HIGH MITOCHONDRIAL ACTIVITY BUT INCOMPLETE ENGAGEMENT OF THE CYANIDE-RESISTANT ALTERNATIVE PATHWAY IN GUARD-CELL PROTOPLASTS OF PEA

The respiratory properties of guard cell protoplasts (CCP) were examin ed in comparison with those or mesophyll protoplasts (MCP) from the sa me leaves of pea (Pisum sativum L. cv Arkel). The rates of respiratory O-2 uptake by GCP were extremely high (280 mu mol mg(-1) Chl h(-1)) a nd were several times greater than those of MCP. On the other hand, th e rates of photosynthetic O-2 evolution by GCP were similar to those o f MCP. Also on the basis of protoplast volume, the respiratory rates o f GCP were higher: more than three times those of MCP. The enzymes of the tricarboxylic acid cycle, per unit protein or unit protoplast volu me, had a 2- to 5-fold higher activity in GCP than in MCP, indicating an enrichment of mitochondrial activity in GCP relative to that in MCP . Respiratory inhibitors were used to assess the activity of the cytoc hrome (cyanide-sensitive) and alternative (cyanide-resistant) pathways in GCP and MCP. The inhibition of respiration by KCN or antimycin A w as more in GCP than that in MCP. The marked inhibition of respiratory O-2 uptake by salicylhydroxamic acid in the presence of KCN showed the presence of the cyanide-resistant pathway in GCP. The activity of the cyanide-resistant electron transport path constituted only one-third of total respiration in GCP but accounted for two-thirds of respiratio n in MCP. The alternative pathway was not completely engaged in GCP bu t reached its full capacity in MCP.