LIGHT MANAGEMENT AND PHOTOINACTIVATION UNDER DROUGHT STRESS IN PEACH.

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.