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growth. Heat stress reduced leaf Chl content by 23 and 48% in C306 at anthesis and 15 days (d)
after anthesis respectively, while in PBW343, 29 and 61% reduction in Chl content at anthesis
and 15 d after anthesis, respectively, was observed under HT as compared with normal tem‐
perature [51]. As a result, HT significantly reduced the leaf photosynthetic rate in both geno‐
types at all three stages of plant growth compared to their respective control. In another study
Chl
a
in the
L. esculentum
leave was reduced by 10-32%, Chl
b
by 10% and 5% reduction in the ra‐
tio of Chl
a
and Chl
b
observed after 2-d of heat treatment [52]. However, the effect of HT on the
photosynthesis of the crop also depends on other climatic parameters. In addition it is not al‐
ways obvious that HT reduces the rate of photosynthesis. For instance, HT had no effect on the
photosynthetic temperature response of potato [53] and pea leaves [54]. Furthermore, growth
of maize leaves at HT had no effect on their rates of photosynthesis [55].
2.5. Water relations
Plant water status is considered as the most important variable under changing ambient
temperatures [56]. Plant water relation is more affected under the combined heat and drought
stress, than the condition of heat and sufficient moisture level. High temperatures affect
seedlings, first, by increasing evaporative demand and tissue damage. High temperatures-
induced increased transpiration and water transportation is another necessary tool for plant
survival under extreme temperatures. Death of a large number of
Pinus ponderosa
seedlings
were observed at 63°C but among those a few were survived those maintained basal stem
temperatures as much as 15°C lower than the surrounding air by keeping higher
g
s
, transpi‐
ration rate and water transportation. Here, water transport through seedling stems may help
to cool plant by the heat transferring mechanism. Heat exchange calculations demonstrated
that rapid water flow through seedling stems can absorb sufficient energy to reduce the stem
temperature by 30°C during peak sunlight hours [57].
Triticum aestivum
and
Hordeum vulgare
were grown in soil that was well watered or not watered in controlled chambers at 15/10, 25/20,
35/30 and 40/35°C day/night temperatures. After two days soil water content, leaf relative
water content, leaf water potential, leaf osmotic potential, leaf turgor potential and osmotic
adjustment were nearly constant at all temperatures when soil was well watered but were
affected strongly by HT when water was withheld [58]. Morales et al. [59] indicated that HT-
induced reduction in leaf water status was caused mainly due to reduction in hydraulic
conductance leading to decrease in water absorption or due to reduced
g
s
. In
Lotus creticus
elevated night temperatures caused a greater reduction in leaf water potential in water-
stressed as compared to well-watered plants [60]. In sugarcane, leaf water potential and its
components were changed upon exposure to heat stress even though the soil water supply
and relative humidity conditions were optimal, implying an effect of heat stress on root
hydraulic conductance [61].
2.6. Dry matter partitioning
Dry matter (DM) partitioning varied widely under different temperatures and crops. Stresses
like water deficit and heat slower down the assimilation process and the mineral uptake during
the grain filling period. Assimilates those are transferred directly to kernels and remobilization
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