COMPARATIVE STUDIES ON THE PHOTOSYNTHESIS OF HIGHER PLANTS : IV. Further Studies on the Photosynthetic Sugar Formation Pathway in C_4-Plants

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In this paper, studies were carried out to confirm whether carbon atoms except C-4 of C_4-compounds were involved in the photosynthetic sugar formation in C_4-plants. In feeding of uniformly-labeled malate to maize leaves, sugar formation under aerobic conditions was 3 times as large as that under anaerobic conditions. There was no detectable difference in the amount of activity in the sugar formed from β-carboxyl-labeled malate between aerobic and anaerobic conditions ; however. Under anaerobic conditions, sugar was formed from alanine-1-^14C in maize but not in rice leaves. Sugar formation of this case might have occured by the direct conversion of pyruvate to sugar via PEP and PGA. From these results, we assume that the following three pathways function cooperatively in the photosynthetic sugar formation in C_4-plants. 1) One carbon atom at number 4 in C_4-dicarboxylic acid is transferred to RuDP, resulting in the formation of PGA and this is metabolized into sugar. 2) After transferring C-4 of C_4-dicarboxylic acid, the remaining C_3-compound is introduced into the TCA cycle and completely degradated there, and thus-produced CO_2 is refixed by PEP carboxylase in the mesophyll and metabolized into sugar via the same pathway as in atmospheric CO_2 fixation. 3) The remaining C_3-compound is directly converted to PEP and then to sugar via PGA.
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