Induction of nitrate uptake and nitrate reductase activity in trembling aspen and lodgepole pine

(NO3-)-N-13 influx into the roots and in vivo nitrate reductase activity (N RA) in the roots and leaves have been measured in trembling aspen (Populus tremuloides Michx.) and lodgepole pine (Pinus contorta Dougl.) seedlings af ter exposure to either 0.1 or 1.5 mol m(-3) NO3- for varying periods up to 20 d. Both NO3- influx and NRA were inducible in these species and, in trem bling aspen, peak induction of nitrate influx and NRA were achieved within 12 h, compared to 2-4 d for influx and 4-12 d for NRA in lodgepole pine. In trembling aspen, approximate to 30% of the total N-13 absorbed during a 10 min influx period followed by 2 min of desorption was translocated to the shoot. In lodgepole pine, by contrast, translocation of N-13 to the shoot w as undetectable during the same time period. Root NRA as well as NO3- influ x from 0.1 mol m(-3) NO3- were substantially higher in trembling aspen than in lodgepole pine at all stages of NO3- exposure, i.e. during the uninduce d, the peak induction, and steady-state stages. In order to examine whether the lower rates of NO3- influx and NRA were related to proportionately few er young (unsuberized) roots in lodgepole pine, we determined these paramet ers in young and old (suberized) roots of this species separately. Inductio n of influx and NRA were initially greater in young roots but at steady-sta te there were only minor differences between the young and the old roots. H owever, even the elevated initial rates in the young roots of lodgepole pin e were substantially lower than those of aspen. In pine, influx at 1.5 mol m(-3) NO3- was similar to 6-fold higher than at 0.1 mol m(-3) NO3- and appe ared to be mostly via a constitutive system. By contrast, in aspen, steady- state influxes at 0.1 and 1.5 mol m(-3) were not significantly different, b eing similar to the rate attained by pine at only the higher [NO3-]. In asp en, leaf NRA was similar to 2-fold higher than that of roots. In lodgepole pine NRA of the needles was below the detection limit. These results show t hat trembling aspen seedlings are better adapted for NO3- acquisition and u tilization than lodgepole pine seedlings.