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

Abstract

A , net CO 2 assimilation rate E , leaf transpiration ETR , electron transport rate F s , fluorescence yield at steady state F m and F m ', maximal fluorescence levels when all PSII reaction centres are closed in dark‐ and light‐acclimated leaves, respectively F o and F o ', initial fluorescence levels when all PSII reaction centres are closed in dark‐ and light‐acclimated leaves, respectively F v / F m , efficiency of excitation capture by open PSII in dark‐adapted leaves ΔF / F m ', actual photochemical efficiency of PSII g , stomatal conductance NPQ , non‐photochemical quenching of chlorophyll fluorescence PPFD , photosynthetic photon flux density Ψ PD and Ψ MD , leaf water potential at pre‐dawn and midday, respectively Rl , estimated photorespiration rate I 1 and I 2 , Irrigation treatments R, Recovery treatment D 1 and D 2 , drought treatments HD 1 and HD 2 , hard drought treatments 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 CO 2 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 CO 2 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 CO 2 assimilation and electron transport rate significantly correlated with pre‐dawn water potential ( Ψ 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 Ψ PD was not maintained, although net CO 2 assimilation was similarly correlated with Ψ PD . Interestingly, the steady‐state chlorophyll fluorescence yield was a good indicator of plant water stress.