The effect of root temperature on the induction of nitrate reductase activities and nitrogen uptake rates in arctic plant species

We investigated whether six arctic plant species have the potential to induce nitrate reductase (NR) activity when exposed to $NO_3^ - $-nitrogen under controlled environment conditions, using an in vivo assay that uses the rate of $NO_2^ - $-accumulation to estimate potential NR activity. We also assessed the effect of low root temperatures on NR activity, growth and nitrogen uptake (using ¹⁵N applications) in two of the selected species. Five of the six species (Cerastium alpinum, Dry as intergrifolia, Oxyria digyna, Saxífraga cernua and Salix arctica) were capable of inducing NR activity when exposed to solutions containing 0.5 mM $NO_3^ - $at 20°C for 10 days. Although in vivo NR activity was not induced in Saxífraga oppositifolia under controlled conditions, we conclude that it was capable of growing successfully on$NO_3^ - $, due to the presence of moderate rates of NR activity observed in both $NO_4^ + $-grown and $NO_3^ - $-treated plants. Exposure of O. digyna and D. integrifolia to 3°C root temperatures for two weeks, with the shoots kept at 20°C, resulted in root and leaf NR activity rates of $NO_3^ - $-treated plants being reduced to rates exhibited by $NO_4^ + $-grown plants. Although these decreases in NR in both species appeared to be due to limitations in $NO_3^ - $-uptake and growth rate (rather than direct low-temperature inhibition of NR synthesis per se), direct low-temperature inhibition of root NR synthesis could not be ruled out. In contrast to the temperature insensitivity of $NO_4^ + $ uptake in D. integrifolia, $NO_3^ - $-uptake in D. integrifolia was inhibited by low root temperatures. We conclude that the selected arctic species have the genetic potential to utilize $NO_3^ - $-nitrogen, and that low root temperatures, in conjunction with other environmental limitations, may be responsible for the lack of induction of NR in D. integrifolia and Salix arctica under field conditions.