Accumulation of ribulose-1,5-bisphosphate carboxylase/oxygenase in potato plants and nitrogen distribution in each organ during growth in relation to productivity

To analyze the effect of nitrogen application at the tuber swelling stage, 15N assimilated at various growth stages was traced during growth, and the amounts of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and chlorophyll were determined during growth. The results obtained were as follows. 1. 15N assimilated after the lowering stage was immediately translocated to the tubers.2. The percentage of soluble 15N, which consisted mainly of free amino acids or amides, to total 15N at 7 d after 15N assimilation was constant in leaves regardless of the growth stages, and increased with growth in stems and tubers, indicating that the rate of protein synthesis in stems and tubers was decreased with growth.3. Based of field experiments on the effect of the topdressing of nitrogen at the flowering stage, it was observed that the amount of Rubisco-N increased, while the crop growth rate per Rubisco-N unit decreased, and the photosynthetic function of leaves was depressed by nitrogen application after the flowering stage.4. The percentage of Rubisco-N to total leaf nitrogen and the ratio of Rubisco-N to chlorophyll decreased when nitrogen was applied after the flowering stage. Thus a large proportion of nitrogen absorbed after the flowering stage was not incorporated effectively into the Rubisco-proteins, but incorporated into proteins other than Rubisco-protein and chlorophyll. Thus, it is considered that nitrogen applied after the flowering stage (tuber swelling stage) had no effect on the increase of the tuber yield because nitrogen application did not enable to stimulate the Rubisco accumulation and nitrogen was distributed to the tubers for storage, but not to the leaves for maintaining the physiological function of the leaves.