Influence of transient shade periods on the effects of drought on photosynthesis, carbohydrate accumulation and lipid peroxidation in sunflower leaves

The effects of a slow-imposing two-week soil drying period, and subsequent re-watering, on leaf water potential (Ψ), gas exchange rates, chlorophyll fluorescence and on the concentrations of malondialdehyde (MDA) and non-structural carbohydrates (starch, hexose and sucrose) were determined in mature leaves of sunflower plants growing under controlled-environmental conditions. To assess how transient shade periods, associated with increased cloud cover, may influence drought-induced effects on carbon assimilation, measurements were carried out both in plants kept under the photosynthetic photon flux density (PPFD) prevailing during the growth period and stress imposition (750 μmol m−2 s−1), and in plants subjected to a 5-h long period under a lower PPFD (200 μmol m−2 s−1). In plants kept under high PPFD, Ψ, stomatal conductance (g), net CO2 uptake rate (A), the quantum yield of photosystem II electron transport (Φe), the photochemical efficiency of open PSII reaction centres (F′v/F′m) and the diurnal accumulation of total non-structural carbohydrates (TNC) were significantly depressed at the end of the soil drying period, whereas non-photochemical quenching (NPQ), the concentrations of MDA and the predawn pools of soluble sugars were found to increase. Under high-light level, drought-induced effects on lipid peroxidation, chlorophyll fluorescence parameters and gas exchange rates were fully reversed upon re-watering. However, the inhibition of diurnal accumulation of TNC still persisted two days following re-watering, suggesting that carbohydrate export rates were enhanced following stress relief. An overall positive effect was found upon transferring water-stressed plants to low light level, as indicated by the increases in Ψ, intrinsic water use efficiency (A/g), Φe and F′v/F′m, as well as the reversal of drought-induced enhancement of both NPQ and MDA concentration. Despite g being similar in shaded well-watered and re-watered plants, the latter exhibited net CO2 uptake rates below those found in well-watered leaves under the same light conditions, together with a diurnal decrease in the concentration of TNC that was mainly attributable to the depletion of starch and sucrose. These results indicate that, contrasting with the positive effects of shading on water-stressed plants, low PPFD may negatively affect the recovery of net photosynthesis following stress relief.