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bility increased in both sensitive and tolerant plants under HT stress. However, under the HT
stress, the tolerant plants showed tightly arranged mesophyll cells in flag leaves, fully
developed vascular bundles and some closed stomata, whereas the sensitive plants suffered
from injury because of the poor structures of these organs [38]. Recently, Johkan et al. [22]
observed that the number of tillers in wheat plants decreased in response to HT, especially
high night-time temperatures, however shoot elongation was promoted.
2.3. Physiological effects
Physiological processes of plants are largely affected by the alteration of surrounded environ‐
mental temperature. The ability of plants to cope with extreme temperature is a complex
process and is determined by environmental factors and also by the genetic capability of the
plant. In general, stability of life processes in most plants is comparatively wide which ranges
from several degrees above zero to around 35°C [6]. The increase of temperature up to a certain
level increases plant growth, photosynthesis, respiration and enzyme activity and after that
these parameters tend to decline (Fig. 1). Respiration rapidly increases with temperature and
drops drastically after an extreme tolerable temperature. Photosynthesis is a comparatively
less sensitive than respiration process but its declining pattern is as like as respiration. The
average rate of enzymatic reactions increases twofold with every 10°C increase in temperature
within the range. The optimal temperature for structural integrity and activity of most enzymes
are within the range of 30-45°C; and enzymes are irreversibly denatured and inactivated at
temperatures higher than 60°C with the exception of thermophilous organisms. Thus each life
process has its own referred critical or lethal temperature after that it can not proceeded and
causes permanent damage to cell structures and ultimately the cell, plant death as well [6].
Growth rate
s
Enzyme activity
Respiration
Photosynthesis
Temperature
Figure 1. Schematic illustration of the effect of temperature on major physiological processes of plants [6, 39]
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