Relationship between Respiration and Photosynthesis in Guard Cell and Mesophyll Cell Protoplasts of Commelina communis L

A mass spectrometric method combining ¹⁶O/¹⁸O and ¹²C/¹³C isotopes was used to quantify the unidirectional fluxes of O₂ and CO₂ during a dark to light transition for guard cell protoplasts and mesophyll cell protoplasts of Commelina communis L. In darkness, O₂ uptake and CO₂ evolution were similar on a protein basis. Under light, guard cell protoplasts evolved O₂ (61 micromoles of O₂ per milligram of chlorophyll per hour) almost at the same rate as mesophyll cell protoplasts (73 micromoles of O₂ per milligram of chlorophyll per hour). However, carbon assimilation was totally different. In contrast with mesophyll cell protoplasts, guard cell protoplasts were able to fix CO₂ in darkness at a rate of 27 micromoles of CO₂ per milligram of chlorophyll per hour, which was increased by 50% in light. At the onset of light, a delay observed for guard cell protoplasts between O₂ evolution and CO₂ fixation and a time lag before the rate of saturation suggested a carbon metabolism based on phosphoenolpyruvate carboxylase activity. Under light, CO₂ evolution by guard cell protoplasts was sharply decreased (37%), while O₂ uptake was slowly inhibited (14%). A control of mitochondrial activity by guard cell chloroplasts under light via redox equivalents and ATP transfer in the cytosol is discussed. From this study on protoplasts, we conclude that the energy produced at the chloroplast level under light is not totally used for CO₂ assimilation and may be dissipated for other purposes such as ion uptake.