Water stress induces different levels of photosynthesis and electron transport rate regulation in grapevines

Diurnal time courses of chlorophyll fluorescence and gas-exchange rates were measured in young potted grapevines (Vitis vinifera L. cv. Tempranillo) subjected to different conditions of water supply under Mediterranean summer conditions. The irrigated plants exhibited typical diurnal patterns for all measured parameters, showing a correspondence between electron transport rate, net CO2 assimilation and stomatal conductance. Mild decreases in soil-water availability led to different degrees of down-regulation of photosynthesis and increased nonphotochemical quenching of chlorophyll fluorescence. A good correspondence between electron transport rate and CO2 assimilation was still maintained, suggesting a coregulation of both photosynthetic processes. In contrast, a severe water deficit induced a drastic down-regulation of photosynthesis and breakage of the above-mentioned link. Both midday net CO2 assimilation and electron transport rate significantly correlated with pre-dawn water potential (Psi(PD)) (r(2) = 0.65 and r(2) = 0.92, P < 0.001, respectively). However, when field data were analysed, the relationship between electron transport rate and Psi(PD) was not maintained, although net CO2 assimilation was similarly correlated with Psi(PD). Interestingly, the steady-state chlorophyll fluorescence yield was a good indicator of plant water stress.