Mild Water Stress Affects Water Relations, Gas Exchanges and Vascular Flows of 'Hayward' Kiwifruit Branches during the Second Stage of Berry Development

Kiwifruit production requires high amounts of water as this species is very sensitive to drought stress, especially during the initial stages of fruit growth and strategies to reduce the water use of Actinidia orchards are highly needed. This work investigates the physiological effects of reduced water supply to 'Hayward' kiwifruit vines to test the possibility to reduce water use during the second stage of fruit development. At 11 weeks after full bloom, the daily pattern of leaf gas exchanges, leaf and stem water potential, sap flow, fruit pressure potential, fruit growth, fruit vascular, and transpiration flows were compared between two irrigation regimes corresponding to a water restitution of 100% and 70% of the estimated Etc, respectively. All physiological parameters were monitored simultaneously on one fruiting branch per vine, on three vines per treatment. Reducing irrigation decreased leaf carbon assimilation during the afternoon, following the decrease in stomatal conductance. Reduced irrigation vines showed lower stem and leaf water potentials, with no changes in the relative stem-to-leaf water potential gradient which, for both treatments, increased during the day, reaching a maximum around 15.00 hour. This was in accordance with sap flow, whose daily peak was reached at the same time, but showed lower values in 70% irrigated branches. Fruit daily growth pattern was highly affected by water shortage: stressed berries showed significant shrinkage during midday hours which was then recovered during the afternoon when higher growth rates were recorded. Berry shrinkage was due to strong xylem backflow from fruit to leaves, which along with transpiration determined berry dehydration and decreased its pressure potential during midday hours. Such decrease probably created favorable conditions for passive phloem unloading, as shown by the subsequent higher phloem flows to stressed berries. These latter re-hydrated in the afternoon thanks to the higher xylem flows recorded at this time of the day, which followed the afternoon peak in the branch sap flow. On a whole daily basis, reduced irrigation decreased the xylem but increased the phloem flows to the berry, with no apparent consequences in daily fruit growth which resulted similar between treatments. These data suggest that water restrictions at this time of the season may leave fruit growth apparently unaffected but it may reduce fruit Ca uptakes, while it may have positive effects on fruit dry matter concentration.