Interactive effects of nitrogen and potassium on photosynthesis and photosynthetic nitrogen allocation of rice leaves

BACKGROUND: Nitrogen (N) and potassium (K) are two important mineral nutrients in regulating leaf photosynthesis. Studying the interactive effects of N and K on regulating N allocation and photosynthesis (Pₙ) of rice leaves will be of great significance for further increasing leaf Pₙ, photosynthetic N use efficiency (PNUE) and grain yield. We measured the gas exchange of rice leaves in a field experiment and tested different kinds of leaf N based on N morphology and function, and calculated the interactive effects of N and K on N allocation and the PNUE. RESULTS: Compared with N0 (0 kg N ha⁻ ¹) and K0 (0 kg K₂O ha⁻ ¹) treatments, the Pₙ was increased by 17.1 and 12.2% with the supply of N and K. Compared with N0K0 (0 kg N and 0 kg K₂O ha⁻ ¹), N0K120 (0 kg N and 120 kg K₂O ha⁻ ¹) and N0K180 (0 kg N and 180 kg K₂O ha⁻ ¹), N supply increased the absolute content of photosynthetic N (Nₚₛₙ) by 15.1, 15.5 and 10.5% on average, and the storage N (Nₛₜₒᵣₑ) was increased by 32.7, 64.9 and 72.7% on average. The relative content of Nₚₛₙ was decreased by 5.6, 12.1 and 14.5%, while that of Nₛₜₒᵣₑ was increased by 8.7, 27.8 and 33.8%. Supply of K promoted the transformation of Nₛₜₒᵣₑ to Nₚₛₙ despite the leaf N content (Nₐ) was indeed decreased. Compared with N0K0, N180K0 (180 kg N and 0 kg K₂O ha⁻ ¹) and N270K0 (270 kg N and 0 kg K₂O ha⁻ ¹), K supply increased the relative content of Nₚₛₙ by 17.7, 8.8 and 7.3%, and decreased the relative content of Nₛₜₒᵣₑ by 24.2, 11.4 and 8.7% respectively. CONCLUSIONS: This study indicated the mechanism that K supply decreased the Nₐ but increased the Nₚₛₙ content and then increased leaf Pₙ and PNUE from a new viewpoint of leaf N allocation. The supply of K promoted the transformation of Nₛₜₒᵣₑ to Nₚₛₙ and increased the PNUE. The decreased Nₛₜₒᵣₑ mainly resulted from the decrease of non-protein N. Combined use of N and K could optimize leaf N allocation and maintain a high leaf Nₚₛₙ content and PNUE.