Water Relations and Physiological Response to Water Deficit of 'Hass' Avocado Grafted on Two Rootstocks Tolerant to R. necatrix

Avocado (Persea americana Mill.) cultivation has spread to many countries from the tropics to the Mediterranean region, where avocado crops commonly face water shortages and diseases, such as white root rot (WRR) caused by Rosellinia necatrix. The use of drought- and WRR-tolerant rootstocks represents a potential solution to these constraints. In this research, water relations and the morpho-physiological response of avocado 'Hass' grafted on two selections of R. necatrix-tolerant rootstocks (BG48 and BG181) were evaluated under well-watered (WW) and at two soil-water-availability conditions (WS, similar to 50% and similar to 25% field capacity). Under WW, scion water use was markedly affected by the rootstock, with BG48 displaying a water-spender behavior, showing higher water consumption (similar to 20%), plant transpiration rates (similar to 30%; E-plant) and leaf photosynthetic rates (similar to 30%; A(N)) than BG181, which exhibited a water-saving strategy based upon a trade-off between leaf-biomass allocation and tight stomatal control of transpiration. This strategy did not reduce biomass, with BG181 plants being more water use efficient. Under WS, BG48 and BG181 exhibited a drought-avoidance behavior based on distinct underlying mechanisms, but increases in leaf mass area (similar to 18-12%; LMA), and decreases in E-plant (similar to 50-65%), plant hydraulic conductance (similar to 44-86%; K-h) and leaf water potential (similar to 48-73%; Psi(w)) were observed in both rootstocks, which aligned with water stress severity. After rewatering, photosynthetic rates fully recovered, suggesting some ability of these rootstocks to withstand water stress, enabling the 'Hass' variety to adapt to region-specific constraints.