Capacity of cytochrome and alternative path in coupled and uncoupled root respiration of Pisum and Plantago

The control of electron flow through the cytochrome and alternative pathways was studied in intact roots of Plantago lanceolata L., P. major L. ssp. major and Pisum sativum L. cv. Rondo, grown either on nitrate or on ammonium. Root respiration in these species was highly resistant to 0.2 mM cyanide and sensitive to 25 mM salicylhydroxamate (SHAM). The uncoupler carbonyl cyanide‐m‐chlorophenyl‐hydrazone (CCCP) stimulated root respiration to the same extent in nitrate‐ and ammonium‐grown plants. In the presence of SHAM, root respiration was not stimulated by CCCP. It is, therefore, suggested that in the presence of SHAM the cytochrome path operates at a rate determined by the Vmax of at least one of its carriers. The sum of the capacities of the alternative and the cytochrome pathways, as determined with 0.2 mM KCN and 25 mM SHAM respectively, was less than the uncoupled rate of root respiration. It is therefore suggested that part of the root oxygen uptake is sensitive to both KCN and SHAM. With increasing age the activity of the alternative path decreased in both Plantago species.