Metabolism of Corn Roots in Malonate

The rate of O(2) uptake by sub-apical corn roots is largely resistant to 0.1 m Na-malonate at pH 5.0. The resistance of this tissue, in which the tricarboxylic acid cycle is very active, is not due to the compensatory induction of another oxidative pathway as seems to be the case in fresh potato slices. In corn roots malonate inhibits succinate utilization as expected and the smallness of the effect on O(2) uptake is due to the utilization of endogenous (cytoplasmic) malate as acetyl acceptor and its conversion to succinate. Malonate uptake stops after 2 to 3 hours when only a fraction (roughly 20%) of the root volume has equilibrated with external malonate. After this time the accumulated succinate is apparently able to overcome the malonate block, the ability to oxidize acetate to CO(2) is largely regained and O(2) uptake is maintained at about 80% of the control level.Malonate sensitivity at high external concentrations of malonate and conditions appropriate for its uptake is therefore fully expressed only under conditions where the production or availability of extra-mitochondrial malate (or perhaps other precursors of oxaloacetate) is at a minimum.