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 irra diance (about 1 000 mu mol.m(-2).s(-1)) on the effects of O-3 (150 nL.L-1 f or 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 wo rk shows that changes occur in photosynthesis-related parameters due to lig ht stress. Photoinhibition was detected as a decrease in the F-v/F-m ratio. This indicates that the light treatment considerably exceeded the photopro tective capacity and resulted in significant photon damage. Moreover O-3 fu migation alone determined an impairment of the photosynthetic process, that in this case is related to the dark reactions of photosynthesis as also sh owed by the strong increase in (1-q(p)). The data presented in this paper i llustrate the complexity of photosynthetic response to high light intensiti es and ozone, which are two stress factors that often occur simultaneously under natural conditions. High light can modify the reaction of leaves to o zone treatment by reducing the chloroplastic electron transport rate and th e quantum yield for PSII photochemistry. Thus, high light seems to enhance the detrimental effects of ozone on photosynthesis. (C) 2000 Editions scien tifiques et medicales Elsevier SAS.