Optimizing deficit irrigation in Monastrell vines grafted on rootstocks of different vigour under semi-arid conditions

A progressive irrigation reduction was applied during five years (from 2017 to 2021) by using regulated deficit irrigation (RDI) and partial root-zone drying irrigation (PRD) in field-grown mature Monastrell grapevines grafted on five different rootstocks (140Ru, 1103P 41B, 110R, and 161-49C) in the semi-arid wine-growing region of D.O. Bullas, South-Eastern (SE) Spain. The main goal was to adjust the irrigation volume applied during the growing season for each rootstock depending on the drought tolerance/vigour/productivity of the rootstock. Thus, the annual irrigation volume was progressively reduced (from 104 mm year(-1)[2017] to 45 mm year(-1) [2018-2019] and 15 mm year(-1) [2020-2021] in the most drought-tolerant, vigorous, and productive rootstocks [1103P, 140Ru], and from 104 mm year(-1) [2017] to 74 mm year(-1) [2018-2019] and 58 mm year(-1) [20202021] in the less drought-tolerant, vigorous, and productive rootstocks [110R, 161-49C, 41B]. A severe reduction of the irrigation in 140Ru and 1103P substantially reduced vigour, productivity, and berry weight and increased water productivity (WP) (220% and 97%, respectively) and phenolic berry quality (50% and 92%, respectively). Medium/low-vigour rootstocks also had a decrease in vigour, yield, and berry weight in response to an irrigation reduction, enhancing WP (between 17% [41B] and 41% [161-49C] and phenolic berry quality (between 11% [161-49C] and 46% [110R] With a low water volume DI (53-77 mm year(-1)), vines grafted on 161-49C delivered the highest berry quality compared to other rootstocks. Physiological changes produced by very low water volume PRD were reflected in a reduced root water and nutrient uptake and leaf gas exchange and a slightly increased technological berry quality compared to RDI. However, PRD did not produce a significant improvement in WP, phenolic concentration, and global berry quality compared to RDI.