Some crucial arguments against the acid growth theory of auxin action (U Kutschera, P Schopfer [1985] Planta 163: 483-493) have been reinvestigated by simultaneous measurements of proton fluxes and growth of maize (Zea mays L.) coleoptiles. Special care was taken to obtain a mild, effective, and reproducible abrasion of the cuticle. Proton secretion rates were determined in a computer-controlled pH-stat. In some experiments, equilibrium pH was measured. Growth rates were determined simultaneously in the same vessel using a transducer-type auxanometer. It was found that (a) the timing of auxin and fusicoccin-induced (FC) proton secretion and growth matches well, (b) the equilibrum external pHs in the presence of IAA and FC are lower than previously recorded and below the so-called 'threshold-pH,' (c) neutral or alkaline unbuffered solutions partially inhibit FC and IAA-induced growth in a similar manner, (d) the action of pH, FC, and IAA on growth are not additive. It is concluded that the acidgrowth-theory correctly describes incidents taking place in the early phases of auxin-induced growth.

The O2 dependence of net H(+) efflux of maize coleoptiles has been investigated. Below 100 μM O2, H(+) efflux in young (1 cm long) coleoptiles is markedly decreased while old (7 cm long) coleoptiles show a decline only at 10 μM O2. Old coleoptiles show the same decrease in net H(+) efflux as young ones if treated with fusicoccin. The ratio of alteration of CO2 production to the change in net proton efflux is about 1:1 at 40-80 μM O2 but not at 10 μM O2. An influx can be observed at 10 μM O2 in young as well as in old coleoptiles if the H(+) concentration is held at values below pH 6.5. Lower O2 concentrations lead to an increase of net H(+) efflux, which might be caused by leaching of organic acids resulting from anaerobic processes, but CO2 production is not significantly changed at these values. It is proposed that more than one system is responsible for proton translocation across the plasmalemma. One of the systems has a high sensitivity to reduced O2 concentration which is within the same range as the high Km of the alternative path.

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