Changes in photosynthetic limitation of soybean under different water nitrogen interactions and their relationships with leaf water use efficiency

The single factor effects of soil water and nitrogen on plant photosynthesis limitation (mesophyll limitation, l_m; stomatal limitation, l_sc; biochemical limitation, l_b) have been extensively studied, but their response to water nitrogen interaction and the relationship with leaf water use efficiency (WUE) have been rarely reported. Dryland soybeans in northern China is selected as experimental materials and photosynthesis limitation is taken as the objects in this study. A soil water-nitrogen interaction experiment is designed to explore the interaction of photosynthetic limitation and its correlation with leaf instantaneous (WUE_ins) and intrinsic water use efficiency (WUE_int). The results showed that (1) The CO₂ diffusion limitation (i.e., l_m and l_sc) gradually increased with water stress, while l_b gradually decreased. (2) The interaction of water and nitrogen resulted in a decrease in l_m and l_sc, while l_b significantly increased. Among them, a fluctuating pattern of first decreasing and then increasing and first increasing and then decreasing with nitrogen addition were found in l_m and l_b of MW plants, respectively, while l_sc decreased overall. The l_m of SW plants increased while its l_b decreased with nitrogen addition, while l_sc did not show any statistical difference between treatments. (3) l_m and l_b showed significant negative and positive correlations with WUE_ins and WUE_int, respectively (P < 0.05), while l_sc showed no significant correlation with the two. (4) Compared to l_sc and l_b, l_m makes a greatest contribution to soybean photosynthetic carbon assimilation and leaf water use efficiency, and is the dominant limiting factor for the photosynthetic and water use capacity.