Flood-induced Shift of Electron Flow Between Cyanide-sensitive and Alternative Respiratory Pathways in Roots of Tolerant and Intolerant Tree Species1

Effects of cyanide (as KCN) and/or salicylhydroxamic acid (SHAM) on respiratory rates (Q O 2 ) of excised tree roots were evaluated before and after flooding intact plants. CN, an inhibitor of cytochrome oxidase, stimulated Q O 2 in flood-intolerant sugar maple ( Acer saccharum Marsh.) and Japanese yew ( Taxus cuspidata Sieb. and Zucc.), but not in flood-tolerant red maple ( Acer rub rum L.), and inhibited Q O 2 in very flood-tolerant baldcypress ( Taxodium distichum (L.) Rich). Eight days of flooding eliminated CN-stimulation of Q O 2 in sugar maple. SHAM, an inhibitor of the alternative oxidase, had little effect on Q O 2 of any species when used alone, but greatly inhibited Q O 2 of all species when combined with KCN, indicating the presence of substantial amounts of both respiratory pathways in flood-tolerant and intolerant species, as well as the ability of electrons to flow from one pathway to another when one is blocked by a specific inhibitor. Flooding reduced root respiration capacity in all species, affecting intolerant species the most and tolerant the least. The exception was Q O 2 of dormant Taxus roots, which was not inhibited by flooding. CN-sensitive Q O 2 was lost from roots of intolerant species in particular, and to a lesser extent from more tolerant species. A limiting leveL of O 2 (0.5%) suppressed SHAM-sensitive Q O 2 more than CN-sensitive Q O 2 indicating a greater affinity for O 2 by the CN-sensitive path-way. The evidence is consistent with a model for relative flood tolerance based upon differential damage to the aerobic respiratory mechanism.