Regulatory Mechanism of Photosynthesis that Depends on the Activation State of Rubisco under Sink Limitation

Knowledge of regulatory mechanism of plant photosynthesis under sink-limitation occurring under various environmental conditions is still insufficient. In the present study, two different degrees of sink-limitation were imposed on soybean plants by removing either half or all developing pods and after that, photosynthetic rate and various other characteristics were investigated in fully expanded source leaves of the depodded and control plants. It was found that a larger degree of pod removal (sink-limitation) resulted in lower leaf photosynthetic rate, stomatal conductance and activation ratio (a percentage of initial activity to total activity) of Rubisco and higher leaf sucrose and starch contents without decreasing leaf intercellular CO2 concentration and affecting leaf water, chlorophyll, total protein and Rubisco protein contents. In addition, there were agreements between activation ratios of Rubisco and photosynthetic rates of the depodded plants relative to controls. These results showed a regulatory mechanism (a down regulation mechanism) of leaf photosynthetic rate in soybean, which depends on the activation state (a decrease in activation ratio) of Rubisco rather than leaf Rubisco content or leaf intercellular CO2 concentration under sink-limitation.