Combined Stresses in Plants: Physiological, Molecular, and Biochemical Aspects

5 Tolerance to Combined Stress of Drought and Salinity in Barley 97

5.6 Effects of Drought and Salinity Stress on Plant

Morphology and Yield

5.6.1 Growth and Development

Plant responses to drought and salinity are complex and involve adaptive changes
and/or deleterious effects. The decrease in the water potential occurring in both
abiotic stresses results in reduced cell growth, root growth, and shoot growth and
also causes inhibition of cell expansion and reduction in cell wall synthesis (Chai-
tanya et al. 2003 ). According to these authors, drought (likely to salinity) affects
the regular metabolism of the cell such as carbon-reduction cycle, light reactions,
energy charge, and proton pumping and leads to the production of toxic molecules.
Literature has affirmed that plant responses to salt and water stress have much in
common. For example, according to Munns ( 2002 ), salinity brings a decrease in
water uptake by plants as the osmotic potential in the root vicinity will become high
and a kind of exosmosis may occur. This will slow down the growth rate, along
with a suite of metabolic changes identical to those caused by water stress. Ahmed
et al. (2013a) observed that barley plants treated with single or combined stress of
salinity (S) and drought (D) showed a significant decrease in plant height, shoot,
and root dry/fresh weights, with the largest reduction in the combined stress (D + S).
Therefore, most mechanisms to tolerate abiotic stresses like drought and salinity are
detrimental to plant development (Fig. 5.1).

Stress induced

injury

Drought Salinity

Disruption of membrane

integrity, dehydration, solute

leakage and metabolic

dysfunction

Excessive toxicity of Na+and

Cl_^ ions to cells, physiological

drought to plant

Activation

of stress

genes

Activation

of stress

genes

Growth

inhibition

Detoxication

signaling

Damage

control and

repair

Stress

tolerance

Restructuring of

cell membrane

and synthesis of

osmolytes

Ionic and osmotic

homeostasis via

SOS pathway or

related pathways

Regulation of cell

division and

expansion

Fig. 5.1 Possible drought and salt stress tolerance mechanisms in barley plants