Among the enzymes of ammonium assimilation, GS appears to be sensitive to higher temperatures,
whereas GDH is comparatively heat stable (50 to 70°C) in many plant species [105]. Stability of GDH at
higher temperatures appears to be of adaptational significance for plants growing at elevated temperatures
as such plants may possibly assimilate NH 4 by the GDH pathway instead of the normal GOGAT/GS
pathway.
- Low Temperature
Low-temperature treatment of plants below the optimum growth temperature reduced N uptake
[72,142,143], decreased N partitioning in the young shoots [144], induced remobilization of N from older
leaves to younger ones [144], and adversely affected the process of N assimilation [145].
InLolium multiflorumandLolium perennegrasses, a decrease in the rate of NO 3 uptake was ob-
served with short-term exposure of roots to low-temperature treatment by decreasing the temperature
from 25 to 15°C [72]. A similar decrease in NO 3 uptake by Cicer arietinumplants was observed at 16°C
soil temperature compared with 22°C [142]. Macduff and Jackson [143] observed that when the root tem-
perature of barley plants was lowered by 3°C, maintaining a common day-night air temperature of
25/15°C, NO 3 uptake by the roots decreased with a concomitant decrease in the total-N content of the
plants. In barley plants, at all temperatures tested, NH 4 uptake was more than NO 3 uptake [143]. At low
root temperatures, NH 4 is regarded as a safe source of N, whereas it appears to be harmful at higher tem-
peratures [65]. Decreased uptake of NO 3 with decreasing temperature indicates that NO 3 uptake is sen-
sitive to temperature.
Low root temperatures drastically affect the partitioning of N within the whole plant [144]. Walsh
and Layzell [144] reported that when 35-day-old soybean plants were exposed to 15°C temperature for 4
days, N partitioning in the young shoots decreased 52 to 61% compared with that in control plants grown
at 25°C. In treated plants, mature leaves maintained an N level similar to that in controls. In another ex-
periment, Rufty et al. [128] observed a similar N partitioning pattern in soybean plant when roots were
treated with low temperature. Besides reduced N uptake and disproportionate partitioning of N, low-tem-
perature treatment of roots caused remobilization of N from older leaves to the young shoots. Walsh and
Layzell [144] observed about 22% remobilization of N from mature leaves of soybean plants by 11 days
of temperature treatment at 15°C compared with N present in leaves at 4 days of treatment. It appears that
the remobilized N from older leaves supports growth of the new shoots under low-temperature stress con-
ditions. Increased remobilization of N to the new shoots and proportionally less N partitioning indicates
that cold-tolerant cultivars have increased partitioning of N in the shoots. This also suggests that tolerance
to low temperature can be increased by increasing the N supply to young shoots [144].
Low-temperature treatment of roots decreased the rate of NO 3 flux to the leaves and, in turn, de-
creased NR activity [64]. Barley and maize seedlings, when grown at 20°C for 7 days, showed a drastic
reduction in NR activity compared with seedlings grown at 28°C [138]. Because NR is a substrate-in-
ducible enzyme, the level of NO 3 in the active pool has a major role in regulating leaf NR activity. De-
creased NO 3 uptake by the roots as a result of the chilling treatment would ultimately lead to a decrease
in NR activity. However, contrary to this in certain cases, an increased rate of ion uptake and root NR ac-
tivity has been observed at a low temperature [145]. Vogel and Dawson [145] reported that when 2-week-
old black alder (Alnus glutinosa) seedlings were exposed to chilling temperatures of 1 to 4°C for 2 hr
during the night, immediately after chilling in vivo, NR activities of roots and shoots increased signifi-
cantly compared with activities in prechilled plants. The apparent increase in NR activity following chill-
ing appears to be due to the increased activity of a constitutive NR enzyme that is reported to be present
in many N 2 -fixing plants.
E. Metal Toxicity
Heavy metals such as Cd^2 , Zn^2 , Cu^2 , Pb^2 , and Al^3 are major environmental pollutants that spread
to the soil via sewage sludge, waste disposal practices, or airborne pollution. They cause plant growth to
deteriorate, cause reduced NO 3 uptake by plants, and have direct inhibitory effects on enzymes of N as-
similation. In corn seedlings, a direct adverse effect of cadmium on NO 3 uptake was reported by Volk
and Jackson [146]. Industrial areas in many countries that are polluted with the heavy metals show re-
duced N concentration in leaves of plants. Pahlsson [147], while investigating the effects of pollutants in
two industrialized belts of Sweden, observed reduced N content in the leaves of polluted trees compared
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