Effects of Postharvest Water Deficits on the Physiological Behavior of Early-Maturing Nectarine Trees

The physiological performance of early-maturing nectarine trees in response to water deficits was studied during the postharvest period. Two deficit irrigation treatments were applied, moderate and severe, and these were compared with a control treatment (fully irrigated). Stem water potential and leaf gas exchange (net CO₂ assimilation rate, A_CO2; transpiration rate, E; and stomatal conductance, g_s) were measured frequently. Drought avoidance mechanisms included a decrease in stomatal conductance, especially in the case of the severe deficit treatment, which also showed a strong dependence of A_CO2 on g_s. Intrinsic water-use efficiency (A_CO2/g_s) was more sensitive than instantaneous water-use efficiency (A_CO2/E) as an indicator to detect water deficit situations in nectarine trees. However, in contrast to the results obtained for other deciduous fruit trees, a poor correlation was found between A_CO2/E and A_CO2/g_s, despite the important relation between E and g_s. A_CO2/E was also weakly correlated with g_s, although this relationship clearly improved when the vapor pressure deficit (VPD) was included, along with g_s as the independent variable. This fact reveals that apart from stomatal closure, E depends on the boundary layer conductance (g_b), which is mediated by VPD through changes in wind speed. This suggests low values of the decoupling coefficient for this water-resilient species.