Ribulosebisphosphate carboxylase activity and photosynthetic o(2) evolution rate in vicia guard-cell protoplasts

Activities of ribulose-1,5-bisphosphate carboxylase and rates of photosynthetic O(2) evolution were measured in guard-cell and mesophyll protoplasts from Vicia faba. The ribulose-1,5-bisphosphate carboxylase activity of guard-cell protoplasts was 30% of that of mesophyll protoplasts; however, the O(2) evolution rate was 3 times higher in guard-cell protoplasts than in mesophyll protoplasts on a chlorophyll basis. When the dark-adapted, guard-cell protoplasts were illuminated by red light, O(2) was evolved with an induction period, which became shorter when the protoplasts were reilluminated. High activity of irreversible NADP-glyceraldehyde-3-phosphate dehyrogenase was found in guard-cell protoplasts. Several lines of evidence revealed that there was virtually no contamination by mesophyll cells in guard-cell preparations. These results indicate that guard cells fix CO(2) photosynthetically and imply that the cells utilize a considerable proportion of reducing equivalents from water for reactions other than CO(2) fixation.