Down-regulation of photosynthesis by drought under field conditions in grapevine leaves

The importance of both stomatal closure and reduced carboxylation efficienc y on the photosynthesis decline in response to long term water stress was p reviously measured in field-grown grapevines. Here we address the question of whether water stress affects the photochemical capacity of leaves, measu ring gas-exchange rates and chlorophyll fluorescence under drought and mode rate irrigation at intervals through the summer season during three consecu tive years. We conclude that usually water stress does not induce photoinhibition in fi eld-grown grapevines, even when stomatal conductance and photosynthesis are reduced to very low values. Moreover, down-regulation of photochemical rea ctions is low, leading to a general pattern of photosynthetic response to d rought consistent in large reductions of stomatal conductance (g), followed by a consistent decrease of CO2 assimilation (A) but with a much lower eff ect on electron transport rate (ETR). In consequence, the intrinsic water-u se efficiency (A/g) increased, as well as the ratio ETR/A. It is suggested that increased electron transport to alternative pathways, such as photores piration, prevented further down-regulation of ETR under drought conditions . These results are in agreement with our previous reports for potted plants. However it is clear that, under field conditions with a much more slowly d eveloped water stress, ETR reductions are more attenuated than in potted pl ants, reducing their incidence in carbon assimilation, which seems to be ma inly regulated by stomatal closure.