Stomatal aperture rather than nitrogen nutrition determined water use efficiency of tomato plants under nitrogen fertigation

Fertigation can improve water use efficiency (WUE) compared with conventional separate supply of water and fertilizers to plants. Yet the mechanisms underlying the improved WUE under fertigation remain largely elusive. Therefore, the impact of water and nitrogen (N) on leaf gas exchange, plant water relations, ABA signaling and WUE as well as leaf δ ¹³C and δ ¹⁸O were investigated in order to unravel how water and N modulate plant WUE. Results showed that reduced soil water regimes under N fertigation caused partial closure of stomata via decreased plant water status and intensified root-to-shoot ABA signaling, resulting in improved intrinsic WUE (WUE _i). Decreased soil water regimes increased plant WUE (WUE _p) and leaf δ ¹³C, and the increased leaf δ ¹³C was due to reduced g _s and/or higher specific leaf N content enhanced photosynthetic capacity. Leaf δ ¹⁸O and δ ¹³C further indicated that the significant increase in leaf δ ¹³C under the reduced water regimes was caused primarily by reductions in g _s compared with N nutrition. Therefore, g _s rather than N nutrition predominated regulation of plant WUE under fertigation. Moderate soil water regimes with sufficient N supply are recommended for fertigation in terms of achieving high fresh fruit yield, WUE and nutrient uptake.