Afternoon Depression In Photosynthesis in Grapevine Leaves—Evidence for a High Light Stress Effect

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Abstract
A midday depression in net photosynthesis and in stomatal conductance was observed when leaves of well-watered Vitis vinifera plants were subjected to a diurnal pattern of variation in leaf temperature and leaf-to-air water vapour pressure difference similar to a summer day, while photon flux density was kept constant at 1450 µmolm−2 s−1,. When leaves were kept at constant leaf temperature (22.5°C) and leaf-to-air water vapour presure difference (8.5 Pa kPa−1) at the same light intensity, stomata opened with the onset of illumination and maximal conductance and photosynthesis values were observed about 1 h later. Subsequently, conductance and photosynthesis decreased gradually. Leaf water potential never dropped below ¯0.3 MPa. Leaves kept under constant environmental conditions showed an afternoon decline in photosynthesis at high internal CO2, in carboxylation efficiency and in maximum conductance as well as an increase in stomatal sensitivity to CO2. When the photon flux density during the day was reduced to 750 µmol m−2 s−1, the afternoon depression in gas exchange rates was attenuated. To evaluate the possible effects of high light stress on changes in chloroplastic behaviour we compared light response curves of photosynthesis determined with an oxygen electrode, in the morning and in the afternoon after the plants were exposed to either high or moderate photon flux densities. A significant depression in photosynthetic capacity was found by this method in high light treated leaves, but not in leaves previously exposed to moderate photon flux density. Apparent quantum yield decreased in the afternoon, particularly after exposure to high light. Maximum chlorophyll a fluorescence at 22°C was reduced and the quenching of fluorescence after reaching the peak was slower in the afternoon than in the morning, especially in high light-treated leaves. Changes in the patterns of chlorophyll fluorescence kinetics were observed after light treatment, i.e. in the afternoon, with oscillations either absent (after high light) or significantly reduced (after moderate light) in comparison to the morning. The significance of these results is discussed and it is suggested that a direct inhibitory effect of high light at the chloroplast level provides the best interpretation for the observed afternoon decline in photosynthetic rate.