5 Tolerance to Combined Stress of Drought and Salinity in Barley 99
5.7 Physiological and Biochemical Bases for Drought
and Salinity Tolerance in Barley
5.7.1 Plant Water Relations
Leaf water potential, relative water content (RWC), stomatal movements,
transpiration, leaf and canopy temperatures are the important characteristics that
influence plant water relations. RWC represents plant water status including water
uptake by the roots as well as water loss by transpiration, thus reflect the metabolic
activity in plant tissue, and hence used as a most meaningful index for water stress
tolerance. A decrease in the RWC in response to drought stress has been noted in
a wide variety of plants (Nayyar and Gupta 2006 ). Furthermore, an exposure of
plants to drought stress substantially decreased the leaf water potential, RWC, and
transpiration rate, with a concomitant increase in leaf temperature as documented in
the previous study (Siddique et al. 2001 ). Although the components of plant water
relations are affected by reduced availability of water, stomatal opening and clos-
ing are more strongly affected. In barley, the application of the different watering
regimes decreased the RWC, midday leaf water potential ( ψw), and leaf osmotic
potential ( ψo) (Robredo et al. 2010 ).
Osmotic effects of salt on plants are due to the lowered soil water potential in the
root zone and thus resemble drought stress by affecting the ability of plants to extract
water from the soil and to maintain turgor (Sohan et al. 1999 ). However, at low or
moderate salt concentrations (higher soil water potential), plants accumulate solutes
and maintain a potential gradient for the influx of water. Under such conditions,
Shannon et al. ( 1984 ) reported that growth may be moderated, but unlike drought
stress, the plant is not water deficient. Several authors found that water potential and
osmotic potential of plants became more negative with an increase in salinity, where-
as the turgor pressure increased (Meloni et al. 2001 ; Gulzar et al. 2003 ). Vysotskaya
et al. ( 2010 ) reported a similar decrease in leaf water potential with increasing salt
concentration in wild barley species (“20–45” and T-1). At 75 mM NaCl, “20–45”
plants were characterized by less inhibition of leaf area, root fresh weight, leaf water
content, and leaf water potentials than T-1 species and were, therefore, considered
more tolerant to salt stress. According to Vysotskaya et al. ( 2010 ), these investiga-
tors, it was concluded that, under high salt concentration, plants (1) sequester more
NaCl in the leaf that lower the osmotic potential and (2) reduce the root hydraulic
conductance causing water stress in the leaf tissue. The combined stress of drought
and salinity depressed water potential, RWC in cultivated barley, but was unchanged
in Tibetan wild barley relative to control (Ahmed et al. 2013a).
5.7.2 Photosynthesis
Photosynthesis, together with cell growth, is among the primary processes to be af-
fected by drought (Chaves 1991 ) or by salinity (Munns et al. 2006 ). The effects can
be direct, as the decreased CO 2 availability caused by diffusion limitations through