Plant growth based on interrelation between carbon and nitrogen translocation from leaves

In individual leaves, the photon-saturated photosynthetic activity (P-sat, expressed on a dry mass basis) was closely related to the nitrogen content (Nc) as follows: P-sat = Cf Nc + P-sat0, where Cf and P-sat0 are constants. On a whole plant basis, the relative growth rate (RGR) was closely related to Nc in canopy leaf as follows: RGR = DMf Nc + RGR(0), where DMf and RGR(0) are constants. However, the coefficients Cf and DMf were markedly different among plant species. To explain these differences, it is suggested that carbon assimilation (or dry matter production) is controlled by both the Nc in a leaf (or leaves) and by the net N translocation from leaves. This is supported by the finding that P-sat is related to the rate of S-35-methionine translocation from leaves. We propose another estimation method for the net N translocation rate (NFR) from leaves: Ne, after full leafing, is expressed as a function of time: Nc = (Nc(0) - Ncd) exp(-Nft) + Ncd, where Nf is a coefficient, t is the number of days after leaf emergence, Nc(0) is the initial value of Nc, and Ncd is the Nc of the dead leaf. The NFR is then calculated as NFR = Delta Nc/Deltat = -Nf (Nc - Ncd). Thus Nf is the coefficient for the NFR per unit Nc. NFR is a good indicator of net N translocation from leaves because NFR is closely related to the rate of S-35-methionine translocation from leaves. Since P-sat is related to the C-14-photosynthate translocation rate, Cf (or DMf) corresponds to the coefficient of saccharide translocation rate per unit amount of Nc. Cf (or DMf) is closely related to the Nf of individual leaves (or the Nf of canopy leaf). This indicates that C assimilation and C translocation from leaves are related to Nc. and N translocation from leaves (net translocation of N). Cf and Nf are negatively correlated with leaf longevity, which is important because a high or low CO2 assimilation rate in leaves is accompanied by a correspondingly high or low N translocation in leaf, and the degree of N translocation in leaves decreases or increases leaf longevity. Thus, since a relatively high P-sat (or RGR) is accompanied by a rapid Nc decrease in leaves, it is difficult to maintain a high P-sat (or RGR) for a sustained time period.