Combined Stresses in Plants: Physiological, Molecular, and Biochemical Aspects

5 Tolerance to Combined Stress of Drought and Salinity in Barley 103

salinity-induced oxidative stress (Fig. 5.2). Different antioxidants have roles in pro-
tecting cells in specific compartments and in particular conditions. It is generally
accepted that O 2 − might be converted to H 2 O 2 and then metabolized to water by APX
and GR in plants to maintain membrane structures (Foyer and Fletcher 2001 ). Like-
wise, several other antioxidant enzyme molecules are responsible to counteract the
deleterious effects of ROS. Initially, SOD catalyzes the conversion of O 2 − to H 2 O 2
that is further reduced to water by APX by using ascorbate as an electron donor
(Scandalios 2005 ). Elevated accumulation of antioxidant enzymes such as SOD,
CAT, GR, APX, and POD is involved in lowering oxidative injury in caper bush
seedlings under drought stress (Ozkur et al. 2009 ). Yang et al. ( 2009 ) reported an
increase in the activity of CAT, SOD, POD, APX, and GR at 25 % field capacity as
compared with 100 % field capacity. Seckin et al. ( 2010 ) observed the opposite pat-
terns in the activities of SOD, CAT, POD, APX, and GR enzymes in response to NaCl
stress in H. marinum and H. vulgare. Thus, the antioxidant system of H. marinum

Drought and salinity

stress

Excess ROS production

(^1 O 2 , H 2 O., OH., O 22 - , H 2 O 2 )

Proteins, lipids,

enzymes, DNA

Reduction of oxidative damage

Stress tolerance

Antioxidant defense

AsA,

GABA, α-toc, GSH

Carotenoids,

Alkaloids,

Phenols

POD

GR. CAT, SOD,

GST, APX

DHAR, MDHAR

Fig. 5.2 Role of antioxidant enzymes in the ROS scavenging mechanism. Exposure to drought
and salinity leads to generation of ROS, including singlet oxygen ()^1 O 2 , perhydroxyl radical
( H 2 O), superoxide hydroxyl radicals ()O 22 −, hydroxyl radicals ( OH), and hydrogen peroxide
( H 2 O 2 ). ROS reactive oxygen species, SOD superoxide dismutase, CAT catalase, POD peroxidase,
GR glutathione reductase, APX ascorbate peroxidase, GABA γ-aminobutyric acid, GSH reduced
glutathione, MDHAR monodehydroascorbate reductase, DHAR dehydroascorbate reductase, GST
glutathione S-transferase