Influence of light fleck and low light on foliar injury and physiological responses of two hybrid poplar clones to ozone

Five-month old hybrid poplar clones NE388 and NE359 were exposed to square-wave 30, 55, and 80 ppb O3 (8 h/day, 7 day/week) under constant high light (HL) and light fleck (LF) during 28 May-29 June 1999, and exposed to 30 and 55 ppb O3 under HL, LF, and constant low light (LL) during 22 May-28 June 2000 within Continuously Stirred Tank Reactors (CSTR) in a greenhouse. Ramets of these two hybrid clones received similar total photosynthetically active radiation (PAR) within the LF and LL treatments. Visible foliar symptoms, leaf gas exchange, and growth were measured. More severe O3 induced foliar symptoms were observed on ramets within the LF and LL treatments than within the HL treatment for both clones. The LF treatment resulted in significantly greater foliar injury than the LL treatment for NE388. The LF and LL treatments generally resulted in lower photosynthetic rates (Pn) for both clones, but did not affect stomatal conductance (gwv); therefore, the ratios of gwv/Pn and the O3 uptake/Pn were greatest in plants grown under the LF treatment, followed by those grown under LL treatment; plants grown under HL had the lowest ratios of gwv/Pn and O3 uptake/Pn. Greater ratios of gwv/Pn and O3 uptake/Pn were consistently associated with more severe visible foliar symptoms. The negative impacts of the LF treatment on growth were greater than those of the LL treatment. Results indicate that not only the integral, but also the pattern of photo flux density, may affect carbon gain in plants. Increased foliar injury may be expected under light fleck conditions due to the limited repair capacity as a result of continuity of O3 uptake while photosynthesis decreases under LL conditions. - Intermittent light flecks enhance physiological injury leading to increased foliar symptoms in clones of ozone-sensitive hybrid poplar