102 I. M. Ahmed et al.
contents in the plant tissues diminished under drought, possibly because of low-
ered PO 43 − mobility as a result of low moisture availability (Peuke and Rennenberg
2004 ). In general, drought stress reduces the availability, uptake, translocation, and
metabolism of nutrients. A reduced transpiration rate due to water deficit reduces
the nutrient absorption and efficiency of their utilization (Farooq et al. 2009 ).
Salinity hampers the uptake of macro- and micronutrients and the concentrations
of sodium (Na+) and chloride (Cl−) in the plant increase, and the concentrations of
potassium (K+) and calcium (Ca+) are reduced (Mansour et al. 2005 ). This together
result in inhibition of plant growth due to limitation in the absorption of other ions
and nutrients required for growth. It has also been reported that the accumulation
of Na+ and Cl− in both cellular and extracellular compartments competes with K+,
Ca+, magnesium (Mg2+), and manganese (Mn2+), whereas Cl− restricts the absorp-
tion of nitrate, phosphate, and sulfate ions (Termaat and Munns 1986 ; Romero and
Maranon 1994 ) and ultimately limits plant growth. Further, high levels of salinity
may also affect the transport of Cl− and Na+ by inhibiting the specific transport
systems of these ions (Maathuis 2006 ). Ahmed et al. (2013) reported that combined
stress (D + S) resulted in higher increase in Ca, Mn, and Fe concentrations in shoots
of wild barley (XZ5) than that of cultivated barley (CM72). Concerning root min-
eral concentrations, drought or salinity stress alone and in combination significantly
increased Ca concentrations in both genotypes, while no significant effect on Zn
and Cu concentrations was observed. Drought alone and D + S markedly increased
Mn concentration in XZ5, but had no effect on CM72 under salinity and D + S treat-
ments. Maintaining higher translocation of Ca, Mn, and Fe maybe an important
way to reduce D + S stress or beneficial to improve plant tolerance to drought and
salinity stress (Ahmed et al. 2013a).
5.7.6 Oxidative Stress and Enzymatic Regulation
The generation of reactive oxygen species (ROS) is one of the earliest biochemical
responses of eukaryotic cells to biotic and abiotic stresses (Apel and Hirt 2004 ). The
production of ROS in plants acts as a secondary messenger to trigger subsequent
defense reactions in plants. The most common ROS are hydrogen peroxide (H 2 O 2 ),
superoxide, the hydroxyl radical, and singlet oxygen that formed as a natural by-
product of the normal metabolism of oxygen and is crucial in cell signaling. The
overproduction of ROS leads to oxidative stress and can cause damage to cellular
components.
To minimize the impact of oxidative stress, plants have evolved a complex
system of enzymatic antioxidants, superoxide dismutase (SOD), catalase (CAT),
peroxidase (POD), glutathione reductase (GR), and ascorbate peroxidase (APX),
and nonenzymatic antioxidants, ascorbic acid, α-tocopherol, reduced glutathione,
β-carotene, Polyamines (PAs), salicylates, compatible solutes such as proline (Pro),
glycine betaine (GB), and zeaxanthin that accumulate in higher plants under drought
and salinity stress (Ozkur et al. 2009 ).
Plants enhance the production of antioxidants in order to minimize the detrimental
effects of oxidative stress to normalize their metabolic activities under drought- and