• Plants experience water stress either when the water supply to their roots becomes limiting or when the transpiration rate becomes intense. • Water stress may range from moderate, and of short duration, to extremely severe and prolonged summer drought that has strongly influenced evolution and plant life. • The physiological responses of plants to water stress and their relative importance for crop productivity vary with species, soil type, nutrients and climate.

1. WATER STRESS & PHYSIOLOGICAL
CONSEQUENCES
SYNOPSIS:
 Introduction
 Types and causes of water stress
 Water and plant responses
 Effect of water stress on growth & metabolism
 Conclusion

2. INTRODUCTION
Water stress is one the major abiotic stress factors that affect all organism lives including human in
terms of health and food. Water absence from the soil solutions affect the natural evaporative cycle
between earth and atmosphere that contribute amount of rainfall. Drought occurs when soil moisture
level and relative humidity in air is low while temperature is also high. Water stress resulting from the
withholding of water, also changes the physical environment for plant growth as well as crop
physiology.
Plants experience water stress either when the water supply to their roots becomes limiting or when the
transpiration rate becomes intense. Water stress is primarily caused by the water deficit, i.e. drought or
high soil salinity. In case of high soil salinity and also in other conditions like flooding and low soil
temperature, water exists in soil solution but plants cannot uptake it a situation commonly known as
‘physiological drought’.
Water stress may range from moderate, and of short duration, to extremely severe and prolonged
summer drought that has strongly influenced evolution and plant life. The physiological responses of
plants to water stress and their relative importance for crop productivity vary with species, soil type,
nutrients and climate.
Drought, as an abiotic stress, is multidimensional in nature, and it affects plants at various levels of
their organization. In fact, under prolonged drought, many plants will dehydrate and die. Water stress
in plants reduces the plant-cell’s water potential and turgor, which elevate the solutes’ concentrations
in the cytosol and extracellular matrices.
As a result, cell enlargement decreases leading to growth inhibition and reproductive failure. This is
followed by accumulation of abscisic acid (ABA) and compatible osmolytes like proline, which cause
wilting. At this stage, overproduction of reactive oxygen species (ROS) and formation of radical
scavenging compounds such as ascorbate and glutathione further aggravate the adverse influence.
TYPES AND CAUSES OF WATER STRESS
Trembling aspen seeds under stress condition showed a decline in shoot water potential as a mild stress
effect after 17 hours of stress conditions. A severe stress response was observed after 20 hours of stress,
marked with a rapid decrease in shoot water potential.
MILD WATER DEFICIT STRESS
Mild water deficit stress causes an instant closure of stomata and a significant decrease in
photosynthesis in C3 plants. Mild water stress, when applied to pea plants, increases the rate of
leaf senescence by 15 days compared with the leaves of same age in well‐watered pots. The average
life span of leaves is observed to be 25 days. Effects of mild water stress were observed on rice
seedlings.
The experiment showed a more significant decrease in leaf area compared to the decrease in shoot dry
matter. This proves that the leaf enlargement is more sensitive to water stress than the dry matter

3. accumulation. The cultivars tolerated mild stress, exhibited a high relative transpiration, low specific
leaf weight and high carbon isotope discrimination in leaf and low leaf initial area.
These leaf parameters resulted in keeping moisture content high in leaves to ensure an increase in leaf
area, shoot dry matter, sugar, and starch content. Mild water stress results in increase in water use
efficiency and degradation of sugar compared to the starch in leaf blades, leading to the increased
accumulation of carbohydrates in leaf blades.
MODERATE WATER DEFICIT STRESS
Moderate water deficit stress does not severely affect the plants and the damage caused by
this type of stress can be reversed. In common bean plants subjected to moderate stress, a
decrease in stomatal conductance and net photosynthetic rate is observed.
Photo inhibition is not found. Plants show a reduction in electron transport, which can be
reversed by relieving the plants from the stress. Moderate water stress applied to barley plants
is associated with root dehydration and leaf dehydration may also take place under prolonged
stress.
SEVERE WATER DEFICIT STRESS
Severe water deficit stress on plants results in a reduction of root volume flux density and hydraulic
conductivity resulting in an increase in apoplastic root flow pathway. Severe drought conditions
result in progressive down regulation of metabolic processes, which leads to a decrease in RuBP
content thus causing an inhibition in photosynthesis and carbon dioxide assimilation.
Severe stress conditions observed in trembling aspen are a decrease in root hydraulic conductivity and
increase in apoplastic tracer dye. Severe water stress causes considerable membrane injuries in barley
plants. This is accompanied by an increase in accumulation of proline, which is involved in alleviating
membrane injuries in leaves.
Severely water stressed plants show an increase in the expression of osmolality in sap and concentration
of proline in leaves. Carbon dioxide assimilation and stomatal conductance rate decrease in such a way
that intrinsic water use efficiency is increased.
WATER AND PLANT RESPONSES
Water deficits may occur during a plant’s life cycle outside of arid and semi-arid regions even in
tropical rainforests. Water is progressively lost from a fully “saturated soil”, firstly by draining freely,
under the influence of gravity, and the rate of loss gradually slows down until no further water drains
away, when the soil is said to be at “field capacity”.
Further loss of water by evaporation or by absorption by plant roots reduces the moisture content still
further, until no further loss from these causes can occur, a stage known as the “wilting point” at which
plants can no longer obtain the water necessary to meet their needs and they therefore wilt and die from
moisture starvation.
Initially, stress conditions occur transiently as “cyclic water stress” even under adequate soil moisture
conditions and may prevail certain time in the daytime and normalized after reduction of transpiration
rate by the night.

4. EFFECT OF WATER STRESS ON GROWTH & METABOLISM
o EFFECT ON GROWTH:
When a plant tissue suffers from water stress, there will be a reduction in turgor pressure. Since cell
expansion is influenced by turgor potential, developing cells will expand less and cell size will be
smaller under this condition. However, the critical water potential for inhibition of cell expansion is
different among species and among organs within plants.
o EFFECT ON CELL ULTRA- STRUCTURE:
Water stress affects the structure and function of membranes, which may lead to change in the ultra-
structure of cells. Chloroplast and mitochondrial structure can be damaged by severe water stress. With
the disruption of thylakoid structure, lipases along with fat droplets increase inside chloroplasts.
Associated with reduced water potential, plasto globules derived from thylakoid membrane increase in
number and size. In plants suffering from water stress, the PS II complexes may be disturbed which in
turn may lead to separation of thylakoid membranes from cell membranes.
o EFFECT ON PHOTOSYNTHESIS:
Water deficit has several effects on photosynthesis. However, the initial effect of water limitation is
usually stomatal closure. Any of the factors, viz., a root signal like ABA, or low turgor pressure in
guard cells or increasing vapor pressure gradient between the leaf and air may lead to stomatal closure.
When stomata are closed, it is likely that there will be a depletion of CO2 in the intercellular spaces.
This is termed stomatal inhibition of photosynthesis. Once CO2 has decreased relative to O2, diversion
of carbon from photosynthesis to photorespiration will be stimulated.
If the light intensity is too high, photo-inhibition of photosynthesis may occur resulting in the formation
of free radicals in the chloroplasts, which is caused by failure to utilize all the energy-rich products of

5. electron transport chain. These free radicals, generally active oxygen species, generated inside the
chloroplasts, are harmful to the protein environment in which the chloroplast reactions occur.
Photo-inhibition is the main non-stomatal inhibition of photosynthesis under water stress. In some
cases, however, water stress directly inhibits the photosynthetic apparatus through reduced turgor,
which results in a change in chloroplast pH and ion concentrations.
As a consequence, the activity of rubisco changes along with a few other enzymes of the Calvin cycle.
Other events of non- stomatal impact of water stress are chlorophyll degradation and the concomitant
decrease in light harvesting and electron transport associated with PS II.
o EFFECT ON DARK RESPIRATION & CARBOHYDRATE
METABOLISM:
Dark respiration of whole plants or shoots or mature leaves subjected to moderate water stress either
remain unchanged or increase slightly over unstressed material. With increasing stress severity and
duration, however, the respiration rate has been found to decline
.
Under such water limitation, photosynthesis decreases before respiration. It has been envisaged that
decrease in the ratio of photosynthesis to respiration and increase of both photorespiration and dark
respiration during water stress may also give rise to plant starvation stress.
As regards carbohydrate metabolism, loss of starch and increase in simple sugars are the common
features linked with water limitation. Carbohydrate translocation also decreases during water stress.
The decrease in sucrose translocation is caused by change in source-sink relationships during water
stress. A decrease in the gradient of sucrose between source leaves and photosynthetic sinks is caused
by low CO2 assimilation by leaves and increased respiration in mesophyll cells.
o EFFECT ON NITROGEN METABOLISM:
The movement of nitrogen from roots to leaves slows down and consequently higher concentration of
nitrate accumulates in water-stressed roots than in the roots of irrigated plants.
Under this condition, high nitrogen level in roots inhibits further uptake of nitrogen from the soil. A
low level of both oxidized and reduced forms of nitrogen may be due to higher losses of foliar nitrogen
in water-stressed plants than in well-watered plants.

6. Nitrate reductase activity has been shown to decline when plant water status is lowered whereas the
activity of nitrite reductase appears to be relatively insensitive to water stress. Water stress is also
associated with increase in protein hydrolysis and decrease in protein synthesis along with concomitant
increase in free amino acids.
o ASSIMILATION OF CARBON DIOXIDE:
Water stress results in decrease in photosynthetic assimilation of carbon dioxide. This decrease is due
to two reasons. First, the restricted diffusion of carbon dioxide in leaf is due to the stomatal closure and
secondly, carbon dioxide metabolism is inhibited as a result of water stress.
It is also observed that this stress affects carbon dioxide assimilation more than the oxygen evolution,
thus this decrease in assimilation is not affected by the increase in concentration of carbon dioxide in
the environment.
Experiments conducted on cowpea show that the decrease in carbon dioxide assimilation as a result of
water stress is mainly due to stomatal closure, which reduces internal carbon dioxide and restrict water
loss through transpiration.
o ABA ACCUMULATION:
The plant hormone ABA accumulates under-water deficit conditions and plays a major role in response
and tolerance to dehydration. Closure of stomata and induction of the expression of multiple genes
involved in defense against the water deficit are known functions of ABA. The amount of ABAs in
xylem saps increases substantially under reduced water availability in the soil, and this results in an
increased ABA concentration in different compartments of the leaf.
Another well-known effect of drought in plants is the decrease in PM-ATPase activity. Low PM-
ATPase increases the cell wall pH and lead to the formation of ABA- form of abscisic acid. ABA-
cannot penetrate the plasma membrane and translocate toward the gourd cell by the water stream in the
leaf apoplasm. High ABA concentration around guard cell results in stomata closure and help to
conserve water.
o ROOT IS THE SENSOR OF WATER STRESS OF SHOOT:
Roots are able to sense the drying of the soil and this information is communicated from the root to the
shoot. It is believed that dehydration of roots may generate a chemical response that moves to the shoot.

7. Such root signals are related to some hormones, which are exported to the shoot possibly in the
transpiration stream. Two hormones like cytokinin, the concentration of which- is reduced by water
stress and ABA increased by water stress may be held responsible in signal transduction between
different plant organs.
o ACCUMULATION OF COMPATIBLE SOLUTES:
At relatively mild water stresses (Ψ = -0.2 to -0.8 MPa), the amino acid proline begins to accumulate
rapidly in cytoplasm. Sometimes, its concentration may become as high as 1% of dry weight of tissue.
Other amino acids such as glycine betaine and sugar alcohol sorbitol also accumulate in cytoplasm.
These organic compounds do not interfere with enzyme functions and decrease the water potential of
the cells without accompanying decrease in their turgor. These organic compounds are called as
compatible solutes (or compatible osmotica) which contribute to osmotic adjustment of cells and help
in maintaining plant water balance and thus increase resistance of plant to water stress.
o STOMATAL RESPONSES DUE TO WATER STRESS:
Stomatal closure prevents water loss from transpirational pathways. Stomatal closure is more closely
related to soil moisture content than leaf water status, and it is mainly controlled by chemical signals
such as abscisic acid (ABA) production in dehydrating roots.
This process may be referred to hydro passive closure. It could also be metabolically dependent and
engage processes and result in ion flux inversion, causing stomatal opening.
This process requires ions and metabolites in a process known as hydro active closure and it seems that
the process is mostly regulated by ABA content in plants. This is brought about by regulating a
sophisticated cascade of biochemical events that involve the ABA complex formation. As well as the
ABA, other signaling molecules and important elements are interfered with in changing stomatal status.
The lower amount of nitrogen increases stomatal sensitivity to drought.

8. CONCLUSION
Wherever they grow, plants are subject to stresses, which tend to restrict their development and
survival. Moisture limitation can affect almost every plant process, from membrane conformation,
chloroplast organisation and enzyme activity, at a cellular level, to growth and yield reduction in the
whole plant and increased susceptibility to other stresses. Reduction in photosynthetic activity and
increases in leaf senescence are symptomatic of water stress and adversely affect crop growth. Other
effects of water stress include a reduction in nutrient uptake, reduced cell growth and enlargement, leaf
expansion, assimilation, translocation and transpiration. In research aimed at improvements of crop
productivity, the development of high-yielding genotypes, which can survive unexpected
environmental changes, particularly in regions dominated by water deficits, has become an important
subject.