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

Abstract

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. CO 2 assimilation rate ( A n) and stomatal conductance ( g s) 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; A n and g s exhibited important midday depressions under water shortage. During the day, the quantum yield of PSII electron transport in the light ( ϕ e), the electron transport rate (ETR), the intrinsic efficiency of open PSII reaction centers ( F ′ v / F ′ m ), the photochemical quenching ( q p) 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 ϕ e, F ′ v / F ′ m and ETR, and an increase in NPQ. Simultaneously, an increase in ETR/ A n 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 q p, 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 n suggests that in water stressed plants the excitation energy in the photosynthetic apparatus, that would normally be consumed via CO 2 assimilation, is partially diverted to the photosynthetic reduction of O 2, 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 F v / F m ratio) remained high, which are symptoms of no PSII damage.