Effects of high light and ozone fumigation on photosynthesis in Phaseolus vulgaris

In the present work, the aim was to study the combined effects of high irradiance (about 1 000 mu mol.m(-2).s(-1)) on the effects of O-3 (150 nL.L-1 for 2 h) on the photosynthetic process of primary pinto (Phaseolus vulgaris L.) leaves. The CO2 assimilation rate significantly decreased in high light and/or O-3-treated leaves, as did the stomatal conductance. The present work shows that changes occur in photosynthesis-related parameters due to light stress. Photoinhibition was detected as a decrease in the F-v/F-m ratio. This indicates that the light treatment considerably exceeded the photoprotective capacity and resulted in significant photon damage. Moreover O-3 fumigation alone determined an impairment of the photosynthetic process, that in this case is related to the dark reactions of photosynthesis as also showed by the strong increase in (1-q(p)). The data presented in this paper illustrate the complexity of photosynthetic response to high light intensities and ozone, which are two stress factors that often occur simultaneously under natural conditions. High light can modify the reaction of leaves to ozone treatment by reducing the chloroplastic electron transport rate and the quantum yield for PSII photochemistry. Thus, high light seems to enhance the detrimental effects of ozone on photosynthesis. (C) 2000 Editions scientifiques et medicales Elsevier SAS.