Influence of Mineral Phosphorus Fertilization on Assimilation and Selected Physiological Parameters in Soybean in the Transylvanian Plain

Abstract

Mineral phosphorus fertilization is an effective method for correcting nutrient deficiencies and enhancing the controlled uptake of nutrients by plants, including their translocation from leaves to seeds. In an experiment conducted at SCDA Turda in 2023, the effect of phosphorus fertilization on net CO₂ assimilation rate (A, μmol CO₂ m⁻² s⁻¹), stomatal conductance (gs, mmol H₂O m⁻² s⁻¹), leaf transpiration rate (E, mmol H₂O m⁻² s⁻¹), and leaf vapor pressure deficit (VPD, kPa) in soybean was evaluated during the beginning of seed filling (R5 growth stage). The experiment included four phosphorus fertilization levels (b₂ – 40 kg/ha, b₃ – 80 kg/ha, b₄ – 120 kg/ha, b₅ – 160 kg/ha) and an unfertilized control (b₁), under variable temperature conditions grouped into three intervals (22–24°C, 25–27°C, and 28–30°C). Results showed that phosphorus fertilization significantly improved net CO₂ assimilation rate and leaf transpiration rate, enhancing key physiological processes such as photosynthesis and stomatal regulation. In the moderate temperature range (22–27°C), fertilization contributed to a reduction in leaf vapor pressure deficit and promoted more efficient use of water and nutrients. At higher temperatures (28–30°C), high phosphorus doses (160 kg/ha) led to a decrease in photosynthetic efficiency (VPD) and stomatal activity (gs), indicating greater water stress. These findings highlight the importance of phosphorus fertilization and environmental management practices, which are crucial for optimizing plant physiological processes—particularly in the context of climate change, where efficient water and nutrient management becomes a priority.