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 sepa rate glasshouse experiments using young cuttings of Balsam Spire popla r (Populus balsamifera var. Michauxii (Henry) x Populus trichocarpa va r. Hastata (Dode) Farwell.) grown in sand culture. In the nitrogen exp eriment, trees received either 1 (N1) or 9 mol N.m(-3) (N9) applied ev ery 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 stres s both reduced tree growth and increased dry matter allocation to root s. Treatment effects upon nitrogen allocation were small due to change s 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 resp iration decreased in N1 and WS trees due to a reduction in total bioma ss and lower rates of respiration per unit tissue. In N1 trees, root r espiration per unit tissue decreased whereas in WS trees, it was shoot respiration per unit tissue that decreased. The likely reasons for th ese differences are discussed.