Limitations to carbon assimilation by mild drought in nectarine trees growing under field conditions

The strategies used by nectarine trees (Prunus persica L. Batsch, var. Silver King) to cope with high light and high temperature/vapour pressure deficit conditions were evaluated in field-grown plants in central Portugal. Diurnal time courses of gas exchange rates and chlorophyll fluorescence were measured “in situ” in attached leaves of well-watered or mild water-stressed plants under summer conditions. 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. 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′mF′v/F′m), 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′mF′v/F′m and ETR, and an increase in NPQ. Simultaneously, an increase in ETR/An was observed. Since water stress led to a reduction in the number of PSII centers that remain open 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/An suggests that in water stressed plants the excitation energy in the photosynthetic apparatus, that would normally be consumed via CO2 assimilation, is partially diverted to the photosynthetic reduction of O2, via photorespiration, Mehler-peroxidase reaction or the water–water cycle. Although electrons not consumed in photosynthetic process may generate active oxygen species, this is not likely to occur in water-stressed nectarine leaves, since chlorophyll concentrations were not decreased (there was no chlorophyll bleaching) and the maximum potential PSII efficiency (estimated through the pre-dawn Fv/FmFv/Fm ratio) remained high, which are symptoms of no PSII damage.