Phosphate fertilizers increase CO2 assimilation and yield of soybean plants in a shaded environment

To study how phosphate fertilizer regulates the photosynthetic of summer soybeans in low light environment, this study used Qihuang 34 as the test variety, two light treatments[ambient sunlight (L1) during the whole growth period and 40% shade (L2) from 40 days after sowing to 24 days after flowering] with two phosphate fertilizer treatments including non-phosphate fertilizer application (P0), conventional phosphate fertilizer application (P1) in each light treatment was set up to measure the gas exchange, chlorophyll a fluorescence differences of photosynthetic performance as well as the yield and its components in Qihuang 34. The results showed that phosphorus significantly increased chlorophyll content, photosynthetic rate, and grain yield in shading treatment. The yield of soybean was significantly reduced after shading treatment, which was 29.9%, but compared with L2P0, L2P1 treatment yield increased by 36.7%, which was significantly higher than the 27.1% under L1 treatment. The photosynthetic rate under saturated light (Asat) under L2 treatment decreased by 31.5% and 15.6% in R1 and R3 periods, respectively, and average increased by 27.4% and 45% after phosphorus application under two light condition. Furthermore, photosynthetic measurements demonstrated a more significant increase in the maximum efficiency of CO2 fixation, maximum rates of RUBP-carboxylation (Vcmax) in phosphorous treatment under shading conditions. Moreover, further analysis of OJIP curves revealed that phosphate fertilizer significantly improved the electron transfer and light energy absorption performance of the PSII reaction center in shading. The above results demonstrate that phosphate fertilizer contributes to light energy utilization efficiency, increase the low light utilization efficiency by improving PSII performance, promote RUBP regeneration, ensure the source of carboxylates substrates, and coordinate the balance of the photochemical reaction and the Calvin cycle, then alleviate the reduction of photosynthetic efficiency under shading, increase the rate of dry matter accumulation, and greatly increase yield.