Water use efficiency in peach trees over a four-years experiment on the effects of irrigation and nitrogen application

• The study was conducted for four years on a peach tree orchard.
• Productive and vegetative response depended on both irrigation and nitrogen.
• High yield water productivity was obtained with no water restrictions.
• Water deficit in fruit growth GSII affect fruit size and reduce the water profits.
• Nitrogen was effective without water restrictions on first phases of fruit growth.

The optimization of crop water productivity is a key objective in modern agriculture. Deficit irrigation is a widely used strategy when water and energy are scarce and/or expensive. However, in fruit tree crops, the severity and duration of water stress can affect yield and fruit size, thus imposing severe limitations on the effectiveness of production processes, and on fruit quality and water productivity. Here we analyzed water use efficiency (WUE) of peach trees over four years both in agronomic terms, as water productivity of fruits and biomass (WUEWPy,b), and in physiological terms, as intrinsic and instantaneous leaf WUE (WUEint and WUEins respectively). Our results show a positive and additive effect of water and nitrogen application on WUEWP, reflected by yield enhancement. WUEint and WUEins varied through changes in stomatal conductance and transpiration—two parameters that are determined by plant water status and nitrogen application. Our results suggest that the variation in WUEWPy was caused by complex regulation of plant growth and fruiting, related to water status and nitrogen availability, which both drive plant WUE. In order to evaluate WUE at the canopy scale, we studied δ13C and δ18O in wood. These parameters were found to be related to leaf stomatal conductance and transpiration and also to the effects of irrigation and nitrogen; however, no clear relationship was observed with WUEWPy. Plant water status was also dependent on crop load and vegetative growth and was strongly associated with WUEWPy and WUEWPb. Taken together, our results show that full irrigation and rational nitrogen application can improve WUEWPy, both in agronomic and economic terms. When water deficit conditions are foreseeable, deficit irrigation in fruit growth phase III (GSIII) is a more profitable strategy that when applied in phase II (GSII). In this regard, flexible deficit irrigation in GSIII is an affordable and practical tool to include in strategies aiming to improve water and nitrogen use efficiency in fruit crops when water is scarce.