the area where the temperature is cool or frost limits the early heading trait (Tewolde
et al. 2006; Narayanan et al. 2018).
Time of Day of Flowering It is the time of the day at which anthesis begins in a
crop (Sheehy et al. 2007). After sunrise, the air temperature increase along with
duration exceeds the threshold temperature. Increase the chances of impede in anther
dehiscence, pollen tube growth, thus anthesis in the early hours of the morning
before sunrise helps to escape the HS and reduces spikelet sterility in plants.
Increased Partitioning It is a tolerance mechanism, in which the duration of grain
filling during HS severely affects the quality and quantity of grain as HS affects the
utilization of mobilized stem reserved contributes to grain development. Increases
partitioning of reserves from different plant parts viz., leaves, stem, etc., is a potential
approach to improve grain filling and enhance yield in wheat (Narayanan et al.
2018).
Chlorophyll Fluorescence Measurement It is a high-throughput screening
method that is used to identify the damage in photosynthetic tissue, especially in
thylakoid which is closely associated with chlorophyll loss. Tolerant genotypes
retain the chlorophyll content in HT genotypes. It is a simple quick rapid method
measured by the portable fluorometer to find the ration of a variable to minimum
chlorophyll fluorescence (Fv/Fm).
Membrane Thermostability (MTS) Denaturation of protein and increments of
unsaturated fatty acids in HS leads the electrolyte leakage, confirms the reduction in
cell membrane thermostability (Blum 2017). It is a highly heritable trait, measures
the reduced cell MTS and reflects stress-induced changes made it as an indirect
measure of HS tolerance in diverse plant species (Liu and Huang 2000; Xu et al.
2006). Calculated as MTS ¼ (1 À T 1 /T 2 ) Â 100, where T 1 is conductivity meter
reading afterword heat treatment, and T 2 is conductivity reading afterword
autoclaving (Ibrahim and Quick 2001).
Estimation of Membrane Lipid Saturation Greater amount of saturated fatty
acids in membrane lipids maintain the membrane fluidity at HT stress. Thus, during
HS plant try to increase the content of saturated and monounsaturated fatty acids
(Zhang et al. 2005). In maize Karim et al. (1997, 1999) reported that leaves with a
high fatty acid in the membrane are less prone to damage through HS. In wheat
Narayanan et al. (2016, 2018) identified changes in lipid metabolism with changes
during HT stress.
Visual Evaluation Methods/Morphological Methods Ease method to lookout the
effect of HS in plants when they are subjected to the same environmental condition.
The ideal heat-tolerant maize plant should have a higher amount of leaf wax,
compact tassel, lower leaf, and cob angle. Other visual assessments like pollen
viability, stigma receptivity (Dupuis and Dumas 1990; Ottaviano et al. 1992) leaf
3 Plant Morphological, Physiological Traits Associated with Adaptation Against. . .
63
et al. 2006; Narayanan et al. 2018).
Time of Day of Flowering It is the time of the day at which anthesis begins in a
crop (Sheehy et al. 2007). After sunrise, the air temperature increase along with
duration exceeds the threshold temperature. Increase the chances of impede in anther
dehiscence, pollen tube growth, thus anthesis in the early hours of the morning
before sunrise helps to escape the HS and reduces spikelet sterility in plants.
Increased Partitioning It is a tolerance mechanism, in which the duration of grain
filling during HS severely affects the quality and quantity of grain as HS affects the
utilization of mobilized stem reserved contributes to grain development. Increases
partitioning of reserves from different plant parts viz., leaves, stem, etc., is a potential
approach to improve grain filling and enhance yield in wheat (Narayanan et al.
2018).
Chlorophyll Fluorescence Measurement It is a high-throughput screening
method that is used to identify the damage in photosynthetic tissue, especially in
thylakoid which is closely associated with chlorophyll loss. Tolerant genotypes
retain the chlorophyll content in HT genotypes. It is a simple quick rapid method
measured by the portable fluorometer to find the ration of a variable to minimum
chlorophyll fluorescence (Fv/Fm).
Membrane Thermostability (MTS) Denaturation of protein and increments of
unsaturated fatty acids in HS leads the electrolyte leakage, confirms the reduction in
cell membrane thermostability (Blum 2017). It is a highly heritable trait, measures
the reduced cell MTS and reflects stress-induced changes made it as an indirect
measure of HS tolerance in diverse plant species (Liu and Huang 2000; Xu et al.
2006). Calculated as MTS ¼ (1 À T 1 /T 2 ) Â 100, where T 1 is conductivity meter
reading afterword heat treatment, and T 2 is conductivity reading afterword
autoclaving (Ibrahim and Quick 2001).
Estimation of Membrane Lipid Saturation Greater amount of saturated fatty
acids in membrane lipids maintain the membrane fluidity at HT stress. Thus, during
HS plant try to increase the content of saturated and monounsaturated fatty acids
(Zhang et al. 2005). In maize Karim et al. (1997, 1999) reported that leaves with a
high fatty acid in the membrane are less prone to damage through HS. In wheat
Narayanan et al. (2016, 2018) identified changes in lipid metabolism with changes
during HT stress.
Visual Evaluation Methods/Morphological Methods Ease method to lookout the
effect of HS in plants when they are subjected to the same environmental condition.
The ideal heat-tolerant maize plant should have a higher amount of leaf wax,
compact tassel, lower leaf, and cob angle. Other visual assessments like pollen
viability, stigma receptivity (Dupuis and Dumas 1990; Ottaviano et al. 1992) leaf
3 Plant Morphological, Physiological Traits Associated with Adaptation Against. . .
63