INTERACTIVE EFFECTS OF NITROGEN AND WATER AVAILABILITIES ON GAS-EXCHANGE AND WHOLE-PLANT CARBON ALLOCATION IN POPLAR

Cuttings of balsam spire hybrid poplar (Populus trichocarpa var. Hasta ta Henry x Populus balsamifera var. Michauxii (Dode) Farwell) were gro wn in sand culture and irrigated every 2 (W) or 10(w) days with a solu tion containing either 3.0 (N) or 0.5 (n) mol nitrogen m(-3) for 90 da ys. Trees in the WN (control) and wn treatments had stable leaf nitrog en concentrations averaging 19.4 and 8.4 mg g(-1), respectively, over the course of the experiment. Trees in the Wn and wN treatments had a similar leaf nitrogen concentration, which increased from 12.0 to 15.8 mg g(-1) during the experiment. By the final harvest, mean stomatal c onductances of trees in the wN and wn treatments were less than those of trees in the Wn and WN treatments (1.8 versus 4.6 mm s(-1)). Compar ed to the WN treatment, biomass at the final harvest was reduced by 61 , 72 and 75% in the Wn, wN and wn treatments, respectively. At the fin al harvest, WN trees had a mean total leaf area of 4750 +/- 380 cm(2) tree(-1) and carried 164 +/- 8 leaves tree(-1) with a specific leaf ar ea of 181 +/- 16 cm(2) g(-1), whereas Wn trees had a smaller mean tota l leaf area (1310 +/- 30 cm(2) tree(-1)), because of the production of fewer leaves (41 +/- 6) with a smaller specific leaf area (154 +/- 2 cm(2) g(-1)). A greater proportion of biomass was allocated to roots i n Wn trees than in WN trees, but component nitrogen concentrations adj usted such that there was no Wn treatment effect on nitrogen allocatio n Compared with WN trees, rates of photosynthesis and respiration per unit weight of tissue of Wn trees decreased by 28 and 31%, respectivel y, but the rate of photosynthesis per unit leaf nitrogen remained unal tered. The wN and Wn trees had similar leaf nitrogen concentrations; h owever, compared with the Wn treatment, the wN treatment decreased mea n total leaf area (750 +/- 50 cm(2) tree(-1)), number of leaves per tr ee (29 +/- 2) and specific leaf area (140 +/- 6 cm(2) g(-1)), but incr eased the allocation of biomass and nitrogen to roots. Net photosynthe tic rate per unit leaf nitrogen was 45% lower in the wN treatment than in the other treatments. Rates of net photosynthesis and respiration per unit weight of tissue were 48 and 33% less, respectively, in wN tr ees than in Wn trees.