Phosphorus deficiency limits photosynthetic induction response to light in soybeans under strip compound planting systems

Objective When intercropping with maize (Zea mays L.), the low-tier crop soybean (Glycine max (L.) Merr.) would suffer yield limitations due to fluctuating light conditions. As phosphorous (P) involves in various physiological processions, we studied the effect of P nutrition on photosynthesis during light induction, trying to enhance soybean photosynthetic efficiency and production potential through P nutrient management.

Method Field experiments were conducted in Sichuan Agriculture University, the cropping system was maize and soybean intercropping. P fertilizer application (CK) and no application (-P) on soybean was set up as the treatments. At branching stage of soybean, leaf gas exchange parameters, P content, and the relevant physiological parameters were measured under both steady-state light and light-induced processes. The parameters included net photosynthesis rate (A), stomatal conductance (g_s), intercellular CO₂ concentration (C_i), transpiration rate (T_r), and electron transfer rate (J), as well as P content and photosynthetic pigment levels. Additionally, proportions of stomatal and biochemical photosynthetic limitation factors were calculated.

Results Compared with CK, -P significantly inhibited the growth of soybean, the grain yield and plant dry weight at maturing stage were reduced by 24.5% and 23.9% respectively. -P significantly reduced the total and inorganic P contents of leaves at the branching stage, but did not affect the contents of chlorophyll a, chlorophyll b, and carotenoids. Under steady-state saturated light, -P decreased the leaf A and T_r by 30.9% and 39.5%, but did not impact the g_s, resulting in a significant increase in C_i; -P decreased the stomatal limit value of soybean leaves by 16.7%, implying that non-stomatal factors are the dominant factor causing the decrease in A under steady-state saturated illumination. During the induction period transitioning from low to high light, the induction rates of A and g_s in CK were faster than those in -P treatment, while the induction rate of J showed no significant difference between the two treatments. Photosynthetic limitation analysis revealed no significant difference in cumulative stomatal limitation and biochemical limitation during light induction under CK treatment. However, cumulative stomatal limitation reached 65.7% under -P condition, significantly higher than biochemical limitation. It shows that stomata are the main factor limiting its photosynthetic induction rate under phosphorus deficiency conditions.

Conclusions Phosphorus deficiency mainly limits the response of stomatal conductance of soybean leaves to light induction under strip compound planting, resulting in a decrease in photosynthetic efficiency and yield. Therefore, reasonable phosphorus fertilizer application is of great significance to improve the photosynthetic efficiency of soybeans in soybean-maize strip compound planting under fluctuating light environment and reduce yield loss.