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

Diurnal time courses of chlorophyll fluorescence and gas-exchange rates wer e measured in young potted grapevines (Vitis vinifera L. cv. Tempranillo) s ubjected to different conditions of water supply under Mediterranean summer conditions. The irrigated plants exhibited typical diurnal patterns for al l 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 photosyn thesis and increased nonphotochemical quenching of chlorophyll fluorescence . A good correspondence between electron transport rate and CO2 assimilatio n was still maintained, suggesting a coregulation of both photosynthetic pr ocesses. In contrast, a severe water deficit induced a drastic down-regulat ion 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 relationsh ip 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 pl ant water stress.