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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 (P n ) of rice leaves will be of great significance for further increasing leaf P n , 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 K 2 O ha − 1 ) treatments, the P n  was increased by 17.1 and 12.2% with the supply of N and K. Compared with N0K0 (0 kg N and 0 kg K 2 O ha − 1 ), N0K120 (0 kg N and 120 kg K 2 O ha − 1 ) and N0K180 (0 kg N and 180 kg K 2 O ha − 1 ), N supply increased the absolute content of photosynthetic N (N psn ) by 15.1, 15.5 and 10.5% on average, and the storage N (N store ) was increased by 32.7, 64.9 and 72.7% on average. The relative content of N psn  was decreased by 5.6, 12.1 and 14.5%, while that of N store  was increased by 8.7, 27.8 and 33.8%. Supply of K promoted the transformation of N store  to N psn  despite the leaf N content (N a ) was indeed decreased. Compared with N0K0, N180K0 (180 kg N and 0 kg K 2 O ha − 1 ) and N270K0 (270 kg N and 0 kg K 2 O ha − 1 ), K supply increased the relative content of N psn  by 17.7, 8.8 and 7.3%, and decreased the relative content of N store  by 24.2, 11.4 and 8.7% respectively.    Conclusions  This study indicated the mechanism that K supply decreased the N a  but increased the N psn  content and then increased leaf P n  and PNUE from a new viewpoint of leaf N allocation. The supply of K promoted the transformation of N store  to N psn  and increased the PNUE. The decreased N store  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 N psn  content and PNUE.    Electronic supplementary material  The online version of this article (10.1186/s12870-019-1894-8) contains supplementary material, which is available to authorized users.

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