Abstract

This study reports the effect of increasing drought stress on photosynthetic efficiency, photoprotective mechanisms and amount of photoinactivation in potted peach plants subjected to water stress. A water stress was gradually imposed allowing plants to dry out for 5 days after being irrigated to field capacity, at the end of July. Stomatal conductance (gs) was used as an indicator of the severity of water stress. At mid-morning on each of the 5 days, gas exchanges and chlorophyll fluorescence were measured (Quenching partitioning) on leaves exposed to saturating light (1600 μmol m-2 s-1). The highest stomatal conductance measured during the study was 0.11 mol m-2 s-1. Within this sub-optimal range the photosynthetic efficiency (ΦCO2) responded linearly to gs. Under saturating light, the reduction in photosynthesis augmented the imbalance between the incoming photon pressure and the rate of exciton/electron transports, exposing leaves to an increasing chance of photoinactivation. This risk was alleviated increasing the efficiency of the photoprotective mechanisms, mainly via the light-dependent non photochemical quenching (NPQ). NPQ efficiency (ΦNPQ) increased slightly with reducing stomatal conductance, up to a gs value of 0.06 mol m-2 s-1; afterwards ΦNPQ raised rapidly, dissipating about 73% of the total absorbed light under the most severe water stress conditions. Despite the photoprotective strategy and the effective photosystem recovery mechanism commonly found in plants, a fraction of inactive photosystem II (PSII) was found, increasing with water stress. In high light conditions water stress can reduce the dry matter accumulation and productivity in peach as a consequence of the synergistic action of decreased photosynthetic efficiency and increased amount of photo-assimilates devoted to PSII recovery.