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 (GCP) were examined in comparison with those of mesophyll protoplasts (MCP) from the same leaves of pea (Pisum sativum L. cv Arkel). The rates of respiratory O2 uptake by GCP were extremely high (280 [mu]mol mg-1 Chl h-1) and were several times greater than those of MCP. On the other hand, the rates of photosynthetic O2 evolution by GCP were similar to those of MCP. Also on the basis of protoplast volume, the respiratory rates of GCP were higher: more than three times those of MCP. The enzymes of the tricarboxylic acid cycle, per unit protein or unit protoplast volume, 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 cytochrome (cyanide-sensitive) and alternative (cyanide-resistant) pathways in GCP and MCP. The inhibition of respiration by KCN or antimycin A was more in GCP than that in MCP. The marked inhibition of respiratory O2 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 respiration in MCP. The alternative pathway was not completely engaged in GCP but reached its full capacity in MCP.