104 I. M. Ahmed et al.
functioned at higher rates to suppress an increased ROS formation under salt stress.
The significant increase in the activities of SOD, POD, APX, and GR in the NaCl-
stressed leaves of H. marinum was highly correlated with the temporal regulation
of the constitutive isoenzymes as well as the induction of new isoenzymes. Lower
level of lipid peroxidation also revealed a higher free radical-scavenging capacity
and protection mechanism of H. marinum against high salinity (300 mM NaCl) than
H. Vulgare. Our previous reports (Ahmed et al. 2013b) indicated that CM72 had a
higher malondialdehyde (MDA) content than XZ5 not only under D + S treatments
but also under drought alone, suggesting less oxidative damage in Tibetan wild
barley than cv. CM72. The essential role of antioxidative systems for maintaining a
balance between the overproduction of ROS and their scavenging to keep them at
appropriate levels for signaling and reinstatement of metabolic homeostasis is well
established.
5.7.7 Compatible Solutes
Compatible solutes are low molecular weight and highly water-soluble compounds
that are usually nontoxic even at high cytosolic concentrations. Plants accumulate
compatible solutes, such as Pro and GB, sugars in response to drought and salinity
to facilitate water uptake (Hare et al. 1998 ; Ashraf and Foolad 2007 ). In addition to
osmotic adjustments, these osmolytes were suggested to be important for protecting
cells against increased levels of ROS accumulation under stress conditions. Major
contributors to osmotic adjustment were revealed to be K+ in the early stages of stress
and molecules including GB, Pro, and glucose, in the late stress (Nio et al. 2011 ).
Pro accumulates in the cytosol and the vacuole during stress (McNeil et al. 1999 )
and was shown to protect plant cells against damages caused by^1 O 2 or HO (Matysik
et al. 2002 ). By quenching^1 O 2 and directly scavenging HO, Pro might be able to
protect proteins, DNA, and membranes (Smirnoff and Cumbes 1989 ; Matysik et al.
2002 ). In the recent study, drought stress alone and D + S combined stress caused a
marked increase in GB content in XZ5 and XZ16, more so than in CM72 (Ahmed
et al. 2013b). Enhanced GB levels in Tibetan wild barley may exert protection on
enzyme activity, including enzymes associated with sugar and amino acid metabo-
lism (Chen et al. 2007 ), leading to greater increases in soluble sugars and Pro in Ti-
betan wild barley than control. Thus, it is proposed that the two Tibetan wild barley
genotypes may acquire more protection than cv. CM72 under stressed environment
due to the elevated levels of GB and the greater osmotic protection from higher
levels of soluble sugars and Pro.
5.7.8 Plant Secondary Metabolism
Plant produces a large variety of secondary metabolites through several metabolic
pathways in normal condition. But different stresses either biotic or abiotic trigger
the plant secondary metabolism that results in enhanced production of plant