Modulation of phenology and agronomical performance of Syrah grafted on two rootstocks under combined salinity and water stress conditions: A three-year field study

Climate change is challenging global viticulture through increasing drought and salinization, making rootstock selection critical. However, field-based understanding of rootstock-mediated stress responses remains limited. Here, we examined the mediation of Sélection Oppenheim 4 (SO4) and 1103 Paulsen (PL1103) rootstocks on Syrah grapevines' responses under combined water and salinity stress over 3 years (2022‒2024). We observed that the soil electrical conductivity (EC_e1:1) increased from 2.09 to 8 dS m⁻¹ under severe stress, while soil chloride concentration ([Cl⁻]) reached 716.5 mg L⁻¹ in Syrah grafted onto PL1103 (SY_PL1103) and 954.9 mg L⁻¹ in Syrah grafted onto SO4 (SY_SO4), compared with 90‒162 mg L^‒1 under the control conditions coupled with varied leaf [Cl⁻] between grafts. Salinity delayed flowering time by 7‒11 days when the EC_e1:1 exceeded 2.79 dS m^‒1 in SY_PL1103 and 1.99 dS m^‒1 in SY_SO4. Nevertheless, SY_SO4 maintained greater physiological performance with 56.8 % greater photosystem II efficiency, 48.71 % greater electron transport rate, and higher non-photochemical quenching across conditions than SY_PL1103, effectively managing excess light as heat to prevent photodamage. SY_SO4 vines exhibited a significantly lower yield reduction (36.3 % vs 56.4 % in SY_PL1103) and better salinity tolerance, with yield decreasing by only 2.63 t ha⁻¹ per unit increase in EC_e1:1 above the threshold (SY_PL1103 = 3.47 dS m⁻¹ and SY_SO4 = 2.71 dS m⁻¹) compared with 7.78 t ha⁻¹ in SY_PL1103. SY_SO4 showed smaller photosynthetic and morphological changes to combined stressors, with higher soil and lower leaf [Cl−] indicating better ion exclusion, offering valuable insights and practical solutions for resilient grapevine breeding and vineyard management strategies in saline-prone arid regions.