ANALYSIS OF ASPEN FOLIAGE EXPOSED TO MULTIPLE STRESSES - OZONE, NITROGEN DEFICIENCY AND DROUGHT

Two field experiments were conducted to examine the impact of a foliar stress (ozone, O-3), alone or in combination with a resource stress, on carbon gain of aspen (Populus tremuloides Michx.). The first experiment involved nitrogen deficiency, and the second, drought stress. The patterns of change in leaf area, photosynthetic rate and carbon gain with leaf position from root to apex were altered by O-3 alone, Net photosynthesis and carboxylation capacity of older leaves decreased, and of younger leaves increased, in response to O-3. Thus younger leaves partially compensated for the decreased photosynthetic capacity of older leaves. Physiological maturity and senescence were accelerated by O-3. Whole plant carbon gain declined with O-3 due to reductions in both leaf area and photosynthetic rate. Nitrogen deficiency alone decreased carbon gain via reductions in both leaf area and photosynthetic rate across leaf position. The effects of O-3 and nitrogen deficiency together on both leaf area and photosynthetic rate were additive. Aspen seedlings compensated to O-3 stress regardless of nitrogen level. Plant responses to both of these stresses may have been mediated by ribulose bisphosphate carboxylase/oxygenase (Rubisco) content or activity, and thus indirectly by nitrogen content. In the second experiment, seedlings were subjected to drought during the first six weeks of exposure to O-3, and were measured several weeks after termination of the drought treatment. Drought caused aspen seedlings to produce fewer and smaller leaves. After termination of drought, they produced larger new leaves, and net photosynthesis of most leaves was greater than in continuously well-watered plants. In plants not exposed to O-3, enhanced photosynthetic rate partially compensated for the smaller leaf area. O-3 generally decreased leaf area and photosynthesis, and hence carbon gain. The effects of prior exposure to drought and O-3 together were additive for whole-plant photosynthetic rate, but less than additive for whole-plant leaf area.