Effects of water deficits on whole tree water use efficiency of orange

To study the effects of water deficits on water use efficiency (WUE) of citrus trees, whole tree transpiration and CO2 assimilation were measured in a semi-arid environment during the summer of 2012. Young orange trees “Valencia Late”, either water stressed (DI) and well-irrigated (C), were monitored in selected days using a gas exchange chamber. Tree transpiration was also measured on a continuous basis with sap flow sensors. The water restriction reduced the transpiration of the DI treatment down to 60% of the maximum potential (treatment C) during the peak of water stress. The instantaneous WUE ranged between 1.7 and 79 g CO2 L−1 H2O and was tightly related to the vapour pressure deficit. Differences in instantaneous WUE due to water stress were insignificant. On a daily basis, WUE ranged between 4.9 (7 August) and 8.8 (7 June) g L−1 for the daytime period; and between 4.0 and 8.2 g L−1 for the 24 h period. As water stress was imposed on the DI treatment, a trend of increasing WUE in DI relative to C was observed, reaching, in the maximum stress period, a difference, of 13–15% (daytime) and 20–22% (24 h) although not statistically significant. Partial rewatering returned the WUE to similar values in both treatments. An analysis of the differences in the diurnal patterns of transpiration suggests that the increase in WUE due to water stress in citrus is achieved indirectly by shifting the overall carbon assimilation towards the morning hours of lower evaporative demand.