Regulatory effect of phosphorus rate on the photosynthetic efficiency and P/Fe ratio in soybean

Objectives This study investigated the effects of different P supply levels on physiological traits and seed yield of soybean genotypes with different P efficiencies.

Methods A hydroponic experiment was carried out, using six P-efficient and six P-inefficient genotypes of soybean as test materials. Based on the Hoagland nutrient solution, the P concentration was adjusted into four levels: 0 (CK), 100, 500 and 1000 µmol/L. The photosynthetic characteristics, P/Fe ratio in soybean leaves were determined at early flowering and early podding stages. The seed weight and contents of P and Fe were measured at seed filling stage (R8). Canonical correlation analysis between photosynthetic characteristics and P/Fe ratio was carried out, and stepwise regression equation was established by seed weight per plant, then pathway analysis was performed.

Results Whether in P-efficient or inefficient genotypes, 0 µmol/L of P resulted in a dramatic increase (P < 0.01) in Fo value at early flowering stage; 100 µmol/L P supply treatment significantly improves actual photochemical efficiency of PSⅡ (Φ_PSⅡ) at early flowering stage, which improved the light energy conversion rate, and under this treatment, seed weights per plant of both P-efficient and P-inefficient genotypes were higher. Compared with P 100 μmol/L treatment, both Fe concentration and Fe/P ratio of leaf at early podding stage exhibited a decreasing trend with increasing P supply level. Compared with treatment of P 100 µmol/L, the seed weight per plant of P-efficient genotypes did not increase significantly at P supply of 500 and 1000 µmol/L, but seed weight per plant of P-inefficient genotypes decreased. In addition, SPAD values at early podding attained peaks under P 100 μmol/L treatment, and the SPAD value at seed filling stage stage had a positive correlation with seed weight per plant of both P-efficient and inefficient genotypes.

Conclusions P supply level affects photosynthetic efficiency as well as P/Fe ratio in soybean leaves. Normal P supply level (100 μmol/L) could regulate the chlorophyll content at the early flowering, early podding and seed filling stage, and then regulate the metabolic process, which is beneficial to grain formation in both P-efficient and P-inefficient genotypes. High P supply does not result in substantial increase in seed weight, but may result in a negative effect on seed weight.