Interactive effects of nitrogen and water availabilities on gas exchange and whole-plant carbon allocation in poplar.

摘要

Cuttings of balsam spire hybrid poplar (Populus trichocarpa var. Hastata X Populus balsamifera var. Michauxii) were grown in sand culture and irrigated every 2 (W) or 10 (w) days with a solution containing either 3.0 (N) or 0.5 (n) mol nitrogen m-3for 90 days. Trees in the WN (control) and wn treatments had stable leaf nitrogen 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 conductances 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). Compared with the WNtreatment, biomass at the final harvest was reduced by 61, 72 and 75% in the Wn, wN and wn treatments, respectively. At the final harvest, WN trees had a mean total leaf area of 4750 ± 380 cm2 tree-1 and carried 164 ± 8 leaves tree-1 with a specific leafarea of 181 ± 16 cm2 g-1, whereas Wn trees had a smaller mean total leaf area (1310 ± 30 cm2 tree-1), because of the production of fewer leaves (41 ± 6) with a smaller specific leaf area (154 ± 2 cm2 g-1). A greater proportion of biomass was allocated to roots in Wn trees than in WN trees, but component nitrogen concentrations adjusted such that there was no Wn treatment effect on nitrogen allocation. Compared with WN trees, rates of photosynthesis and respiration per unit weight of tissue of Wn treesdecreased by 28 and 31%, respectively, but the rate of photosynthesis per unit leaf nitrogen remained unaltered. The wN and Wn trees had similar leaf nitrogen concentrations; however, compared with the Wn treatment, the wN treatment decreased mean totalleaf area (750 ± 50 cm2 tree-1), number of leaves per tree (29 ± 2) and specific leaf area (140 ± 6 cm2 g-1), but increased the allocation of biomass and nitrogen to roots. Net photosynthetic rate per unit leaf nitrogen was 45% lower in the wN treatmentthan in the other treatments. Rates of net photosynthesis and respiration per unit weight of tissue were 48 and 33% less, respectively, in wN trees than in Wn trees.