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Comparative effects of partial root-zone irrigation (PRI) and deficit irrigation (DI) on stomatal conductance (gs), nitrogen accumulation and distribution in tomato (Lycopersicon esculentum L.) plants were investigated in a split-root pot experiment. Results showed that both PRI and DI saved 25% water and led to 10.0% and 17.5% decreases in dry biomass, respectively, compared with the fully irrigated (FI) controls. Consequently, water use efficiency (WUE) was increased by 18.6% and 10.8% in the PRI and DI plants, respectively. The highest WUE in the PRI plants was associated with the highest carbon isotope composition (δ13C), indicating that the improvement of WUE might have been a result of long-term optimisation of stomatal control over gas exchange. The constantly higher xylem sap ABA concentration in PRI compared with DI plants was seemingly responsible for the greater control over stomatal …