In the present work, the aim was to study the combined effects of
high irradiance (about 1?000 ?mol·m?2·s?1) on the effects of O3 (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 O3-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 Fv/Fm ratio. This indicates that the light treatment
considerably exceeded the photoprotective capacity and resulted in
significant photon damage. Moreover O3 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-qP). 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.