pH-Dependence of Extension Growth in Avena Coleoptiles and Its Implications for the Mechanism of Auxin Action

The pH-dependence of acid-induced growth in excised segments of Avena sativa coleoptiles has been reinvestigated in the pH range 3 to 7. In contrast to previous reports (e.g. DL Rayle 1973] Planta 114: 63-73), only acidic buffers with a pH below 5.0 induce an extension response. A pH of 3.5 to 4.0 is required to mimic auxin-mediated growth. Very similar pH-response curves are obtained with both intact (abraded) and peeled coleoptiles. These results agree with the recent finding of a similarly low sensitivity to protons in maize coleoptiles. It is shown that the apparently much higher sensitivity to protons previously reported for peeled Avena coleoptiles is due to incubating the tissue in buffer of pH 6.8 between peeling and measuring the effect of acidic buffers. Neutral pH reversibly inhibits the spontaneous extension burst originating on release from tissue tension after removing the epidermis. Reversal of this inhibition can be achieved by buffers of pH 5.0 to 6.0 (or distilled water), thereby simulating an acid-induced growth response in this pH range. It is concluded that true acid-induced wall-loosening generally does not take place above pH 5.0 and that a pH considerably below 4.0 is required in order to stimulate growth to an extent comparable to that obtained in response to auxin. The “acid-growth theory,” which requires an acid-mediated loosening of the cell wall in the pH range 5 to 6, this pH being established by auxin-induced proton excretion, can therefore also not be substantiated in Avena.