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Single-rooted soybean leaves were used to investigate the suppression of photosynthesis through end-product inhibition during acclimation to CO(2 )enrichment. The photosynthetic activity was greater in leaves cultured at a CO(2) partial pressure of 70 Pa (high-CO(2)) than that in the leaves cultured at 35 Pa CO(2) (control) during the initial exposure to CO(2) 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 CO(2) concentration (Ci) in the high-CO(2) 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-CO(2) 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(2) enrichment. The lower rate of photosynthesis in the high-CO(2) leaves could be linearly increased to a rate approaching the control level by increasing the external atmospheric [CO(2)], which thereby compensated for a reduced CO(2) transfer diffusion from the intercellular space to the stroma in chloroplasts. It is consequently concluded that, during the acclimation to CO(2 )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 CO(2) diffusion from the intercellular space to the stroma in chloroplasts brought about by the large accumulation of starch.

The Mechanism to Suppress Photosynthesis Through End-Product Inhibition in Single-Rooted Soybean Leaves during Acclimation to CO2 Enrichment