Hydrogen ion dependence of carrier-mediated auxin uptake by suspension-cultured crown gall cells.

1. The dependence on external hydrogen ion concentration of the carrier-mediated components of the uptakes of indol-3-yl acetic acid (IAA) and 2,4-dichlorophenoxyacetic acid (2,4D) by suspension-cultured crown gall cells from Parthenocissus tricuspidata Planch. is examined. The initiall rates of uptake of IAA and 2,4 D from 0.30 μmol/l solutions exhibit pH optima of approximately pH 5.0 and pH 4.0 respectively.-2. The inherent difficulties, especially at low pH, of estimating the carrier-mediated uptake component in the presence of the large diffusive uptake of undissociated acid are discussed.-3. At pH 5.0, the uptakes of the two auxins are characterised by similar "Michaelis Constants" (0.65 and 0.80 μmol/l) and "Maximum Velocities" (0.87 and 1.0 pmol/min/mg cells) for IAA and 2,4D respectively.-4. A dependence of carrier-mediated auxin uptake on cations other than hydrogen ions could not be demonstrated. It is suggested that uptake of auxin anions may occur by co-transport with hydrogen ions.