Agriculture Reference
In-Depth Information
2.4. Photosynthesis
Temperature plays one of the most important roles in the rate and ability of a plant to photo‐
synthesize effectively. In general, there is a positive correlation between change in temperature
and photosynthesis. But when temperatures exceed the normal growing range (15°C to 45°C)
of plants heat injury takes place and HT hurts the enzymes responsible for photosynthesis.
Even in the absence of heat stress injury, photosynthesis would be expected to decline as
temperature increases because photorespiration increases with temperature faster than does
photosynthesis [40].
In tobacco leaves HT stress (43°C for 2 h) decreased the rate of photosynthesis by 38%
compared with that of the rate of photosynthesis at optimal temperature (25°C). After 1 d
recovery, it reached only about 75% of its control. Under HT condition, the stomatal conduc‐
tance ( g s ) also decreased significantly [41]. Prasad et al. [42] reported that high night temper‐
ature (31.9°C/27.8°C) decreased chlorophyll (Chl) content and photosynthetic rate by 8% and
22%, respectively, compared to optimum night temperature. Deactivation of RuBisCO is one
of the causes associated with the decline in photosynthesis under HT. Many authors reported
that the heat-induced deactivation of RuBisCO is the primary constraint for photosynthesis at
moderately HT and showed that Chl fluorescence signals from PSII are not affected by
temperatures that cause significant deactivation of RuBisCO [43]. While studying with oak
( Quercus pubescens L.) leaves, Haldimann and Feller [43] concluded that regardless of whether
temperature was increased rapidly or gradually, rate of photosynthesis decreased with
increasing leaf temperature and it was reduced more than 90% at 45°C as compared to 25°C.
Stomatal conductance is also an important factor that modulate photosynthesis rate in plants.
Eamus et al. [44] have shown from the stomatal conductance response of Eucalyptus haemas‐
toma leaves to temperature declined with leaf temperatures above about 30-32°C, with a
considerable reduction at 40°C. In Semillon leaves, Greer and Weston [45] observed that 4-d
heat exposure at 40°C caused a sustained reduction in photosynthesis that was 95% attributed
to reduced g s which suggest that stomata of this plant was highly susceptible to heat. Recently,
Greer and Weedon [46] observed that average rates of photosynthesis of Vitis vinifera leaves
decreased by 60% with increasing temperature from 25°C to 45°C. This reduction in photo‐
synthesis was attributed to 15-30% stomatal closure.
It was noted that an increase in temperature of 10°C to 15°C above normal growth temperature
leads to alteration of photosynthetic pigments and thus limiting photosynthesis. The reasons
for decreasing in photosynthetic pigments under HT may be attributed to the inhibition of bio‐
synthesis, changes in ultrastructure of chloroplast, especially the membrane, and photodeter‐
ioration [47, 48]. Tewari and Tripathy [47] observed that heat stress significantly reduces Chl
content in T. aestivum which was due to inhibition of porphobilinogen deaminase activity and
thus reduction in protochlide content in the seedlings upon exposure to short duration of heat
stress (42°C). Heat stress has been reported to reduce Chl content, Chl a / b ratio and Chl:Car ra‐
tio in various plant and tree species like Festuca arundinacea [49] and Solanum spp. [50], T. aesti‐
vum [48]. Recently, Almeselmani et al. [51] investigated the performance of heat tolerant (C306)
and heat susceptible (PBW343) wheat genotypes under HT (35/25°C day/night). They ob‐
served that HT significantly reduced leaf Chl content in both genotypes at any stages of
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