94 I. M. Ahmed et al.
to each of the different stresses individually (Rollins et al. 2013 ; Iyer et al. 2013 ).
Breeding of stress-tolerant crops is the most efficient strategy to maintain yield in
stress-prone marginal land. It is thus important to identify genetic resources with
high tolerance to abiotic stresses, especially those co-occurring in the field, such as
salinity and drought, and to understand its mechanisms.
Barley ( H. vulgare L.) is the fourth most important cereal crop in the world in
terms of production. For its versatile properties, it has been used for animal feed,
human food, and beverage (Koornneef et al. 1997 ). Barley as a staple food is at-
tracting renewed attention, especially in Asia and northern Africa, because of its
nutritional value (Baik and Ullrich 2008 ). In addition to its agricultural importance,
barley is a genetic model for other crops. However, much of the genetic variation
for improving abiotic stress tolerance has been lost during the process of domesti-
cation, selection, and modern breeding (Zhao et al. 2010 ). Even more, barley has
a wider ecological range than any other cereals and is widespread in temperate,
subtropical, and arctic areas, from sea level to heights of more than 4500 m in the
Andes and Himalayas (Bothmer et al. 1995 ). Barley can be grown on soils unsuit-
able for wheat, and at altitudes unsuitable for wheat or oats. Because of its salt and
drought tolerance, barley thrives in nearly every corner of the earth, including ex-
tremely dry areas near deserts. Barley is a short-season, early-maturing, diploid, and
self-pollinating crop, thus it is also an ideal model plant for genetic study of drought
and salinity tolerance (Li et al. 2007 ). Several papers have summarized research on
barley abiotic stress tolerance including drought and salinity tolerance (Zhao et al.
2010 ; Wu et al. 2013 ). In this chapter, we review the impact of salinity and drought
stress applied singly and in combination in barley through morphological, physi-
ological, biochemical, molecular, cellular, and ultrastructural approaches.
5.2 Drought Stress and Tolerance
Drought is a meteorological term and is commonly defined as a period without
significant rainfall or a deficiency of water supply. Generally, drought stress occurs
when the available water in the soil is reduced and atmospheric conditions cause
continuous loss of water by transpiration or evaporation. Hence, a continuous short-
fall in precipitation (meteorological drought) coupled with higher evapotranspira-
tion demand leads to agricultural drought (Mishra and Cherkauer 2010 ). Agricul-
tural drought is the lack of ample moisture required for normal plant growth and de-
velopment to complete the life cycle (Manivannan et al. 2008 ). Although droughts
can persist for several years, even a short, intense drought can cause significant
damage and harm the local economy. Drought is a worldwide problem, constraining
global crop production and quality seriously and recent global climate change has
made this situation more serious (Apel and Hirt 2004 ; Forster et al. 2004 ; Zhao et al.
2010 ; Budak et al. 2013 ).
Drought stress is also considered to be a moderate loss of water, which leads to
stomatal closure and limitation of gas exchange. Desiccation is a much more exten-