Influence of sunflecks on hybrid poplar, black cherry, and red maple response to ozone

Studies were conducted within Continuously Stirred Tank Reactors (CSTR), and within a natural forest to identify the influences of sunflecks (lightflecks) and low light on plant responses to ozone (O₃) exposures in seedlings of black cherry, red maple, and ramets of hybrid poplar clones NE388 and NE359. Seedlings and ramets were exposed to square-wave 30, 55, and 80 ppb O ₃ under constant high light (HL), lightfleck (LF), and constant low light (LL) treatments within the CSTR studies, and exposed to ambient O ₃ under varying light conditions within the forest based studies. Within the CSTR studies, the LF and LL treatments resulted in more severe visible foliar symptoms than the HL treatment, with more severe foliar in being induced by the LF than the LL treatment in NE388. Plants grown under the LF and LL treatments had significantly lower net photosynthetic rates (Pn) than those grown under the HL treatment. However, light treatments did not consistently or significantly affect stomatal conductance (g_wv), as a result, the ratios of g_wv/Pn and O₃ uptake/Pn were highest in plants grown under the LF treatment, followed by those grown under the LL treatment; plants grown under the HL treatment had the lowest ratios of g _wv/Pn and O₃ uptake/Pn. Similar results were observed within the forest based studies. More severe visible foliar injury was observed on plants grown under low light conditions than that observed on plants grown under high light conditions. Plants grown under low light conditions had significantly lower Pn and g_wv, but higher O₃ uptake/Pn than those grown under high light conditions. Under both CSTR and natural forest conditions, higher ratios of g_wv/Pn and O₃ uptake/Pn were consistently associated with more severe visible foliar symptoms. The results indicate that not only the integral but also the pattern of photo flux density may affect plant responses to O₃, and suggest that increased visible foliar injury may be expected under lightfleck conditions due to the fact that plants continued to take up O₃ while the repair and defense capacity is decreased by the significant decrease in photosynthesis under low light.