3.2.2.4 Phytohormones
Plants tend to cope with unfavorable environmental conditions, though the degree of
tolerance dependent upon the type of stress to different crop species. Phytohormones
are the natural molecule in plants conciliate growth, development, nutrient allocation
(Fahad et al. 2015), and acts as a natural defense molecule in plants by maintaining
antioxidants level under stress condition. The involvement of abscisic acid (ABA) in
biochemical pathways is found to cope up with the adverse effects of the HT
(Maestri et al. 2002). In wheat, ABA plays an important role in the regulation of
heat tolerance (Lata and Prasad 2011). Walker-Simmons and Sesing (1990) found
that ABA accumulation gradually increases in the grain at HT (25
C compared to
15
C), however, the increasing level of ABA did not rise because of HS, its role has
been seen after the recovery from stress (Larkindale and Huang 2005). ABA act as a
major chemical for root-to-shoot stress signal (Schachtman and Goodger 2008),
including stomatal closure and inhibition in leaf expansion. Exogenous application
of ABA increases filling rate and sink capacity in grain by regulating endogenous
hormone molecule to accelerate endosperm cell division and accumulation of
photosynthate (Yang et al. 2014), as well as induces Heat shock protein (Hsp101)
transcripts in wheat (Campbell et al. 2001). Jones and Setter (2000) reported a link
between ethylene production and heat susceptibility in wheat and identify several
heat tolerance genes in ethylene biosynthesis/signaling. Ethylene production in
response to HT differs in different crop species (Arshad and Frankenberger 2002)
which helps to regulate seed germination, flowering, and fruiting in addition to
provide tolerance to plant. Lower the content of ethylene in spike correlates with
higher grain yield (Valluru et al. 2017). Fenglu et al. (1997) in maize, studied the
production of ethylene maximum at the top ear and minimum at the middle ear,
indicates the important role of ethylene in assimilate partitioning to grain filling.
Salicylic acid (SA) is a major component of the signaling pathway that react to
hypersensitive response and systemic acquired resistance (Kawano et al. 1998). It
also stabilizes the heat shock transcription factors and helps to bind the heat shock
element with the promoter of heat shock-related genes thus regulate the signaling
pathway in HS condition and promote the growth of the plant. Persistent of HS for
long term withhold by SA along with Ca
2+ homeostasis and antioxidant systems
(Wang and Li 2006). Cytokinin also helps to mitigate heat tress by changing grain
cytokinin content under HS conditions. In dwarf wheat variety at HT decrease in
cytokinin content correlates with reduced grain filling and grain weight (Banowetz
et al. 1999).
3.2.2.5 Oxidative Stress and Antioxidant Defense
Oxidative damage is usually a subsequent stage of most of the abiotic stresses in
plants. Exposure of plants to heat initially causes oxidative damage by the formation
of ROS (Wahid et al. 2007; Liu and Huang 2000). Continual HS in crops may cause
accumulation of ROS involved in proteolysis of protein with depolarization of cell
membrane and trigger of programmed cell death (Qi et al. 2010; Mittler et al. 2011)
which causes premature leaf senescence and root growth inhibition (Miller et al.
2009). Under HS condition, majority of ROS are produced in the PS I and PS II of
60
R. Gajghate et al.
Plants tend to cope with unfavorable environmental conditions, though the degree of
tolerance dependent upon the type of stress to different crop species. Phytohormones
are the natural molecule in plants conciliate growth, development, nutrient allocation
(Fahad et al. 2015), and acts as a natural defense molecule in plants by maintaining
antioxidants level under stress condition. The involvement of abscisic acid (ABA) in
biochemical pathways is found to cope up with the adverse effects of the HT
(Maestri et al. 2002). In wheat, ABA plays an important role in the regulation of
heat tolerance (Lata and Prasad 2011). Walker-Simmons and Sesing (1990) found
that ABA accumulation gradually increases in the grain at HT (25
C compared to
15
C), however, the increasing level of ABA did not rise because of HS, its role has
been seen after the recovery from stress (Larkindale and Huang 2005). ABA act as a
major chemical for root-to-shoot stress signal (Schachtman and Goodger 2008),
including stomatal closure and inhibition in leaf expansion. Exogenous application
of ABA increases filling rate and sink capacity in grain by regulating endogenous
hormone molecule to accelerate endosperm cell division and accumulation of
photosynthate (Yang et al. 2014), as well as induces Heat shock protein (Hsp101)
transcripts in wheat (Campbell et al. 2001). Jones and Setter (2000) reported a link
between ethylene production and heat susceptibility in wheat and identify several
heat tolerance genes in ethylene biosynthesis/signaling. Ethylene production in
response to HT differs in different crop species (Arshad and Frankenberger 2002)
which helps to regulate seed germination, flowering, and fruiting in addition to
provide tolerance to plant. Lower the content of ethylene in spike correlates with
higher grain yield (Valluru et al. 2017). Fenglu et al. (1997) in maize, studied the
production of ethylene maximum at the top ear and minimum at the middle ear,
indicates the important role of ethylene in assimilate partitioning to grain filling.
Salicylic acid (SA) is a major component of the signaling pathway that react to
hypersensitive response and systemic acquired resistance (Kawano et al. 1998). It
also stabilizes the heat shock transcription factors and helps to bind the heat shock
element with the promoter of heat shock-related genes thus regulate the signaling
pathway in HS condition and promote the growth of the plant. Persistent of HS for
long term withhold by SA along with Ca
2+ homeostasis and antioxidant systems
(Wang and Li 2006). Cytokinin also helps to mitigate heat tress by changing grain
cytokinin content under HS conditions. In dwarf wheat variety at HT decrease in
cytokinin content correlates with reduced grain filling and grain weight (Banowetz
et al. 1999).
3.2.2.5 Oxidative Stress and Antioxidant Defense
Oxidative damage is usually a subsequent stage of most of the abiotic stresses in
plants. Exposure of plants to heat initially causes oxidative damage by the formation
of ROS (Wahid et al. 2007; Liu and Huang 2000). Continual HS in crops may cause
accumulation of ROS involved in proteolysis of protein with depolarization of cell
membrane and trigger of programmed cell death (Qi et al. 2010; Mittler et al. 2011)
which causes premature leaf senescence and root growth inhibition (Miller et al.
2009). Under HS condition, majority of ROS are produced in the PS I and PS II of
60
R. Gajghate et al.