Diurnal time courses of chlorophyll fluorescence and gasexchange 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.Key-words: Vitis vinifera L.; Vitaceae; chlorophyll fluorescence; electron transport rate; grapevines; photosynthesis; water stress.Abbreviations: 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; ∆F/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.
INTRODUCTIONDrought is considered to be a predominant factor both for determining the global geographic distribution of vegetation and for restricting crop yields in agriculture (Schulze 1986). Furthermore, water stress is a limiting factor for a wide range of physiological processes in plants (Cornic 1994;McDonald & Davies 1996) There is much evidence that water stress per se does not cause reductions in primary events of photosynthesis, i.e. PSII efficiency (Genty, Briantais & Vieira da Silva 1987;Cornic et al. 1989). However, water stress is often accompanied (particularly under Mediterranean conditions) by other limiting factors such...