EFFECT OF MUTUAL SHADING ON CARBON DISTRIBUTION IN RICE PLANT

The C-14 distribution among organs of rice plant at the ripening stage was determined under different levels of light intensity and mutual shading. The results obtained were as follows. 1) By shading (63%), dry matter accumulation was remarkably affected, whereas the C-14 distribution ratio to shoot, roots, ears, and for respiration was affected slightly by the shading treatment after (CO2)-C-14 feeding, but not by the shading treatment before (CO2)-C-14, feeding. By prolonged mutual shading in the artificial canopy, dry matter accumulation decreased, but the C-14 distribution ratio to each organ and for respiration was affected slightly regardless of growth stages since plants had became adapted to mutual shading for a long period of time. These results indicate that the C-14 distribution pattern was basically unaffected by the light intensity. 2) When (CO2)-C-14, was assimilated in a fully expanded leaf of a tropical rice cultivar Peta under the condition that leaves at a lower position below the fully expanded leaf were completely shaded by aluminum foil, C-14 compounds from the fully expanded leaf translocated slowly into other organs, but a large amount of C-14 compounds from the fully expanded leaf was lost by respiration. When (CO2)-C-14 was assimilated to the flag leaf of temperate rice plants at the late maturation stage, and the side of the plant was shaded at 50% with a cloth, a large amount of C-14 compounds was consumed by respiration in the C-14-fed leaf, but the C-14 compounds were not translocated from the upper leaf to the lower leaves. Thus, it is considered that the lower leaves did not consume the carbohydrates which were assimilated in upper leaves regardless of growth conditions, and that the respiratory activity of the fully light-exposed upper leaf increased due to the imbalance of source ability. 3) By mutual shading, the photosynthetic activity of the lower leaf decreased, and the distribution ratio of the C-14 compounds from leaves in the upper position to organs in a lower position also decreased. Therefore, it was assumed that the root activity decreased by mutual shading, which may account for the retardation of plant growth.