Partitioning of electrons between the cytochrome and alternative pathways in intact roots

To test the hypothesis that the cytochrome pathway is not invariably saturated when the alternative pathway is engaged, we titrated root respiration of several species with KCN (an inhibitor of the cytochrome pathway), both in the absence and presence of an inhibitor of the alternative pathway (salicylhydroxamic acid, SHAM). The slopes of the resultant KCN ρ plots (ρ_cyt) were then used to determine whether the cytochrome pathway was saturated in each species. The species used were Festuca ovina ssp. ovina L., Phaseolus vulgaris I., and six Poa species (Poa pratensis I., Poa compressa L., Poa trivialis L., Poa alpina L., Poa costiniana Vick., and Poa fawcettiae Vick.). Although the cytochrome pathway was saturated in a number of species (i.e. ρ_cyt values were 1.0), several others exhibited ρ_cyt values of less than 0.5. Alternative pathway capacity correlated negatively with ρ_cyt, with ρ_cyt, values of less than 1.0 occurring in tissues in which the alternative pathway capacity was greater than 25 to 30% of total respiration. The species that did not show full engagement of the cytochrome pathway rarely exhibited SHAM inhibition in the absence of KCN. We conclude that this lack of SHAM inhibition is not due to a lack of alternative pathway engagement but rather to the diversion of electrons from the alternative pathway to the unsaturated cytochrome path following the addition of SHAM.