The rate of O2uptake was about 29 times higher in guard cell protoplasts (GCPs) than in mesophyll protoplasts (MGPs) on a Chi basis. The O2uptake was inhibited by respiratory inhibitors, but stimulated by respiratory uncouplers. On a Chi basis, the activities of Cytcoxidase and NADH-Cytcreductase, mitochondrial enzymes, were about 27 and 35 times higher in GCPs than in MCPs. On a Chi basis, the ATP content was about 9 times higher in GCPs. The amount of ATP in GCPs was decreased by respiratory inhibitors, an energy transfer inhibitor, and uncouplers of oxidative phosphorylation. On a volume basis, GCPs had 8- to 10-fold higher respiratory activities than MCPs, but had a low Chi content and lacked the activity of NADP-glyceraldehyde-3-phosphate dehydrogenase (NADP-GAPD), the Calvin cycle enzyme. From these results, we concluded that oxidative phosphorylation plays a main role in ATP production in guard cells and that guard cells have a heterotrophic feature. Salicylhydroxamic acid (SHAM) in combination with KCN or NaN3strongly inhibited O2uptake, indicating the presence of cyanide-resistant respiration in guard cells. Phenylmercuric acetate (PMA), a potent inhibitor of stomatal opening, reduced the ATP content of GCPs by about 90, whereas it had a relatively small effect on the ATP level of MCPs. The specific effect of PMA on GCPs is discussed.
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