Extreme Temperature Responses, Oxidative Stress and Antioxidant Defense in Plants - Abiotic Stress: Plant Responses and Applications in Agriculture - page 172

temperature. Pant et al. [28] observed that when seeds of Cassia tora were incubated under
normal room temperature they exhibited 92% germination but when exposed to 40, 50 and
60°C continuously for 10 d the germination percentage decreased to 85, 63 and 32%, respec‐
tively. Several earlier investigators have suggested that HT may be necessary for adequate
release of energy for germination and growth [29-31], but it can also reduce plant emergence.
Hall [32] stated that the maximum threshold temperatures for germination and emergence are
higher for warm-season than for cool-season annuals. For instance, the threshold maximum
seed zone temperature for the emergence of Vigna unguiculata is about 37°C, whereas in Lactuca
sativa , it is 25-33°C.
2.2. Growth and morphology
The most observed effect of heat stress on plants is the retardation of growth. As heat stress
often occurs simultaneously with drought stress, the combination of drought and heat stress
induce more detrimental effect on growth and productivity of crops than when each stress
was applied individually [24]. In higher plants, heat stress significantly alters cell division and
cell elongation rates which affect the leaf size and weight. However, it was reported that heat
stress resulted in significant increases in leaf numbers, particularly when reproductive
development was arrested without any decrease in leaf photosynthetic rates [20, 24]. Exposure
of plants to severe heat stress decreased the stem growth resulting in decreased plant height
[20]. Rahman [33] reported that plant height of wheat plant ranges from 66.4-97.3 cm and
55.7-82.3 cm in normal and heat stress condition, respectively. While studying with T.
aestivum , Ahamed et al. [34] observed that sowing time mediated heat stress negatively
influenced the plant height and number of tillers of 4 different genotypes. In a recent study,
Al-Busaidi et al. [35] observed that high atmospheric temperature cause significant water loss
which negatively influenced the growth and biomass production in biofuel plant, Jatropha
curcas . Parallel to shoot growth heat stress often decreases root growth, number of roots and
root diameter [36].
High temperature decreased shoot dry weight, relative growth rate (RGR) and net assimilation
rate (NAR) in maize, millet and sugarcane [5, 37]. In their review, Wahid [5] mentioned that
HT can cause considerable pre- and post-harvest damages, including scorching of leaves and
twigs, sunburns on leaves, branches and stems, leaf senescence and abscission, shoot and root
growth inhibition, fruit discoloration and damage. High temperature also alters the internal
morphology (anatomy) of plants and these changes are generally similar to those under
drought stress. Under HT stress, there is a general tendency towards reduced cell size, closure
of stomata and curtailed water loss, increased stomatal density and trichomatous densities,
and larger xylem vessels in both roots and shoots [5]. Several lines of study indicate that
exposure of plants to HT resultes in the disintegration of ultrastructural characteristics, mainly
attributed to a lower stomatal density, larger stomatal chamber with a larger stomatal opening
area, thinner leaves, loose arrangement of mesophyll cells, a partially developed vascular
bundle and unstable organelle structure. Zhang et al. [38] examined the microscopic and
ultrastructural characteristics of mesophyll cells in flag leaves of both HT sensitive and tolerant
rice genotypes grown under heat stress (37/30°C) and reported that the membrane permea‐