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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 δ¹³C and δ¹⁸O 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 δ¹³C, and the increased leaf δ¹³C was due to reduced g_s and/or higher specific leaf N content enhanced photosynthetic capacity. Leaf δ¹⁸O and δ¹³C further indicated that the …