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Photosynthetic Carbon Metabolism in Isolated Maize Bundle Sheath Strands ^1,2

William L. Ogren^b

^a Department of Agronomy, University of Illinois, Urbana, Illinois 61801, ^b United States Regional Soybean Laboratory, North Central Region, Agricultural Research Service, United States Department of Agriculture, Urbana, Illinois 61801

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.

¹ This study was supported in part by Agricultural Research Service, United States Department of Agriculture Cooperative Agreement No. 12-14-100-11,159(34), administered by the Agricultural Research Service, Beltsville, Md.

² Publication No. 763 of the United States Regional Soybean Laboratory, Urbana, Ill.

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