plant growth and sterility (BG
and : 1999, MA, VN et al. 2000, WF, FB et al. 2001). To recycle resources and produce
food, the exploration of space will require the development of Advanced Life Support
Systems (ALS) (Wheeler, M. et al. 2001). ALS will likely combine biotechnology and
physicochemical processes. For air and water purification, the biological component will
include the use of higher plants. It also provide food and psychological benefits (NASA
1998). National Aeronautics and Space Administration (NASA) has a research and

6. development effort to build such systems as part of the NASA Advanced Life Support
System Program (DW and DL 1989, SH and RL 1991, KA, DJ et al. 1997). Plants grown in
space-flight environments, conditions provided to fulfill human environment requirements
and also subject to microgravity (Wheeler, M. et al. 2001). The International Space Station
is expected to have higher CO2 concentrations in its atmosphere than on Earth. Advanced
Life Support System have a potential range of environmental conditions; total gas pressures
60kPa and CO2 700 Pa that is 20 times greater than on Earth (NASA 1998). Plants can
tolerate humidity wide, total gas pressure, relative variation in concentrationof the essential
gases oxygen and carbon dioxide (R and MC 2002). With Advanced Life Support System,
environmental variables have received little research effort; total atmospheric pressure,
high CO2 partial pressure up to 70 Pa or higher, greatly in excess of the range currently
studied by researchers interested in global change differences in humidity, effects of trace
gases – including ethylene under low pressure conditions. Payload volume and mass
required for deployment reduced, because less massive material to be used between plant
growth facility and its external environment due to reduced pressure differential. From
engineering view point, there are important advantages associated with growing plants at
hypobaric conditions in biomass production in extraterrestrial base or space-flight
environments. A hypobaric condition would reduce atmospheric leakage from the facility
to its near 0kPa external environment. Hypobaric conditions in the plant growth facility
would require less N2 gas to be transported or obtained in situ to supplement the
physiologically active gases (CO2 and O2). NASA-sponsored testing reported ethylene
buildup in an 11m2 closed chamber containing wheat plant (Edeen, M.A. et al. 1996).
The internationalSpace Station was required to keep ethylene below 50 ppb through the use
of its trace contaminant control system, and the plant chambers used in space vary in their
degree of closure, with some open to cabin air for cooling and CO2 supply, while others are
relatively closed to provide better environment control levels of <50 ppb reduced seed set in
wheat (Klassen and Bubgee 2002). At the production rates noted for the wheat during rapid
growth, maximum levels in a system with leakage of only 1%vol/d would exceed 1.0ppm,
which could result in little or no seed set (Saliburry and F.B 1997). Related studies with
wheat at NASA’s Johnson Space Center used a more tightly sealed chamber with wheat
plants showed that ethylene levels exceeded 800ppb, resulted in a poor seed production
(Bingham, G. et al. 2000). In addition, experiments on growing wheat to maturity in space
initially failed to produce viable seed, and this was later attributed to the high 1 to 2ppm
ethylene levels in Mir Space Station atmosphere. Subsequent tests in which cabin ethylene
levels were reduced resulted in successful seed production by the wheat plant (Saliburry
and F.B 1997, Bingham, G. et al. 2000) Preliminary tests in our large chamber showed that
potassium permanganate filters were effective for controlling ethylene. Tests with catalytic
burners, UV degradation, selective membranes, different sorbents, or even ethylene
metabolizing microbes also have been considered for ethylene control (Martin and
Sinnaeve 1987, Jr 2002). From number of earlier studies it was demonstrated clearly that
seedling growth and plant germination are possible at hypobaric conditions (J 1973, CBB
and BB 1978, CBB and BB 1978, ME and BR 1988, SH and RL 1991, SH and RL 1991) ,

7. M and D 1992, M and D 1992, E, Y et al. 2002, KA, DJ et al. 2002, KA, DJ et al. 2002, R
and MC 2002). Plants can be grown at high altitude, where pressures are well below 100
kPa (Jr, C et al. 2002), Increasing altitude and decreasing temperature has a invariable
association that confounds the issue of the effect of pressure alone. Ethylene is sensitive in
sealed microenvironments. Hypobaric (as low as 30 kPa total gas pressure) environment
had no adverse effect of ethylene on plant growth, net photosynthesis and stomatal
conductance. Hypobaric may also be influencing enzymes in ethylene biosynthesis. Under
these conditions plant growth increased. Outside the chambers, Li-COR 6400
measurements were taken under ambient conditions on leaves. The increased yield under
hypobaria was more likely due to reduction of ethylene production. (Martin JK and 1987)
grew wheat in a sealed Experimental Soil-Plant Atmosphere System chambers. They
reported that during the first 11 d, ethylene concentration accumulated to 100–200 nmol
mol–1, which was sufficient to affect plant growth (Abeles, B. et al. 1992). In both ambient
and low pressure chambers, Ethylene concentration increased with time (R and M. 2002)
but was not affected by differences in total pressure. However, the volume change per unit
time was greater in their chambers and low pressure was limited to 70kPa. Plants show
enhanced productionof ethylene, experiencing environmental stress frequently (PW and M.
1997). When seedlings Trans plants exposed to total pressures of 30 and 101 kPa, then a
negative linear correlation between chlorophyll content and final ethylene concentration
was developed. Lettuce was more sensitive to ethylene than wheat, because whenboth were
direct seeded and grown for 28d in the same chambers, plant development in wheat was
better than lettuce, especially when ethylene concentration in the chamber was higher.
Hypobaric environments are associated with hypoxia (low O2) condition, particularly when
total gas pressure is reduced below 50 kPa. To avoid hypoxia under hypobaric conditions,
sufficient partial pressures of O2 is supplied. The partial pressure of O2 results in oxidative
phosphorylation (MC and D. 1997) cause potential limitation to plant growthat a hypobaric
condition. Under hypoxic conditions, plant root systems show dramatic ethylene
production rate increase (MB, T. et al. 1985, MC, C. et al. 1989, CJ, M. et al. 1994). The
atmosphere was comprised predominately of oxygen (pO2 = 5 kPa; ≅ 83 % O2), during a
seven-day test at 6 kPa total gas pressure provided. Seedlings germinated and grew in this
environment. Lower pressures and therefore also less O2 seeds failed to germinate
(Schwartzkopf and Mancinelli 1991). When wheat Plants were grown at ambient pressure
in open systems, in which shoots were exposed to normal O2 levels but roots were under
low O2. O2 diffused from leaves to roots through gas channels (aerenchyma) and other plant
structures. Roots and shoots were under the same partial pressure of 6.2 kPa pO2 under a
total gas pressure of 30 kPa, in our tight, closed system. Finally, ethylene concentrations
were reduced by 54.4% under 30 kPa. It can be argued that ethylene concentration
decreased at decreased total pressure due to the hypoxic effect in the chambers. At ambient
pressure, when wheat incubated in four separate air-tight glass containers, flushed with
ambient O2 and6.2 kPa pO2by mixing various amount of compressed air with pre-purified
N2 gas in a closed system. Hypoxia (6.2 kPa) inhibited ethylene production –8.4%. Hypoxia
effect on reducing ethylene was less than than hypobaric. From other studies it was inferred

8. that low total pressure (21–24 kPa) did not inhibit seed germination and initial growth as
long as pO2 was 5 kPa or more (ME, . et al. 1988). One possibility, might be that increase in
oxygen diffusion under hypobaric conditions could have maintained cells under
fully-oxygenated conditions, thereby inhibiting ethylene productionand counterbalance the
effect of low oxygen.
The highest ethylene production rates during vegetative growth ranged from 1.6-2.5
nmol.m-2.d-1 during rapid growth of wheat stands. When photoperiod changed back to 12h
at 61 days, then ethylene levels decreased. Grain filling is an intensive transportation
process regulated by soil drying and plant hormones. The grain-filling rate in wheat is
mediated by the balance between ABA and ethylene in the grains, and an increase in the
ratio of ABA to ethylene increases the grain-filling rate.
Aluminum toxicity is a major constraint limiting crop growth and yield on acid soils
(Kochian, Hoekenga et al. 2004, Kochian, Pineros et al. 2015). Most Al exist in non-toxic
complexes forms when pH is above 5.0. When soil pH drops, Al3+ ions or phytotoxic forms
of Al as [Al(H2O2)6]3+ appeared (Ma, Chen et al. 2014). When it’s concentration was low,
root growth and function was rapidly inhibited, subsequently leaded to poor nutrient
acquisition results in reductionof crop production (Ma 2007, Sun, Lu et al. 2014). Due to its
re-activity, a number of possible mechanisms for Al toxicity have been proposed. It can
interact with multiple root cell sites, including the cell wall, plasma membrane, and inner
side of cytoplasm (symplasm), intracellular components, such as enzymes and proteins,
which lead to the disruption of their functions (Zheng and Yang 2005, Liu, Piñeros et al.
2014)4. Aluminum may also interfere with signal cascades in plants, such as cytosolic Ca2+
and 1, 4, 5-trisphosphate (Matsumoto 2000, Rengel and Zhang2003). Alexclusion from the
root tips based on root exudation of organic acid is the well-characterized mechanism, to
withstand Al stress. Genes involved in the Al-activated organic acid exudation have been
identified in several plant species. For example, Triticum aestivum Al-activated malate
transporter (TaALMT1), which support the Al-induced wheat root malate exudation, has
been identified as the major gene conferring Al resistance in wheat (Sasaki, Yamamoto et al.
2004). The mechanisms of Al toxicity and tolerance are difficult to find, althoughextensive
progresses have been made during the past few years. Under optimal and stressful
conditions, Ethylene established as a vital co-regulator of plant growth and development
(Lin, Zhong et al. 2009, Shan, Yan et al. 2012). Ethylene production has rapidly increased
in plant roots under Al stress (Sun, Tian et al. 2007)–15. Lotus japonicas, Arabidopsis,
Glycine max (Sun, Tian et al. 2007, Kopittke, Moore et al. 2015) used ethylene synthesis
inhibitors or ethylene-insensitive mutants, rapidly produced ethylene contributes to
Al-induced root inhibition. In the roots of maize, enhanced ethylene production did not
play a role in Al toxicity (Gunsé, Poschenrieder et al. 2000). In plants, ethylene is
synthesized from S-adenosylmethionine (SAM) and 1-aminocylopropane-1carboxylic acid
(ACC) respectively catalyzed by 1-aminocylopropane-1carboxylic acid synthase (ACS)
and 1-aminocylopropane-1carboxylic acid oxidase (ACO) (Wang, Li et al. 2002). The
cellular ethylene biosynthesis is under strict metabolic regulation in higher plants, the
enzymes may change to some extent in response to abiotic stress (Schellingen, Van Der

9. Straeten et al. 2014). Down-regulation of ethylene production through manipulating its
biosynthesis enzymes has been considered as an essential strategy to enhance Al tolerance
of crops for example, in Medicago (Chandran, Sharopova et al. 2008). Modulating plant
resistance to Al stress, Putrescine (Put) is an essential signaling molecule (Wang, Huang et
al. 2013). (Lutts, Kinet et al. 1996, Li, Jiao et al. 2004) suggested that Put may interfere with
ethylene biosynthesis or signaling transductions under stress conditions. (Hyodo and
Tanaka 1986) found that Put suppressed ethylene production in a non-competitive manner.
Further studies show that under osmotic stress, Put decreased stress-induced ethylene
production through reducing the level of reactive oxygen species (Li, Jiao et al. 2004).
Since both of Put and ethylene have been implicated in the regulation of Al-induced root
elongation inhibition, it is reasonable to assume that Put may alleviate Al-induced root
inhibition, and subsequently Al toxicity through a mechanism of modulating ethylene
production. Above hypothesis was addressed by using pharmacological agents and two
wheat genotypes differing in Al tolerance (Al-sensitive, Yangmai-5; Al-tolerant, Xi
Aimai-1). The differential Al sensitivity between the wheat genotypes was associated with
their different ethylene production capacities, and that Put promoted root growth under Al
stress by inhibiting ACS-mediated ethylene production. Al tolerance in wheat plants was
shown to be highly associated with the TaALMT1-mediated malate exudation from the root
tips (Sasaki, Yamamoto et al. 2004, Kochian, L. et al. 2015). However, application of Put
had little effect on the expression of TaALMT1 in the root tips of both wheat genotypes
under Al stress, suggested that the Put-related improved Al tolerance might not be
associated with TaALMT1-mediated malate efflux. Put did not affect Al-induced malate
secretion in wheat plants (Yu, Jin et al. 2015). Inhibition of root elongation increased with
increasing external Al concentrations in both Al-sensitive (Yangmai-5) and Al-tolerant (Xi
Aimai-1) wheat genotypes. (Yu, Jin et al. 2015) revealed a marked Put accumulation in the
root tips of wheat plants. Here, exogenous Put significantly alleviated Al-induced root
inhibition in both wheat genotypes. Its effect was more pronounced in Yangmai-5 than in
Xi Aimai-1. (Eticha, Zahn et al. 2010, Yang, Geng et al. 2014) suggested that ethylene may
also regulate root response to Al in plants. Ethylene-releasing substance (ethephon) and an
ethylene precursor (ACC) effected root growth in wheat plants to Al stress. Al caused a
rapid production of ethylene in the root tips of Yangmai-5 after 1.5 h. The Al-elicited
ethylene production reached a maximum after 3 h exposure to Al, and the level was
significantly higher in Yangmai-5 than in Xi Aimai-1. The difference in Al content may not
be responsible for the variation in ethylene production since there was no significant
difference in Al content in the root tips of the two wheat genotypes before 6 h. Application
of ethylene biosynthesis (AVG and CoCl2) or ethylene perception (AgNO3) antagonists
reduced Al-induced ethylene production or blocked its action and efficiently promoted root
growth of both wheat genotypes under Al stress. (Tang and Newton 2005, Swarup, Perry et
al. 2007, Yin, Wang et al. 2014) suggested that Put and ethylene have opposite effects in
many different plant processes and abiotic stress responses. Put had also been reported to
interact with ethylene biosynthesis under various stress conditions (Hyodo and Tanaka
1986, Quinet, Ndayiragije et al. 2010). However, the interaction between Put and ethylene
under Al stress remains undetermined. Exogenous Put inhibited Al-induced ethylene

10. production in the roots of wheat seedling subjected to Al stress, which was similar to the
effects of AVG or CoCl2. Importantly, these changes were related to the recovery from root
inhibition caused by Al exposure. Furthermore, pretreatment with ethephon or ACC was
able to counteract the Put-induced alleviation of root inhibition by Al stress. Put improved
root growth by inhibiting ethylene production under Al stress (Yu, Jin et al. 2016). The
ameliorative effect of Put was more prominent in Yangmai-5, whichproduced higher levels
of ethylene than Xi Aimai-1. (Chen, Xu et al. 2013, Sauter, Moffatt et al. 2013) suggested
that in terrestrial plants, the ethylene and polyamine pathways were generally considered to
be competitive. Although exogenous Put slightly increased spermidine content and
decreased ethylene production, the two pathways were not strictly antagonistic because the
addition of AVG, which inhibits the conversion of SAM (the common precursor of
spermidine and ethylene) to ethylene, did not enhance polyamine biosynthesis. A better
explanation could be that the availability of SAM in vivo is not rate limiting during the
biosynthesis of either ethylene or spermidine, and that both pathways could run
simultaneously (Mehta, Cassol et al. 2002, Dias, Santa-Catarina et al. 2009). Evaluation of
the potential sources of ethylene revealed that Al-induced root inhibition might be due to
the increase in both ACS and ACO activities. However, ACS and ACO had been identified
as two sites where Put can affect ethylene biosynthesis (Lutts, Kinet et al. 1996,
Parra-Lobato and Gomez-Jimenez 2011). The effects of Put on the activities of ACS and
ACO, and ACC content were examined to further unravel how Put decreases ethylene
production under Al stress. Put inhibited ethylene production by directly suppressing ACS
activity at the step where SAM was converted to ACC (Yu, Jin et al. 2016).
The most significant abiotic stress affectingplant growthand limiting crop yields is drought
(Beltrano, Ronco et al. 1999, Ming-Yi and Jian-Hua 2004). In case of wheat, stress caused
by drought at the time of grain-filling usually shortens the grain-filling period and reduces
the grain-filling rate, results in reduction of grain yield (Aggarwal and Sinha 1984, Nicolas,
Lambers et al. 1985, Kobata, Palta et al. 1992, Zhang, Sui et al. 1998). Hormones and their
interactions in plants are suggested to mediate this process (Brenner and Cheikh 1995,
Sharp and LeNoble 2002, Yang and Zhang 2006). The major phyto-hormones induced in
response to stress are ethylene and abscisic acid (WJ and J. 1991, Cheng and Lur 1996,
Gazzarrini and McCourt 2001, Wilkinson and Davies 2002, Davies 2004). Various types of
stresses have been reported to promote ethylene production in different tissues of a number
of plant species (Narayana, Lalonde et al. 1991, Morgan and Drew 1997). An
overproduction of ethylene induced by drought has frequently been related to fruit abortion
in cotton (Guinn 1976) and grain weight reduction in wheat (Xu, Yu et al. 1995, Beltrano,
Ronco et al. 1999). The concentration of ABA, which is generally regarded as an inhibitory
growth hormone (DC 1980, AJ and HG 1991), increases markedly in leaves (Ober and
Setter 1990, Westgate, Passioura et al. 1996), floral organs (HS and DA 1982, Lee, Martin
et al. 1988, Ober, Setter et al. 1991), and developing grains (Goldbach and Goldbach 1977,
Ober, Setter et al. 1991, Kato, Sakurai et al. 1993, Ahmadi and Baker 1999, Yang, Zhang et
al. 2001) in responses to soil drying. Water stress-induced reductions in grain set and kernel
growth in wheat (Morgan 1980, Siani and D.A. 1982, Ahmadi and Baker 1999) have been

11. observed to be associated with elevated concentrations of ABA. It has frequently been
observed, however, that ABA can promote dry matter accumulation in the sink organ and
that ABA concentration is correlated with the growth rate of fruits or seeds (Eeuwens and
Schwabe 1975, Browning 1980, BerÜTer 1983, Schussler, Brenner et al. 1984, Ross and
McWha 1990, Yang, Zhanget al. 2001) indicated that an increased concentrationof ABA is
necessary to prevent excess ethylene production under water stress, and that as a result of
this interaction (Wang, Cook et al. 1987)ABA may often function to maintain, rather than
inhibit, appropriate plant growth. (Yang, Zhang et al. 2000, Yang and Zhang 2006) showed
that mild soil drying imposed during the grain-filling period in wheat can increase carbon
remobilization from vegetative tissues to grains and accelerate the grain-filling rate. A
higher ABA concentration and lower concentrations of ethylene and 1-
aminocylopropane-1-carboxylic acid (ACC) were found in superior grains (within a spike,
those grains that were filled earlier and reached a greater size) than in inferior grains (within
a spike, those grains that were filled later and were smaller), and were associated with a
higher filling rate in the superior grains. The grain-filling rates of both superior and inferior
grains, under either the WW or the MD treatment, were closely associated with ABA
concentrations in grains. The superior grains had higher ABA concentrations and higher
grain-filling rates than the inferior grains. The MD treatment increased the grain-filing rate
in both superior and inferior grains, and this increase was accompanied by an increase in
ABA concentration in these grains. Application of ABA to WW spikes significantly
increased the ABA concentration in grains, and grain-filling rates and grain weights were
significantly increased for both superior and inferior grains. ABA applied to SD spikes also
significantly increased the grain-filling rate and grain weight of the inferior grains.
Application of fluridone, an inhibitor of ABA biosynthesis, had the opposite effect
indicating that a higher concentration of ABA is required to maintain a faster grain-filling
rate, consistent with reports that normal concentrations of endogenous ABA are required to
maintain appropriate shoot and root growth in well-watered and water-stressed tomato
plants (Sharp, LeNoble et al. 2000, LeNoble, Spollen et al. 2004). The mechanism by which
ABA facilitates grain filling was not understood. It had been proposed that ABA had a
major role in relation to sugar-signaling pathways and enhances the ability of plant tissues
to respond to subsequent sugar signals (Rook, Corke et al. 2001, Davies 2004). (Dewdney
and McWha 1979, Ackerson 1985) reported that ABA can enhance the movement of
photosynthetic assimilates towards to developing seeds. The activities of three key enzymes
e.g., SuSase, AGPase and SSSase involved in the sucrose-to-starch pathway in the grains
(Hawker and Jenner 1993, Ahmadi and Baker 2001), were significantly enhanced by
application of ABA, but substantially reduced by application of fluridone, to WW spikes at
the early grain-filling stage. This result suggested that ABA may promote grain filling
through an increase of sink activity by regulation of the key enzymes involved. Ethylene
concentration in grains was rather high at the early grain-filling stage in contrat to ABA.
Its temporal pattern was opposite to that of ABA under WW and MD treatments. The
ethylene evolution rate was correlated with the grain-filling rate, the relationship being one
of exponential decay. Application of an inhibitor of ethylene synthesis (cobalt ion)
increased, while application of an ethylene-releasing agent (ethephon) reduced, the

12. activities of SuSase, AGPase and SSSase in grains, the grain-filling rate and grain weight.
These results suggested that, in contrast to ABA, ethylene plays a role in inhibiting grain
filling, consistent with speculation that ethylene may be a negative regulator of ABA action
in the seed (Ghassemian, Nambara et al. 2000). The effect of water stress on ethylene
production has remained a matter of debate (Morgan and Drew 1997). The concentrations
of ethylene and ACC in grains were reduced under the MD treatment and greatly increased
under the SD treatment, indicating that the production of ethylene in wheat grains may
depend on the severity and duration of soil drying. The temporal pattern of ACC
concentration was very similar to that of ethylene, and that they were very significantly
corrected, suggesting that the increase in ethylene production may be attributable to the
increase in ACC concentration in grains under the SD treatment. The ABA concentration
was greatly increased, whereas the grain-filling rate was decreased, under the SD treatment.
A probable explanation is that ethylene production outperforms ABA accumulation under
such conditions. (Davies 2013)proposed that plant hormones can act either synergistically
or antagonistically and it is the balance between promoting and inhibiting agents that
ultimately determines the path of plant growth and development. The grain-filling rate was
not only correlated with the concentrations of ABA and ethylene, but also correlated with
the ratio of ABA to ACC. Under the SD treatment, concentrations of both ABA and
ethylene were increased in the grains. However, the ratio of ABA to ACC was greatly
reduced, suggested that soil drying had a greater effect on ethylene release than on ABA
accumulation in these grains. When these spikes were treated with cobalt ion, ethylene
production was reduced and grain-filling rate and grain weight were increased. In contrast,
grain-filling rate and grain weight were decreased when application of fluridone resulted in
a reduction in ABA and an increase in ethylene production. The reduction in the
grain-filling rate and grain weight under the SD treatment was mainly attributable to an
increase in ethylene production, and that antagonistic interactions between ABA and
ethylene mediate the effects of soil drying on grain-filling rate in wheat. The correlation
between ABA concentration and grain-filling rate showed a polynomial relationship, and
the regression between the ratio of ABA to ACC and the grain-filling rate exhibited a
hyperbolic curve. Such curves suggested that the maximum grainfilling rate (3.4–3.50 mg
per kernel d−1) was achieved only, when the ABA concentration in grains is 3.41 nmol g−1
DW and the ratio of ABA to ACC is 0.13. It indicated that a higher ABA concentration and
a higher ratio of ABA to ACC are necessary to increase the grain-filling rate. Such
enhancement would be most effective when the ABA concentration was low or the ethylene
(ACC) concentration was high. However, when soil drying was too severe, a large increase
in water stress-induced ABA and consequently a very high ABA concentration and a very
high ABA to ACC ratio will not produce a high grain-filling rate. Other factors, such as
inhibited photosynthesis and phloem translocation, might limit grain filling. In addition,
very severe soil drying and very large amounts of ABA may have anadverse effect on grain
filling as a result of a shortened grain-filling period. An increase in ABA concentration and
reductions in ethylene and ACC concentrations in grains under MD conditions increased
the grain-filling rate, whereas much higher ethylene, ACC and ABA concentrations under
SD conditions reduced the grain-filling rate. In conclusion, the slower grain-filling rate of

13. the inferior grains compared with the superior grains was associated with a lower ABA
concentration and higher concentrations of ethylene and ACC in the grains. It was found
that moderate soil drying imposed during the grain-filling period can accelerate the
grain-filling rate. The accelerated grain-filling rate was attributable, at least in part, to an
increased ABA concentration and decreased ethylene production in the grains. Under
severe soil drying, a greater increase in ethylene production than in ABA accumulation
contributed to a reduction in the grain-filling rate and grain weight. A higher ratio of ABA
to ethylene in wheat grains is required to increase the grain-filling rate (Yang, Zhang et al.
2006).
Gibberellic acid (GA3) acts on barley aleurone tissues to induced de-novo synthesis of
several hydrolases, including α-amylase and protease. ABA inhibits this response. A
similar system of enzymatic induction had been characterized in wheat. Since the synthesis
and secretion of protease by the barley aleurone layer precisely parallels that of α-amylase,
both in response to Gibberellin and in its time course of release, it had been suggested that
control of synthesis of two hydrolases by GA3 was exerted via the same mechanism. It was
reported that although ethylene enhanced the release of α-amylase from barley aleurone, it
had no effect upon total amylase activity induced by GA3. Ethylene has been reported to be
active in stimulating Gibberellic acid-induced alpha amylase production by wheat aleurone
cells. Ethylene stimulated Gibberellic acid-induced protease synthesis in isolated wheat
aleurone layers was due to a stimulation of enzyme formation rather than an increase in
enzyme stability, since the decrease in protease enzyme activity observed during the last
12h of incubation was more rapid in ethylene treated tissue. The rapid increase in protease
activityat the start of inductionperiod indicated that ethylene not only stimulated the rate of
enzyme formation, but was also acting to reduce the lag period of hydrolase synthesis,
Since ethylene itself was without effect upon protease activity in the absence of GA3, its
effect upon the wheat aleurone layer was probably related to some primary biochemical
event induced by this latter hormone. Although ethylene brought about a large quantitative
change in protease activity, it had no effect upon the qualitative aspects of this phenomenon.
Thus the mode of action of ethylene was likely to be associated with the overall synthetic
system, rather than direct activation of genes specific for one or more different forms of
protease.
Respiration record of germinating pea seedling was due to apple emanations, (Smith and
Gane 1932) reported as the effects of ethylene on plants. (Roberts 1951) reported that,
Apple emenations inhibited the growth of pea seedlings without influencing their
respiration record. But wheat seedlings growth not effected. Ethylene as active substance,
chemically identified in apple emanations when mixture of Ethylene, Nitrogen and Oxygen,
wheat seedlings growth inhibited (Mack and Livingston 1933). Rate of Carbon dioxide
evolution could be lowered or above that in untreated seedlings by altering their contents of
the gas stream. Rate of carbon dioxide emission was similar for treated and untreated
seedlings, for an air stream containing 20% oxygen. Ethylene is a normal metabolite in
plants and plant parts under some circumstances. Ethylene is given off by ripening apples
(Elmer 1936), plums, peaches (Isaac 1938), citrus fruits, bananas (Niederl, Brenner et al.

14. 1938), pears, tomatoes, cantaloupe, Squash, eggplant, avocado, loquat, leaves ofdandelions,
peonies, and rhubarb, asparagus shoots, dandelion flowers, and seeds from fresh unripe
pods of lima beans and peas (Denny and Miller 1935). It might associated with auxin
(Borgström 1939). Dry weights of the endosperm and embryos of treated and un-treated
germinating seeds were nearly the same at comparable ages, it was apparent that ethylene
treatment did not with hydrolyze of carbohydrates and proteins in the endosperms and with
the translocation of with the products of hydrolysis to the embryos. In case of protein by
nitrogen determination, it was confirmed and protein synthesis in the protein not disturbed.
At the fourth day, weight of water per embryo in the treated seedlings in which water was
uptake by embryo, only about 62% of that was in the normal seedlings at that stage. By the
fifth day, the quantity of water per treated seedlings had risen to slightly above that in
normal seedling. Increase in water and dry matter in the treated and normal seedlings was
similar but the treated seedlings tend to lag behind the normal seedlings. Water contents of
ethylene treated plants vary. (Isaac 1938) Found lower water content while (Harvey 1915)
found no change in water content in potatoes due to treatment with ethylene. By the fifth
day, masses of treated and untreated seedlings were similar. So unless there was a big
difference in the intracellular air spaces of the two types of seedling then the external
volumes must be similar. Since the length of treated seedlings was less than the radial
dimensions must be greater. Two factors could obscure the visual manifestation of this
difference in the growth habit of the shoots and roots. The tissues of treated embryo were
somewhat more compact readily. They were certainly different in texture and rigidity. It
was very unlikely that the cells were more compact longitudinally. The other factor was the
arithmetic fact that the volume increased as the square of the radius but only as the first
power of length. (Isaac 1938) Showed that the shape of cortical cells in the treated plants
was different from that in normal plants. The greater diameter of treated cells would be
expected on a weight for weight basis to result in reduced elongation. By the fifth day, the
treated seedlings were found to have a mass similar to that of untreated seedlings but the
treated shoots and roots were shorter. Thus the radial dimensions of treated plants must be
greater unless the densities of the treated and un-treated cells were very different. Greater
differences in density were not likely to occur. Growth inhibition of gassed wheat seedlings
was due to the failure of cells to elongate during differentiation. This was supported by the
greater density of the root hairs in the treated seedlings. It was apparent that the effects of
ethylene on germinating wheat were complex. Ethylene produced two distinct types of
effects on Agrostemma seedlings, a direct effect on elongation, and indirect effect on the
cotyledons and growth point (SF and HK 1978).
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