Positional differences in nitrogen and sugar concentrations of upper leaves relate to plant N status in rice under different N rates

This research was undertaken to investigate the spatial differences of protein-N, non-protein-N, total N, soluble sugar and starch concentrations in the four upper fully expanded leaves, and their relationships with plant N concentration in rice (Oryza sativa). Plant and leaf samples were analyzed for concentrations of N and C components from the field experiments with six rice genotypes and 5 N fertilization rates in two growing seasons at Nanjing, China. The results showed that protein-N, non-protein-N and total N concentrations in rice leaves of all genotypes increased significantly with increasing N rates, and they could be all used as indices for diagnosis of plant nitrogen nutrition. Leaves at different positions of all genotypes responded differentially to the increasing N rates. The differences in each chemical component of the leaf tissue were smaller among the first, second and third leaf from the top (1–3 LFT), but greater between the 4 LFT and the 1–3 LFT from the top. The differences between the 4 LFT and the 1–3 LFT in tissue nitrogen of all genotypes decreased with increasing N rates, and could be well characterized by the relative positional difference index (PDI) between the 4 LFT and the 1–3 LFT. The PDI indices were all exponentially related to plant nitrogen concentrations, which could be grouped into two regression curves for Japonica rice and Indica rice, respectively. In particular, the PDI between 4 and 3 LFT could accurately indicate plant nitrogen status of rice genotypes under varied nitrogen supply levels over different growth stages in two growing seasons. It is concluded that the leaf positional difference index in relation to N concentration between the third and fourth leaves should be ideal indicator of plant nitrogen status in rice.