Effects of N Application Rates and Ratios on Photosynthetic Characteristics and Dry Matter Production of Different Rice Varieties.

Nitrogen (N) fertilizer regulates leaf photosynthesis and dry matter production in rice, and extensive genotypic differences in N responsiveness exist among cold-region japonica rice cultivars. A two-year field experiment was conducted in the city of Jiamusi, Heilongjiang Province, during 2023-2024, using two representative japonica cultivars: the small-panicle cultivar Longjing 47 (LJ47) and the large-panicle cultivar Longjing 3010 (LJ3010). A split-plot design was adopted with four N application rates (0, 110, 138, and 166 kg N·ha-1) and four basal-tiller to panicle-grain N ratios (10:0, 8:2, 7:3, and 6:4). The results showed that increasing N supply significantly enhanced SPAD values, photosynthetic capacity, leaf area index, and total dry matter accumulation in both cultivars. LJ47 maintained high photosynthetic capacity under moderate N supply, whereas LJ3010 required higher N input to sustain superior photosynthetic performance. From heading to maturity, dry matter accumulation, photosynthetic potential, crop growth rate, net assimilation rate, N uptake, and grain yield all increased with an increasing ratio of panicle-grain N fertilizer, with more pronounced improvements observed in LJ3010. LJ47 achieved optimal physiological performance and grain yield at 138 kg N·ha-1 (moderate N), with no further promotion under high-N supply. In contrast, LJ3010 required 166 kg N·ha-1 (high-N) to maintain photosynthetic advantages and achieve higher grain yield in the late growth stage. Increasing the proportion of basal-tiller N fertilizer favored the source sink balance of LJ47, while increasing panicle-grain topdressing delayed leaf senescence and promoted post-heading dry matter translocation in LJ3010. For a target grain yield of 9 t·ha-1, the recommended N rate is 138 kg·ha-1 with a basal-tiller to panicle-grain N ratio of 8:2-7:3 for LJ47, and 166 kg·ha-1 with a ratio of 7:3-6:4 for LJ3010. This study provides a theoretical basis and data reference for the formulation of precise N fertilization strategies that match fertilizer management with rice cultivar characteristics in cold regions.