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

¹³NO₃^– influx into the roots and in vivo nitrate reductase activity (NRA) in the roots and leaves have been measured in trembling aspen (Populus tremuloides Michx.) and lodgepole pine (Pinus contorta Dougl.) seedlings after 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 trembling 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, ≈ 30% of the total ¹³N 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 ¹³N to the shoot was undetectable during the same time period. Root NRA as well as NO₃^– influx 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 uninduced, the peak induction, and steady-state stages. In order to examine whether the lower rates of NO₃^– influx and NRA were related to proportionately fewer young (unsuberized) roots in lodgepole pine, we determined these parameters in young and old (suberized) roots of this species separately. Induction of influx and NRA were initially greater in young roots but at steady-state there were only minor differences between the young and the old roots. However, even the elevated initial rates in the young roots of lodgepole pine were substantially lower than those of aspen. In pine, influx at 1·5 mol m^–3 NO₃^– was ~ 6-fold higher than at 0·1 mol m^–3 NO₃^– and appeared 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, being similar to the rate attained by pine at only the higher NO₃^–]. In aspen, leaf NRA was ~ 2-fold higher than that of roots. In lodgepole pine NRA of the needles was below the detection limit. These results show that trembling aspen seedlings are better adapted for NO₃^– acquisition and utilization than lodgepole pine seedlings.