DEPRESSION OF PHOTOSYNTHESIS IN RICE PLANT WITH LOW ROOT ACTIVITY FOLLOWING SOLUBLE STARCH APPLICATION TO THE SOIL

Soluble starch was applied to soil in a pot where rice plant grew at the 10th leaf stage and the initial heading stage. When root activity and leaf bleeding rate were lowered; in the treated plant, photosynthesis:and stomatal conductance were investigated in the morning. Immediately after that initial Rubisco carboxylation activity and Rubisco content were determined to elucidate what was predominantly responsible for the depressed photosynthesis due to low root activity. Accompanied by a decrease in root activity and leaf bleeding rare during 2-9 days after soluble starch application, the photosynthesis and stomatal conductance in the treated plants were all lowered while the initial Rubisco carboxylation activity and Rubisco content in their leaves were approached or even higher than that in the control plant. The photosynthesis was an extremely significant positive curvilinear correlation with the stomatal conductance. These results show that the depression of photosynthesis in the low-root activity plant introduced by soluble starch application is due to lower stomatal conductance in the treated plants and not due to a lower internal CO2 assimilation capacity. The decline of photosynthesis in the treated plants, however, was mitigated, in whole or in part, with prolonging the time after soluble starch application - perhaps through absorbing the additional nitrogen, which was supplied with the soluble starch.