Reduced irrigation decreases the optimal nitrogen threshold of greenhouse tomatoes by altering gas exchange and stomatal morphology

Abstract

In arid regions, agricultural water shortages and excessive nitrogen fertilization severely restrict the sustainable development of the tomato industry. To elucidate the effects of water-nitrogen interactions on the yield and photosynthetic physiological characteristics of tomatoes, a two-year (2024 and 2025) split-plot greenhouse experiment was conducted. The main plots evaluated two irrigation lower limits at 75% (W1) and 65% (W2) of field capacity (FC), and the subplots consisted of four nitrogen application rates: 0, 150, 300, and 450 kg ha-1 (N0, N1, N2, and N3). The results indicated that: (1) The responses of fruit yield, aboveground biomass, and water productivity (WP) to the nitrogen application rate all exhibited quadratic parabolic trends. Under W2, the N1 and N2 eliminated the yield discrepancy with W1, and increased WP and agronomic nitrogen use efficiency by 9.9%–11.7% and 30.4%–35.1%, respectively. Compared to N0, the N1 and N2 increased fruit yield by 33.3%–34.9% and 52.8%–53.1%, respectively. (2) W2 caused the optimal nitrogen thresholds to shift toward lower nitrogen levels. The optimal nitrogen rates for fruit yield and WP under W1 were 344.2 and 346.1 kg ha-1, respectively, which decreased to 279.9 and 264.8 kg ha-1 under reduced irrigation (W2). Concurrently, the optimal nitrogen rate for net photosynthetic rate (Pn) declined from 394.6 kg ha-1 under full irrigation to 300.1 kg ha-1. (3) A random forest model demonstrated that the SPAD value, intrinsic water use efficiency, Rubisco activity, and stomatal width were the core factors determining Pn. Moderate nitrogen application (W2N2) induced a “high-density and small-aperture” stomatal morphological trait and significantly enhanced Rubisco activity. (4) Under W2, the N3 led to a significant accumulation of malondialdehyde concentration, accompanied by marked decreases in the SPAD value and Rubisco activity, ultimately causing a decline in Pn and a significant yield reduction. In conclusion, setting the irrigation lower limit to 65%FC combined with a nitrogen rate of 265–280 kg ha-1 constitutes a precision management strategy for synergistically achieving water conservation, stable yields, and high water-fertilizer use efficiency in greenhouse tomatoes.