level of NR and NIR in plants, but a unified mechanism leading to the mode of action of light in regulat-
ing the activities of these enzymes is still awaited.
D. Temperature
Nitrogen uptake, metabolism, and assimilation, like any other physiological and biochemical processes,
are strongly related to temperature. Optimum absorption and assimilation occurs at the normal tempera-
ture. Any deviation from the normal temperature range adversely affects N absorption, metabolism, and
assimilation.
- High Temperature
High temperatures affect seed germination and plant growth, yield attributes and induce many metabolic
alterations in crops. Different plant species have different optimum temperature ranges for growth and
yield. Even a small increase in soil temperature affects the growth and nutrient uptake in plants. A rise in
temperature beyond the optimum growth temperature impairs the rate of uptake as well as the assimila-
tion of nutrients by exerting a profound influence on the activities of assimilatory enzymes. In many plant
species, the effects of day-night temperatures on the uptake of various nutrients have been studied exten-
sively [137,138].
C 4 plants grown under conditions of high temperature and high humidity show enhanced efficiency
in N use compared with C 3 plants [138]. C 4 plants such as corn (Zea maysL.) and sorghum (Sorghum bi-
colorL.) and C 3 plants such as barley, rice, wheat, and oats (Avena sativaL.) were grown either for 7 days
at 20 or 28°C or for 3 weeks at 26°C. Greater accumulation of NO 3 was observed in C 3 than in C 4 plants
under any of the three conditions tested [138]. However, N supplied as NO 3 was more efficiently assim-
ilated into protein in C 4 than in C 3 plants [138]. Lowering the temperature in both sets of plants from 28
to 20°C caused accumulation of NO 3 as well as a lower protein-to-nitrate ratio [138]. The greater effi-
ciency of C 4 cereals toward NO 3 uptake and assimilation compared with C 3 plants at all tested tempera-
ture levels appears to be due to the highly organized cellular structure and spatial organization of N as-
similatory enzymes in C 4 plants [138].
In young corn seedlings, day-night temperatures of 30/30°C are regarded as optimum for NO 3 up-
take [137]. Polisetty and Hageman [137], while examining the effects of three temperature treatments,
30/20°C, 30/30°C, and 35/35°C day-night temperatures, on NO 3 uptake in corn seedlings, observed that
the amount of NO 3 taken up during the night was about 4- and 3-fold greater for 30/30°C over 30/20°C
and 35/35°C, respectively, whereas during the light period, NO 3 uptake increased by 1.5- and 1.3-fold,
respectively. This suggests that optimum NO 3 uptake by corn seedlings occurs at 30/30°C and that either
an increase or decrease in the temperature leads to a decrease in NO 3 uptake. The decreased NO 3 uptake
above optimum growth temperatures appears to be due to the impairment of the NO 3 uptake process as
well as the inhibition of root and shoot development due to the increase in temperature [137].
Partitioning of N in different parts of the plants is affected by increasing temperature. In rice, the
absolute N content per kernel was comparatively stable in the temperature range from 24/19°C to
33/28°C, whereas beyond this temperature a decline in the N content of the kernels was observed [139].
The varieties of rice differ in sensitivity to higher temperature. Japonica varieties of rice were more sen-
sitive than indica types during kernel development [140]. The highest concentration of N in terms of
percentage of dry weight was recorded in rice kernels in the temperature range from 33/28°C to
39/34°C [139]. A decrease in the N content of shoots in soybean plants was reported by Hafeez et al.
[141] due to an increase in temperature from 30 to 48°C compared with the control plants growing at
30°C.
The NR is sensitive to higher temperatures. Temperatures above a certain optimum affect the level
of NR in plants as well as inhibit its activity. The magnitude of inactivation or lowering of the NR level
by higher temperature varies according to the species [142]. Chandra and Pareck [142] observed that in
sorghum plants an increase in temperature by 6 to 11°C caused more than a 60% reduction in NR activ-
ity at the vegetative stage and 30% at the anthesis stage. Corn seedlings maintained at 15–20°C showed
six times more NR activity than at 25 to 30°C [63]. In barley seedlings, induction of NR did not take place
when seedlings were maintained at 41°C. Seedlings growing at 24°C, when transferred to 43°C, lost 70%
of their NR activity [63]. Whether high temperature causes inactivation, decreased synthesis, or increased
degradation of NR still remains to be investigated.
646 DUBEY AND PESSARAKLI