Effects of different water and nitrogen treatments on greenhouse tomato yield, water use efficiency, and physiological responses

This study aimed to explore how the yield, water use efficiency (WUE), and physiological characteristics of greenhouse-cultivated tomatoes in Northeast China respond to different water and nitrogen treatments during the flowering to fruit set stage (Stage 1), fruiting stage (Stage 2), and ripening stage (Stage 3). Three irrigating upper and lower limits were set at 65% θ fc and 75% θ fc (W1), 75% θ fc and 85% θ fc (W2), and 85% θ fc and 95% θ fc (W3), where θ fc represents the field capacity. Three levels of N application were used: N1 (180 kg ha −1 ), N2 (240 kg ha −1 ) and N3 (360 kg ha −1 ). Variance analysis and quadratic curve fitting were employed to characterize the effects of different water and nitrogen treatments on various physiological indicators. The results showed that in Stage 1, the irrigation water use efficiency (WUE i ) under N3 was not affected by irrigation water. In Stage 2, there was no significant difference in the photosynthetic rate (Pn) between W3N2 and W2N2. However, the transpiration rate (Tr) in W3N2 was 13.23% higher than that in W2N2, resulting in higher instantaneous water use efficiency (WUE ins ) under the W2N2 treatment. The diurnal variation in the sap flow rate (SFR) under different water and nitrogen conditions was primarily characterized by a multimodal curve. A quadratic relationship was found between tomato Pn and SFR under different water and nitrogen treatments. In Stage 2, the increase in Pn was accompanied by a greater needed SFR, indicating a decreasing efficiency of photosynthesis in utilizing sap flow. Based on the strength of the correlations between WUE ins , intrinsic water use efficiency (WUE in ), and WUE i and yield, emphasis should be placed on WUE ins during Stage 1 and on WUE i during Stages 2 and 3. The results contribute to improving irrigation and fertilization practices for tomatoes in regions with similar environmental conditions and scientifically promoting transpiration and WUE.