Two energy-dissipating systems, an alternative oxidase and an uncoupling protein, are known to exist in plant mitochondria. In tomato fruit mitochondria linoleic acid, a substrate for the uncoupling protein, inhibited the alternative oxidase-sustained respiration and decreased the ADP/O ratio to the same value regardless of the level of alternative oxidase activity. Experiments with varying concentrations of linoleic acid have shown that inhibition of the alternative oxidase is more sensitive to the linoleic acid concentration than the uncoupling protein activation. It can be proposed that these dissipating systems work sequentially during the life of the plant cell, since a high level of free fatty acid-induced uncoupling protein activity excludes alternative oxidase activity.433237-40