Irrigation schedule on table grapes by stem water potential and vapor pressure deficit allows to optimize Water Use

This study tested the relationship between stem water potential (ψstem) and vapor pressure deficit (VPD) in order to evaluate its use as reference for irrigation control of table grape (Vitis vinifera) cvs. Thompson Seedless and Redglobe. Two trials were carried out on consecutive seasons. In the first season the treatments consisted of four different irrigation regimes: T1, plants irrigated at 100% of ETc; T2, irrigated until fruit set the same as T1 but after fruit set every other time T1; T3, plants with no irrigation from fruit set to harvest; and T4, plants irrigated at 50% of ETc throughout the growing season. Vines from T1 always showed a higher ψstem for a wide VPD range, therefore the ψstem and VPD relationship was established using T1 vines, obtaining a logarithmic function with a high determination coefficient (R2=0.85). The following season this relationship was used to control irrigation frequency on two table grape cvs. Irrigation started when vines had 20 cm shoots and then the irrigation amount was set to replenish 10% of soil available water. Trial 1 was conducted on cv. Thompson Seedless and consisted of two treatments: T1 plants irrigated to satisfy 100% ETc and T2 plants irrigated according to the previously obtained ψstem-VPD reference line. Trial 2 was established with Redglobe cv. plants using three treatments: T1 plants irrigated to satisfy 100% Etc; T2, plants irrigated the same as T1 but after fruit set every other time T1; and T3, plants irrigated according to the ψstem-VPD reference line previously obtained. Irrigation frequency determined using the ψstem-VPD relationship reduced total water volume compared to T1 on both trials, with no effects on yield or quality, showing the feasibility of using this relationship to control irrigation frequency.