Ammonium nutrition improves the yield of drip-irrigated rice by prolonging leaf photosynthetic capacity and enhancing source‒sink performance

The effects of the application of different nitrogen (N) forms via drip-irrigation on the physiological photosynthetic characteristics of rice and their contributions to yield remain incompletely understood. To address this knowledge gap, a field drip-irrigation experiment was conducted from 2020 to 2022 using a high-NUE cultivar (T-43) and a low-NUE cultivar (LX-3). Using a no-nitrogen treatment (CK) as the control, the effects of three nitrogen fertilizer treatments—urea (Urea), ammonium (NH₄⁺), and nitrate (NO₃⁻)—were compared. The main results are as follows. (1) The application of nitrogen significantly increased the biomass (26.74%–30.71%) and yield (40.35%–84.91%). Compared with Urea and NO₃⁻, NH₄⁺ increased the chlorophyll content (Chl) and N metabolic enzyme activity, decreased the abscisic acid (ABA) content, and improved stomatal conductance (Gs). (2) T-43 presented relatively high levels of indole-3-acetic acid (IAA), Chl a + b, and Gs, and a relatively high photosynthetic rate (Pn), whereas LX-3 presented increased ABA levels and reduced Chl a + b, and Gs. (3) Partial least squares path model revealed that under NH₄⁺, IAA was positively correlated with Chl b, Gs, and Fv/Fm, whereas ABA was negatively correlated with Y(II), and Chl a + b. Conversely, the Chl contents decreased, and the ABA and Chl a/b increased in the Urea and NO₃⁻ treatments, thereby increasing the contribution of NH₄⁺ (GoF = 0.714) to yield when compared to the yields in the Urea (0.614) and NO₃⁻ (0.594). In summary, NH₄⁺ synergistically enhances Chl content, photochemical efficiency, antioxidant capacity, and regulates hormone balance while increasing source–sink dry matter accumulation and distribution, thereby achieving efficient production of drip-irrigated rice in arid areas.