Time of irrigation affects vine water relations and the daily patterns of leaf gas exchanges and vascular flows to kiwifruit (Actinidia deliciosa Chev.)

The cultivation of Actinidia deliciosa requires high amounts of water and innovative approaches for optimizing its irrigation scheduling are needed. This paper tests the possibility to improve the kiwi vine physiological performances by changing the timing of water supply during the day. Two irrigation treatments were applied in a 'Hayward' commercial orchard, both providing 100% of crop evapotranspiration (ETc) but differing for the time of the day when irrigation was applied: early in the morning (M) or at midday/early afternoon (A). The daily patterns of soil water content, stem, fruit-bearing branch, leaf and fruit water potentials, sap flow along the branch, leaf gas exchanges, fruit growth, phloem, xylem and transpiration flows to/from the fruit were monitored at different times during the season: 6, 9, 11 and 18 weeks after full bloom (WAFB). In most dates A irrigation increased trunk, stem and leaf water potentials, as well as leaf gas exchanges, during the afternoon, i.e., when the highest air vapor pressure deficits (VPDs) were recorded. As fruit pressure potential was unaffected by the treatments, fruit from "A" irrigated vines took advantage of a higher stem-to-fruit water potential gradient. In response to this, at 11 WAFB, when the highest VPDs were recorded, "A" fruit experienced a higher xylem flow during the afternoon. Fruit growth was slightly improved at this time of the day but did not show any significant difference between treatments on a cumulative daily basis, probably due to the amount of water supplied (100% ETc). This work shows how the hour of irrigation can affect the vine physiological performances. Although further investigation is needed, supplying water in the afternoon, when environmental conditions are more stressing, has the potential of improving kiwifruit vine water status, leaf gas exchanges, and xylem flows to the fruit, and this may provide a new approach at increasing water use efficiency in this crop.