DEVELOPING A PHYSIOLOGICAL BASIS FOR MODELING PEACH CANOPY PHOTOSYNTHESIS UNDER WATER STRESS CONDITIONS

Abstract

Daily leaf photosynthesis was reported to be linearly related with daily leaf PAR (photosynthetic active radiation) interception, in peach trees. This relationship was considered as a fundamental step for modeling whole peach canopy photosynthesis. However, data reporting linear relationships was obtained under non-water-stressed trees. Although commercial peach orchards are usually managed under full irrigation, reduced irrigation might become inevitable in water limited areas. Water shortage is becoming increasingly frequent in peach producing areas with Mediterranean climates. Modeling the effect of water stress, on whole canopy photosynthesis, requires studies on the effect of water stress on the relationship between daily leaf photosynthesis and daily leaf light interception. We, therefore, studied the effects of water stress on the linear relationship between daily photosynthesis and light interception in a commercial ‘Ryan’s Sun’ peach orchard. Water stress was imposed during the final stage of fruit development (50 days before harvest), because irrigation restrictions usually occur mid-summer. Under full irrigation conditions, there was a linear relationship between daily intercepted PAR and daily photosynthetic CO2 assimilation. This relationship had approximately a zero y-intercept. Under water stress conditions, the y-intercept was maintained, but there was a significant decrease in daily photosynthetic CO2 assimilation rates, at high levels of daily intercepted PAR. The higher the level of water stress, the higher the reduction in CO2 assimilation. The main consequence is that linearity between daily intercepted PAR and daily photosynthetic CO2 assimilation was not maintained under severe water stress conditions. Photosynthetic radiation use efficiency (PhRUE), for all leaves of the canopy, was similar under full irrigation conditions. However, under water stress conditions, the leaves that intercepted high values of PAR had significantly reduced PhRUE. These results can facilitate modeling the effect of water stress on whole canopy photosynthesis, in peach trees.