The mechanism to suppress photosynthesis through end-product inhibition in single-rooted soybean leaves during acclimation to CO2 enrichment

Single-rooted soybean leaves were used to investigate the suppression of photosynthesis through end-product inhibition during acclimation to CO2 enrichment. The photosynthetic activity was greater in leaves cultured at a CO2 partial pressure of 70 Pa (high-CO2) than that in the leaves cultured at 35 Pa CO2 (control) during the initial exposure to CO2 enrichment but then decreased rapidly with a large accumulation of starch, to well below the level of the control leaves. The response curve of photosynthesis (A) to the intercellular CO2 concentration (Ci) in the high-CO2 leaves cultured long-term exhibited a significantly low initial gradient. However, on exposure to darkness for 48 h, the initial gradient of the A to Ci curve and rate of photosynthesis were completely restored, and almost all of the accumulated starch was expended. The ribulose bisphosphate carboxylase (RuBPcase) content and activation ratio in the high-CO2 leaves remained high and roughly constant during the experiment, and were unchanged by the exposure, while this enzyme was slightly inactivated or inhibited after long-term exposure to CO, enrichment. The lower rate of photosynthesis in the high-CO2 leaves could be linearly increased to a rate approaching the control level by increasing the external atmospheric [CO2], which thereby compensated for a reduced CO2 transfer diffusion from the intercellular space to the stroma in chloroplasts. It is consequently concluded that, during the acclimation to CO2 enrichment, the suppression of photosynthesis through end-product inhibition was mainly caused by a lowering of the carboxylation efficiency of RuBPcase due to hindrance of CO2 diffusion from the intercellular space to the stroma in chloroplasts brought about by the large accumulation of starch.