Agriculture Reference
In-Depth Information
increased with the increase in N level. However, the enzymic
activities were low in all treatments of plants grown at low
temperature. Nitrate and nitrite reductase activities were
enhanced by N concentration. No detectable activity of nitrate
reductase was found in rice roots grown at N levels below 20
ppm N (Shieh and Liao 1987).
Many factors, including exogenous carbon metabolites
infl uence the expression of enzymes involved in nitrogen
metabolism, but little is known about the effect of low
temperature on the expression of those enzymes. It appears
that rice seedlings grown at optimal temperature (30ºC) were
subjected to low temperature (20ºC) stress and several key
enzymes involved in ammonium assimilation and carbon
metabolism, including glutamine synthetase, NADP-dependent
isocitrate dehydrogenase and NADH-dependent glutamate
dehydrogenase for rice roots.
The nutrient uptake is reduced due to restricted root
growth, poor tillering and leaf discolouration. The early
vegetative growth stage of the plant life cycle is vulnerable to
cold stress marked injuries that have been observed on rice
seedlings planted in early spring in temperate and subtropical
environment (Andaya et al. 2003). However, the degree of injury
due to cold stress varies with duration of exposure, variety and
stage of development. Both root and shoot development have
been shown to be very sensitive to cold stress at seedling stage.
In poorly developed root system, absorption and translocation
of nutrients and water is hindered affecting shoot development.
Moreover, cold stress has been shown to arrest leaf growth by
extending the duration of meristematic cycles (Rymen et al.
2007). It reduces root hydraulic conductance resulting in low
leaf water and larger potential and ultimately reduces growth
at once. This becomes irreversible, it ends up with cell death.
