chloroplasts, mitochondria, plasma membrane, apoplasts, peroxisomes, and endoplasmic reticulum while a little amount is also produced by microbodies (Soliman
et al. 2011). There are two ways for the production of ROS either enzymatic or
nonenzymatic (Apel and Hirt 2004). The increased level of ROS in the cell acts as
toxic element which induces oxidation of protein and damages nucleotides. This
causes lipid peroxidation of cell membrane and pigments destruction which makes a
serious threat to the cell functioning (Apel and Hirt 2004; Xu et al. 2006;
Hasanuzzaman et al. 2012). HS induced membrane peroxidation and aggravated
membrane injury in wheat (Savicka and Skute 2010) and maize (Kumar et al. 2012).
Oxidative stress is an aversive effect of HS in plant cells due to the production of
singlet oxygen (O
2 ), superoxide radical (O
2À ), hydrogen peroxide (H 2 O 2 ), and
hydroxyl radical (OH
À
) notably enhance in membrane peroxidation and reduction
in membrane thermostability in many crops including wheat (Savicka and Skute
2010), leads to cell injury and even cell death (Marutani et al. 2012; Suzuki et al.
2012). Among the ROS, O
2À is formed by photo-oxidation reactions (flavoprotein,
redox-cycling) through Mehler reaction in chloroplasts, during mitochondrial electron transport chain reactions and glyoxisomal photorespiration, by nicotinamide
adenine dinucleotide phosphate (NADPH) oxidase in plasma membranes, xanthine
oxidase, and membrane polypeptides. The scavenging of O
2À by superoxide
dismutase (SOD) results in the production of H 2 O 2 , which is removed by ascorbate
peroxidase (APX) or catalase (CAT). However, both O
2À and H 2 O 2 are not much
toxic as the OH
À
, which is formed by the combination of O
2À and H 2 O 2 in the
presence of trace amounts of Fe
2+ and Fe
3+ by the Haber–Weiss reaction. OH
À react
with almost all constituents of cells and can impaired chlorophyll, lipids, nucleic
acid, protein, and other macromolecules cause premature leaf senescence, root
growth inhibition, affects plant metabolism, reduce growth and yield (Sairam and
Tyagi 2004; Miller et al. 2009; Qi et al. 2010; Mittler et al. 2011).
HS tolerant plants mediate antioxidant defense mechanism which works either
enzymatic or nonenzymatic way. The enzymatic defense system is considered to be
the most effective (Farooq et al. 2008). Major enzymes involved as protectants are
SOD, APX, CAT, and peroxidase. SOD is an elementary antioxidant enzyme, which
converts O
2À into H 2 O 2 and O 2 , APX neutralizes H 2 O 2 by using ascorbate as a
substrate, CAT breaks down H 2 O 2 , glutathione reductase which reduces glutathione
disulfide to the sulfhydryl from glutathione, and peroxidase has ameliorating effects
of HS in wheat and maize (Suzuki et al. 2011; Caverzan et al. 2016). The nonenzymatic defense system involves reduction in glutathione, tocopherols, ascorbic acid,
and carotenoids. Tiwari and Yadav (2019) describe the role of the ascorbateglutathione cycle in maize crops in terminal (reproductive) HS. Therefore, the
enrichment of antioxidants in cell is a better approach by the crop to conflict the
effects of ROS (Sharma and Dubey 2005). Reassurance against oxidative stress is a
major key to determine the endurance of a crop under HS. Understanding the
expression, accumulation, and developmental pathway of antioxidants under environmental stress condition helps to improvise and make a significant step towards
the development of heat-tolerant lines.
3 Plant Morphological, Physiological Traits Associated with Adaptation Against. . .
61
et al. 2011). There are two ways for the production of ROS either enzymatic or
nonenzymatic (Apel and Hirt 2004). The increased level of ROS in the cell acts as
toxic element which induces oxidation of protein and damages nucleotides. This
causes lipid peroxidation of cell membrane and pigments destruction which makes a
serious threat to the cell functioning (Apel and Hirt 2004; Xu et al. 2006;
Hasanuzzaman et al. 2012). HS induced membrane peroxidation and aggravated
membrane injury in wheat (Savicka and Skute 2010) and maize (Kumar et al. 2012).
Oxidative stress is an aversive effect of HS in plant cells due to the production of
singlet oxygen (O
2 ), superoxide radical (O
2À ), hydrogen peroxide (H 2 O 2 ), and
hydroxyl radical (OH
À
) notably enhance in membrane peroxidation and reduction
in membrane thermostability in many crops including wheat (Savicka and Skute
2010), leads to cell injury and even cell death (Marutani et al. 2012; Suzuki et al.
2012). Among the ROS, O
2À is formed by photo-oxidation reactions (flavoprotein,
redox-cycling) through Mehler reaction in chloroplasts, during mitochondrial electron transport chain reactions and glyoxisomal photorespiration, by nicotinamide
adenine dinucleotide phosphate (NADPH) oxidase in plasma membranes, xanthine
oxidase, and membrane polypeptides. The scavenging of O
2À by superoxide
dismutase (SOD) results in the production of H 2 O 2 , which is removed by ascorbate
peroxidase (APX) or catalase (CAT). However, both O
2À and H 2 O 2 are not much
toxic as the OH
À
, which is formed by the combination of O
2À and H 2 O 2 in the
presence of trace amounts of Fe
2+ and Fe
3+ by the Haber–Weiss reaction. OH
À react
with almost all constituents of cells and can impaired chlorophyll, lipids, nucleic
acid, protein, and other macromolecules cause premature leaf senescence, root
growth inhibition, affects plant metabolism, reduce growth and yield (Sairam and
Tyagi 2004; Miller et al. 2009; Qi et al. 2010; Mittler et al. 2011).
HS tolerant plants mediate antioxidant defense mechanism which works either
enzymatic or nonenzymatic way. The enzymatic defense system is considered to be
the most effective (Farooq et al. 2008). Major enzymes involved as protectants are
SOD, APX, CAT, and peroxidase. SOD is an elementary antioxidant enzyme, which
converts O
2À into H 2 O 2 and O 2 , APX neutralizes H 2 O 2 by using ascorbate as a
substrate, CAT breaks down H 2 O 2 , glutathione reductase which reduces glutathione
disulfide to the sulfhydryl from glutathione, and peroxidase has ameliorating effects
of HS in wheat and maize (Suzuki et al. 2011; Caverzan et al. 2016). The nonenzymatic defense system involves reduction in glutathione, tocopherols, ascorbic acid,
and carotenoids. Tiwari and Yadav (2019) describe the role of the ascorbateglutathione cycle in maize crops in terminal (reproductive) HS. Therefore, the
enrichment of antioxidants in cell is a better approach by the crop to conflict the
effects of ROS (Sharma and Dubey 2005). Reassurance against oxidative stress is a
major key to determine the endurance of a crop under HS. Understanding the
expression, accumulation, and developmental pathway of antioxidants under environmental stress condition helps to improvise and make a significant step towards
the development of heat-tolerant lines.
3 Plant Morphological, Physiological Traits Associated with Adaptation Against. . .
61