Improved plant nitrogen nutrition contributes to higher water use efficiency in tomatoes under alternate partial root-zone irrigation

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 (delta C-13), 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 conductance during the treatment. At the end of the experiment, N accumulation and N-15 recovery was highest in FI, intermediate in PRI and lowest in DI. In addition, PRI plants consistently allocated more N into the upper and middle leaf layers than in the FI and DI treatments. The improved N nutrition and distribution in the canopy may indicate that PRI plants have a greater photosynthetic capacity than DI plants; this is confirmed by the observed positive linear relationship between specific leaf N content and d13C. It is concluded that PRI improves N nutrition and optimises N distribution in the canopy, which might have been partly contributed to the high WUE in PRI tomato plants.