AbstractSome dihydrochalcones and related flavonoids were tested Cor their effects upon indoleacetic acid oxidase (mainly from pea roots) and upon oxidative phosphorylation in mitochondria from cucumber hypocotyls. Also their influence upon the growth of wheat roots and upon the absorption of14C‐labelled sugars and 2, 4‐dichloro‐phenoxyacetic acid in wlieat roots was studied.The dihydrochalcone glucoside phloridzin known for its strong inhibitory effects upon sugar transport in some animal tissues gives no corresponding specific effects in higher plants. Phloridzin and some related dihvdrochalcone glycosides as well as the structurally similar compounds naringenin and 2′, 4, 4′‐trihydroxychalcone are potent stimulators of indoleacetic acid oxidase. Sieboldin, which compared to phloridzin has an additional hydroxyl group (giving an ortho‐diphenolic substitution in the B‐ring) is a strong inhibitor of indoleacetic acid oxidase (10−6Mgives about 50 per cent Inhibition). Phloridzin and sieboldin replace each other in different groups of species in the genusMalus.The aglycones phloretin, naringenin and 2′, 4, 4′‐trihydroxychalcone stimulate wheat root growth (especially in the presence of auxins or growth‐inhibilory sugars) and inhibit the absorption of sugars and of 2, 4‐dichlorophenoxyacetic acid. They also inhibit oxidative phosphorylalion and give a distinct uncoupling effect. The glycosides are as a rule less active in all these respects and sieboldin is almost inactive.There is a rather good correlation between uncoupling of oxidative phosphorylation and effects upon root growth, whereas there is no agreement in the details between the effects upon root