5 Tolerance to Combined Stress of Drought and Salinity in Barley 95
sive loss of water that can potentially lead to gross disruption of metabolism and cell
structure and eventually to the cessation of enzyme-catalyzing reactions. Drought
is characterized by the reduction of water content, turgor, total water potential, wilt-
ing, closure of stomata, and decrease in cell enlargement and growth. Barley is one
of the most important cereal crops grown in many developing countries, where it is
often subject to extreme drought stress that significantly affects production (Cec-
carelli et al. 2007 ). Investigating the drought-tolerance mechanisms in barley could
facilitate a better understanding of the genetic bases of drought tolerance, and fa-
cilitate the effective use of genetic and genomic approaches for crop improvement.
5.3 Salinity Stress and Tolerance
Salinity-affected soils are classified into two types: saline and sodic soils. Some-
times, a third type can be categorized as saline-sodic soils. Salt’s negative effects
on plant growth have initially been associated with the osmotic stress component
caused by decreases in soil water potential and, consequently, restriction of water
uptake by roots.
In agriculture, salt stress severely affects the growth and economic yield of many
important crops (Maas and Hoffman 1977 ). Compared with other cereal crops, in-
cluding wheat, rice, rye, and oat, barley is highly tolerant to salinity, thus offering
a means for efficient utilization of saline soil and improvement of productivity in
these environments. However, barley still suffers from salt toxicity in many areas of
the world. On the other hand, dramatic differences can be found among and within
the barley species, providing the potential for developing cultivars with improved
salt tolerance. It is predicted that the genetic improvement of salt tolerance will be
an important aspect of barley breeding in the future.
5.4 Overlap Between Salinity and Drought Stresses
Salinity and drought stress show a high degree of similarity with respect to physi-
ological, biochemical, molecular, and genetic effects (Sairam and Tyagi 2004 ).
Physiological drought occurs when soluble salt levels in the soil solution are high
enough to limit water uptake due to low water potential, thereby inducing drought
stress (Lee et al. 2004 ). The major difference between the low-water-potential en-
vironments caused by salinity versus drought is the total amount of water available.
During drought, a finite amount of water can be obtained from the soil profile by
the plant, causing ever-decreasing soil water potential. In most saline environments,
a large amount of water is at a constant, but under low water potential. Plants have
a chance to adjust their osmotic potential, which prevent loss of turgor and gener-
ate a lower water potential that allows plants to access water in the soil solution for
growth (Taiz and Zeiger 2006 ).