96 I. M. Ahmed et al.
Both stresses lead to cellular dehydration, which causes osmotic stress and re-
moval of water from the cytoplasm into the intracellular space resulting in a reduc-
tion of the cytosolic and vacuolar volumes. Early responses to water and salt stress
are largely identical except for the ionic component in the cells of plants under salt
stress. These similarities include metabolic processes, e.g., a decrease of photosyn-
thesis or increase in the levels of the plant hormonal processes, such as abscisic
acid (ABA). High intracellular concentrations of sodium and chloride ions are an
additional problem of salinity stress (Bartels and Sunkar 2005 ). Plants use com-
mon pathways and components in response to stresses, a concept known as cross-
tolerance, which allows plants to acclimate to a range of different stresses after
exposure to one specific stress (Pastori and Foyer 2002 ; Tuteja et al. 2007 ). Thus, a
salinity-tolerant species could also be drought tolerant or vice versa, and has similar
mechanisms to cope with those stresses (Ashraf and O’Leary 1996 ).
5.5 Mechanisms of Acclimation or Adaptation to Drought
and Salinity Stress
Drought and soil salinity are among the most damaging abiotic stresses affecting to-
day’s agriculture. It is understandable that plants are under periodic water stress be-
cause of the unpredictable nature of rainfall. Salt stress is often observed in irrigated
areas, hydraulic lifting of salty underground water, or spread of seawater in coastal
areas. Plants have evolved mechanisms to perceive the incoming stresses and to
cope with them by rapid regulation of their physiology and metabolism. Very often,
such regulations and responses include feed-forward mechanisms for stress reduc-
tion that are in addition to the responses that are seen after stresses have caused irre-
versible damage to physiological functions. A good example of such a feed-forward
mechanism is the ability of plants to regulate their water loss through partial closure
of stomata and/or reduced leaf development, long before there is a substantial loss
of their leaf turgor or some irreversible damage to inner membrane systems (Zhang
et al. 2006a). The physiological responses of plants to survive under water stress
include leaf wilting, a reduction in leaf area, leaf abscission, and the stimulation of
root growth by directing nutrients to the underground parts of the plants. Besides,
the effects of water deficit become more detrimental during reproductive stages of
the plant (flowering and seed development), as the translocation of photosynthetic
assimilates from leaf to root is reduced which cannot grow more deep in search of
water and nutrients. In addition, ABA, the plant stress hormone, induces the closure
of leaf stomata, thereby reducing water loss through transpiration, and decreasing
the rate of photosynthesis. These responses improve the water-use efficiency of the
plant on the short term (Muhammad and Asghar 2012 ).