Nitrogen distribution in leaf canopies of high-yielding rice cultivar Takanari

Takanari, an indica cultivar of rice (Oryza sativa L.), is one of the Japanese cultivars with the highest yield. Although previous studies have indicated that Takanari has a greater photosynthetic capacity in canopy top leaves, which is achieved partly by its higher nitrogen (N) content per leaf area, N distribution at the whole-canopy level has not been studied. We addressed following questions: (i) Does Takanari have a higher N uptake rate? (ii) Does Takanari have more leaf N in the whole canopy? (iii) Does Takanari have higher leaf N content per leaf area in the whole canopy? (iv) Does Takanari reallocate more N from old to canopy top leaves? We studied N distribution in stands of Takanari and a popular japonica rice cultivar Koshihikari. Difference in the total plant N and leaf N per plant between Takanari and Koshihikari was small. Whereas Takanari had a lower leaf N per plant N than Koshihikari, Takanari had a steeper vertical gradient of leaf N content than Koshihikari. The slope of vertical distribution of leaf N content (K_N) was positively correlated with the light extinction coefficient (K_L) in each cultivar. Whereas the K_N–K_L relationship of Koshihikari was similar to that of the general trend found in a recent meta-analysis, Takanari had a significantly higher K_N value than others when compared at a given K_L. The K_N values in Takanari were considered to be closer to the optimal K_N than those in other species, which may contribute to higher productivity and higher N use efficiency in canopy photosynthesis.