The quantitative and chronological relationship between IAA-induced H(+)-pump activation and elongation growth studied by means of xylem perfusion [on Vigna unguiculata]

The relationship between IAA-induced elongation growth and membrane hyperpolarization in Vigna unguiculata was further examined by means of the xylem perfusion method simultaneously measuring elongation growth and membrane potentials. The membrane hyperpolarization induced by various concentrations of IAA (10−6 M to 10−3 M) was always followed the acceleration of elongation growth, and had a quantitative correlation with the induced growth rate. The length of lag times for these responses shortened with the increase of IAA concentration. These results indicate that the membrane hyperpolarization is a primary event in an IAA-induced series of reactions resulting in the acceleration of elongation growth. The IAA-induced membrane hyperpolarization seems to be a more appropriate parameter for examination of the binding properties of IAA-receptor. The dose-response curve for IAA-induced growth was similar to the well known “bell-shaped” curve, but that for IAA-induced hyperpolarization was somewhat different, and can be approximated by Michaelis- Menten kinetics. C50 values for the hyperpolarization of Vpx and Vps are calculated to be 7.5 × 10-7 M and 8.7 × 10−7 M, respectively. These values are higher than the C50 values which were obtained with the classical growth bioassay.