Photosynthetic Carbon Metabolism in Isolated Maize Bundle Sheath Strands 12

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Photosynthetically active bundle sheath strands capable of assimilating up to 8 micromoles CO2 per milligram chlorophyll per hour have been isolated from fully expanded leaves of Zea mays L. Mesophyll cell contamination of the preparations was negligible, as evidenced by light and electron microscopy and by a high ratio of chlorophyll a to chlorophyll b in the strands. Ribose 5-phosphate markedly stimulated the rate of photosynthetic 14CO2 fixation by the isolated strands. In contrast, both pyruvate and phosphoenolpyruvate had a comparatively small stimulatory effect on bundle sheath 14CO2 fixation. After 5 minutes of photosynthesis in 14C-bicarbonate, 95% of the incorporated 14C was found in compounds other than C4-dicarboxylic acids, most notably in 3-phosphoglycerate and sugar phosphates. A similar distribution of 14C was observed in the presence of exogenous ribose 5-phosphate. Extracts of bundle sheath strands contained high specific activities of “malic” enzyme, phosphoglycolate phosphatase, hydroxypyruvate reductase, and ribulose 1,5-diphosphate carboxylase, whereas the specific activities of NADP+-malate dehydrogenase and phosphopyruvate carboxylase were extremely low. These results indicate that the Calvin cycle occurs in the bundle sheath cells of maize.

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