Drought and saltstress leads to significant loss in crop production thereby affecting the globalfood security index.
Geographically 70% of the earth’s surface is covered bysaline water. The major ions that contribute to salinity includes Na+, Ca2+,Mg2+, K+, Cl?, SO4 2?, HCO3 ?, CO3 2– and NO3 2? (Flowers et al., 2000). Salinationis referred to excess accumulation of salts in soil or water.
The factorsfavoring salination includes mineral weathering, dust, precipitation and inwardmovement of salt towards land surface along with groundwater.Drought on theother hand is a condition where plants are subjected to inadequate water supplylower than the optimum required for growth and survival. Less rainfall,irregular rainfall pattern and wrong irrigation practices contribute towardsdrought (Tuberosa and Salvi, 2006). When plants are exposed to prolongeddrought, it leads to wilting and drop in photosynthetic activities in thewater-stressed plants and finally resulting in plant death.Salt tolerance response is bestdescribed by the biphasic response mechanism proposed by Munns and Termaat(1986), Munns et al. (1995), Munns (2002). The osmotic (first) phase of growthresponse initiates as high levels of salt accumulates outside the plant. TheNa+ and Cl? ion levels outside the cell are altered increases drasticallywhereas the ion concentration inside the cell does not vary.
This lowers thewater intake capacity of plants leading to water-deficit and is characterizedby reduced leaf and root growth. The initial plant responses observed duringthis phase is stomatal closure. The plant responses in terms of cellular andmetabolic processes are similar to those affected by drought. The ionic(second) phase of the growth response is induced by the adverse toxic effect ofhigh salt (ion) accumulation. Prolonged exposure of plants to high salinityleads to entry of salts into the plants which accumulate in the older leavesresulting in very high cytosolic Na+ and Cl? ions. Subsequently, increased saltion concentrations disrupt membrane structure and cellular organelles; alterphotosynthetic and transpiration rate; inhibit enzyme activity; and eventuallydeath of the leaves. As the rate of leaf death exceeds the rate of new leafformation, the survival of the plant is minimized (Gilroy et al., 2014).
The biphasic response mechanism undersalinity has been documented in maize (Zeamays L.) (Fortmeier and Schubert, 1995) , wheat (Wakabayashi et al., 1997),A. thaliana (J. Li et al. (2015), tomato(Maggio et al., 2007), O. sativa (Negrãoet al.
, 2011), barley (Adem et al., 2014) and other plants of economic andmedicinal importance. The biphasic model contributed immensely in understandingthe possible cross-talks surrounding all abiotic stresses. Moreover ithighlighted that gene expression levels also altered in response to abioticstress conditions. Adem et al. (2014) compared three salt stressed barleycultivars with their wild-type counterparts.
They observed rapid initialosmotic stress responses that lead to three to five fold biomass loss and plantgrowth was inhibited. The second phase is triggered when the plants wereexposed to 4 weeks of saline stress. All barley cultivars exhibited low chlorophyllcontent, high oxidative stress and elevated levels of cellular Na+accumulation.Plants evolved three vital strategiesthat help them overcome biphasic growth inhibition and survive in salineconditions. Plants use ion exclusion, compartmentalization and osmoprotectionto maintain optimum cellular ionic concentrations, Ion exclusion strategyregulates the overall exclusion of toxic ions accumulated via ion exchangers(NHX). Compartmentalization of Na+ ions into vacuoles improves tissue toleranceand is accomplished by major ion channels (V-PPases, V-ATPases, Na+/K+antiporters). The third strategy involves cellular osmotic adjustment andosmoprotection by upregulating the expression of candidate genes involved in the synthesis of osmolytes, aquaporins, antioxidant enzymesthat contribute to salt tolerance to (Chakraborty and Roychoudhury, 2013).The physiological traits or parametersthat contribute to salinity tolerance and used for quantifying the degree ofstress response includes the maintenance of plant relative water content (RWC),transpiration (T) and transpiration use efficiency (TUE) (Harris et al., 2010;This et al., 2010; Barbieri et al., 2012); leaf area (Maggio et al., 2007);seed germination (Foolad and Lin, 1997); production of antioxidants (Ashraf,2009); early seedling growth (Kingsbury and Epstein, 1984); and harvest index(HI) (Gholizadeh et al., 2014).Shoot and root fresh and dry matter, root and shootlength, relative growth rate, net assimilation rate, relative water content andwater use efficiency.