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

Abstract

The importance of both stomatal closure and reduced carboxylation efficiency on the photo- synthesis decline in response to long term water stress was previously measured in field-grown grapevines. Here we address the question of whether water stress affects the photochemical capacity of leaves, measuring gas-exchange rates and chlorophyll fluorescence under drought and moderate irrigat- ion at intervals through the summer season during three consecutive years. We conclude that usually water stress does not induce photoinhibition in field-grown grapevines, even when stomatal conductance and photosynthesis are reduced to very low values. Moreover, down-regulat- ion of photochemical reactions is low, leading to a general pattern of photosynthetic response to drought consistent in large reductions of stomatal conductance (g), followed by a consistent decrease of CO2 assimilation (A) but with a much lower effect on electron transport rate (ETR). In consequence, the intrinsic water-use efficiency (A/g) increased, as well as the ratio ETR/A. It is suggested that increased electron transport to alternative pathways, such as photorespiration, 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 developed water stress, ETR reductions are more attenuated than in potted plants, reducing their incidence in carbon assimilation, which seems to be mainly regulated by stomatal closure.