Activation state of Rubisco decreases with the nitrogen accumulation during the reproductive stage in soybean [Glycine max (L.) Merr.]

The CO2 assimilation rate, N content, Rubisco activation state, and protein distribution to Rubisco and Rubisco activase in the leaf were analyzed for four soybean genotypes at two growth stages under field conditions. The CO2 assimilation rate was largely constant except for one genotype during the reproductive stage. Rubisco activation state significantly decreased with the increase of N content during this period in three of four genotypes. When standardized by total soluble protein amount, Rubisco activase amount was constant during this period in three genotypes. The results in the present study showed the deactivation of Rubisco with the N accumulation, which can result in the constant rate of CO2 assimilation during the reproductive stage of soybean. The deactivation of Rubisco was attributed to an unknown mechanism other than to the deficiency of Rubisco activase.