LIGHT ENVIRONMENT ALTERS RESPONSE TO OZONE STRESS IN SEEDLINGS OF ACER-SACCHARUM MARSH AND HYBRID POPULUS L .1. IN-SITU NET PHOTOSYNTHESIS,DARK RESPIRATION AND GROWTH

Hybrid poplar (Populus tristis Fisch. x P. balsamifera L., cv. Tristis ) and sugar maple (Acer saccharum Marsh.) seedlings were grown under c ontrasting light and ozone treatments to investigate the role of the l ight environment in their response to chronic ozone stress. In consecu tive growth chamber experiments, cuttings of shade-intolerant poplar a nd 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-2 d-1 PPFD or 7 % of full sunlight) and six-fold greater ir radiance (c. 16.6 mol m-2 d-1 PPFD or 45 % of full sunlight) in combin ation with low (< 10 nl l-1) and elevated levels of ozone (c. 99-115 n l l-1). In unshaded poplar plants, ozone exposure reduced root dry mas s by 33 % at final harvest, while shaded plants had no such response. By comparison, sugar maple root dry mass was reduced by ozone in shade d plants by 1 0 %, but was unaffected by ozone in unshaded plants. In poplar, leaf area: plant dry mass ratios were unaffected by ozone, whe reas in sugar maple ozone-exposed plants had a 24 % lower leaf area: p lant dry mass ratio in the shaded treatment. In shade-grown sugar mapl e, ozone doubled dark respiration rates of leaves, but in unshaded see dlings ozone had no effect on respiration. In comparison, in poplar pl ants ozone exposure resulted in greater increases in dark respiration under unshaded than shaded conditions. In unshaded plants, ozone treat ment resulted in lower in situ net photosynthesis in poplar, but not i n sugar maple. Overall, shade-grown sugar maple appeared more sensitiv e to ozone stress than unshaded plants in terms of lower leaf area: pl ant dry mass ratio and root growth and higher leaf respiration. In pop lar on the other hand, root growth, leaf respiration and photosynthesi s 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 elevate d levels of ozone than plants under the opposite light conditions.