Main effects of nitrogen supply and drought stress upon whole-plant carbon allocation in poplar

Effects of nitrogen supply and drought stress were studied in two separate glasshouse experiments using young cuttings of Balsam Spire poplar (Populus balsamifera var. Michauxii (Henry) x Populus trichocarpa var. Hastata (Dode) Farwell.) grown in sand culture. In the nitrogen experiment, trees received either 1 (N1) or 9 mol N.m(-3) (N9) applied every 2 days. In the drought experiment, all trees received 3 mol N.m(-3), applied every 2 days for well-watered trees (WW) and every 10 days for drought-stressed trees (WS). Low nitrogen supply and drought stress both reduced tree growth and increased dry matter allocation to roots. Treatment effects upon nitrogen allocation were small due to changes in component nitrogen concentrations. Leaf area per tree was reduced in stressed treatments, although the effects upon leaf number, weight, and specific leaf area differed between treatments. Whole-plant respiration decreased in N1 and WS trees due to a reduction in total biomass and lower rates of respiration per unit tissue. In N1 trees, root respiration per unit tissue decreased whereas in WS trees, it was shoot respiration per unit tissue that decreased. The likely reasons for these differences are discussed.