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Metabolism and Enzymology

Red Light-Dependent CO₂ Uptake and Oxygen Evolution in Guard Cell Protoplasts of Vicia faba L.: Evidence for Photosynthetic CO₂ Fixation ¹

Department of Biological Sciences, Stanford University, Stanford, California 94305

Suspensions of dark-adapted guard cell protoplasts of Vicia faba L. alkalinized their medium in response to irradiation with red light. The alkalinization peaked within about 50 minutes and reached steady state shortly thereafter. Simultaneous measurements of O₂ concentrations and medium pH showed that oxygen evolved in parallel with the red light-induced alkalinization. When the protoplasts were returned to darkness, they acidified their medium and consumed oxygen. Both oxygen evolution and medium alkalinization were inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). In photosynthetically competent preparations, light-dependent medium alkalinization is diagnostic for photosynthetic carbon fixation, indicating that guard cell chloroplasts have that capacity. The striking contrast between the responses of guard cell protoplasts to red light, which induces alkalinization, and that to blue light, which activates proton extrusion, suggests that proton pumping and photosynthesis in guard cells are regulated by light quality.

³ Present address: Biological Sciences, 273 Applied Sciences Bldg., University of California, Santa Cruz, CA 95064.

¹ Supported by the National Science Foundation grant PCM 82-14378, the Department of Energy grant DE-AT03-84ER10924, and the Department of Agriculture grant 84-CRSR-2-2495.

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