| Diurnal time course of gas exchange rates and chlorophyll fluorescence were measured in situ in attached leaves of nectarine trees (Prunus persica L. Batsch, var. Silver King) subjected to different water availabilities under summer conditions in central Portugal. CO2 assimilation rate (An) and stomatal conductance (gs) of well-watered trees decreased along the day in response to high temperature and vapour pressure deficit. Soil water deficit increased the sensitivity of leaf gas exchange to summer atmospheric conditions: An and gs exhibited important midday depressions under water shortage. In spite of a very good correlation between the decline in An and reductions in gs observed over the course of the day, intercellular CO2 concentration became higher in the late afternoon. During the day, the quantum yield of PSII electron transport in the light (Φe), the electron transport rate (ETR), the intrinsic efficiency of open PSII reaction centers (F'v/F'm), and the photochemical quenching (qp) and non-photochemical quenching (NPQ) of chlorophyll fluorescence remained constant in well-watered trees, in spite of some decrease in stomatal conductance in the afternoon. Water stress induced after midday a large, but reversible, decrease of Φe, F'v/F'm and ETR, and an increase in NPQ. Simultaneously, an increase in ETR/A was observed. Because water stress led to a reduction in the size of the pools of electron acceptors after midday, as indicated by the decrease in qp, the contribution of thermal de-excitation at PSII (given by NPQ) in the protection against photoinhibition became more important in stressed trees. The increase in ETR/A suggests that in water stressed plants the excitation energy in the photosynthetic apparatus is partially diverted to the photosynthetic reduction of O2, via photorespiration, Mehler-peroxidase reaction or the water-water cycle. |
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