Ecological strategy for regulating carbon and nitrogen balance in plants.

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Ontogenetic changes of photosynthesis (Pnet) of individual leaf were examined in relation with nitrogen content (Nc, mg g^<-1>) in individual leaf. During the lag phase of NC at the leaf expanding stage, CFR (net carbon flux rate) related negatively to Nc. since Pnet relates to photosynthates translocation from leaf (CFR,which was estimated by using isotope technique), it was assumed that CFR could be an indicator of net carbon efflux rate from leaf. After the lag phase of Nc, as Pnet related positively to Nc, CFR could be ascribed as follows ; CFR=CfxNc+CFR_0, where Cf and CFR_0 are coefficients. Therefore, Cf is coefficient of carbohydrate translocation rate per unit Nc. Nc is also expressed as a function of time as follows ; Nc= (Nc_0-Ncd) x exp (-Nf x t) +Ncd, where Nf is noefficient, t is days after leaf emergence, Nc_0 is the Cn of leaf at emergence, and Ncd is the Nc of dead leaf. Then net nitrogen exchange rate (NFR) is calculated as NFR=DELTANc/DELTAt=-Nfx (Nc-Ncd). Therefore, Nf is coefficient for nitrogen translocation rate per unit Nc.Thus, it is hypothesized that Cf and Nf are indicators for carbon and nitrogen efflux from individual leaf (carbon and nitrogen efflux rate per unit Nc), respectively. Cf (or DMf) increased exponentially with the increase of Nf, indicating that the net efflux of carbon and nitrogen interrelate closely. Accordingly, since Nf indicates net nitrogen efflux rate per unit Nc, CFR is regulated not only by Nc, but also by net nitrogen efflux (Nf) from individual leaf.