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

Abstract

A, net CO2 assimilation rate E, leaf transpiration ETR, electron transport rate Fs, fluorescence yield at steady state Fm and Fm', maximal fluorescence levels when all PSII reaction centres are closed in dark‐ and light‐acclimated leaves, respectively Fo and Fo', initial fluorescence levels when all PSII reaction centres are closed in dark‐ and light‐acclimated leaves, respectively Fv/Fm, efficiency of excitation capture by open PSII in dark‐adapted leaves ΔF/Fm', actual photochemical efficiency of PSII g, stomatal conductance NPQ, non‐photochemical quenching of chlorophyll fluorescence PPFD, photosynthetic photon flux density ΨPD and ΨMD, leaf water potential at pre‐dawn and midday, respectively Rl, estimated photorespiration rate I1 and I2, Irrigation treatments R, Recovery treatment D1 and D2, drought treatments HD1 and HD2, 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 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 (ΨPD) (r2 = 0·65 and r2 = 0·92, P < 0·001, respectively). However, when field data were analysed, the relationship between electron transport rate and ΨPD was not maintained, although net CO2 assimilation was similarly correlated with ΨPD. Interestingly, the steady‐state chlorophyll fluorescence yield was a good indicator of plant water stress.