Light environment alters response to ozone stress in seedlings of Acer saccharum Marsh, and hybrid Populus L.: I. In situ net photosynthesis, dark respiration and growth.

Hybrid poplar (Populus tristis Fisch. ×P. balsamifera L., cv. Tristis) and sugar maple (Acer saccharum Marsh.) seedlings were grown under contrasting light and ozone treatments to investigate the role of the light environment in their response to chronic ozone stress. In consecutive growth chamber experiments, cuttings of shade-intolerant poplar and 3-yr-old seedlings of shade-tolerant sugar maple were grown in pots for 6 and 10 wk, respectively, under shaded, low light irradiance (c. 2.5 mol m d PPFD or 7% of full sunlight) and six-fold greater irradiance (c. 16.6 mol m d PPFD or 45% of full sunlight) in combination with low (< 10 nl 1 ) and elevated levels of ozone (c. 99-115 nl 1 ). In unshaded poplar plants, ozone exposure reduced root dry mass by 33% at final harvest, while shaded plants had no such response. By comparison, sugar maple root dry mass was reduced by ozone in shaded plants by 10%, but was unaffected by ozone in unshaded plants. In poplar, leaf area: plant dry mass ratios were unaffected by ozone, whereas in sugar maple ozone-exposed plants had a 24% lower leaf area: plant dry mass ratio in the shaded treatment. In shade-grown sugar maple, ozone doubled dark respiration rates of leaves, but in unshaded seedlings ozone had no effect on respiration. In comparison, in poplar plants ozone exposure resulted in greater increases in dark respiration under unshaded than shaded conditions. In unshaded plants, ozone treatment resulted in lower in situ net photosynthesis in poplar, but not in sugar maple. Overall, shade-grown sugar maple appeared more sensitive to ozone stress than unshaded plants in terms of lower leaf area: plant dry mass ratio and root growth and higher leaf respiration. In poplar on the other hand, root growth, leaf respiration and photosynthesis were more affected by ozone in unshaded than in shaded plants. These findings suggest that shade-grown sugar maple and unshaded poplar may experience greater reductions in carbon gain and growth under elevated levels of ozone than plants under the opposite light conditions.