Background: Water and nitrogen (N) are essential resources influencing plant growth and yield. To improve their efficiencies in crop production is challenging because the physiological mechanisms of water and N coupling and their interactive effect on crop water use efficiency (WUE) are not well understood yet.

Aim: The aim of this study was to investigate the physiological responses and phytohormones signaling in oats in response to soil water status and N supply under fertigation, to explore the mechanisms regulating plant growth and WUE.

Methods: Oat plants were subjected to the factorial combination of three soil moisture regimes (50, 70, and 90% of soil water holding capacity, SWHC) and three N levels (fertilized with 74, 149, and 298 mg kg(-1)).

Results: The stomatal conductance (g(s)) was significantly decreased by soil water deficit, and also by the highest N level, whereas photosynthesis rate (A(n)) was unaffected by neither water nor N. Consequently, intrinsic WUE (WUEint, A(n)/g(s)) was highest under reduced irrigation and high N fertilization. This effect at stomatal level was affirmed by responses in whole plant WUE (WUEb), which was positively correlated with shoot delta C-13. A positive correlation between delta O-18 and delta C-13 in shoots further indicated that decreases of g(s)rather than changes in A(n)contributed to the enhanced WUE.

Conclusion: Moderate soil water deficit and sufficient N supply is recommended for saving irrigation water and improving WUE on fertigated oat plants without compromising biomass accumulation to any large extent.