A dual role of turgor pressure in auxin-induced cell elongation in Avena coleoptiles.

1. Auxin-induced wall loosening, as measured by the Instron extensometer technique, and the conversion of wall loosening into extension, as measured by cell elongation, differ in their relationship to turgor pressure (TP). Wall loosening can occur at any TP greater than zero while rapid cell extension only occurs when the TP exceeds a critical value (Pc). 2. The amount of auxin-induced increase in wall extensibility is proportional to the turgor pressure in the region between Pc and zero. In the absence of auxin, wall extensibility decreases slightly when TP exceeds Pc. 3. A reassessment of the turgor pressure of intact Avena coleoptiles has shown that it is greater than Pc. The TP of intact Avena coleoptiles is sufficient to permit turgor-driven cell elongation to occur. 4. It is proposed that wall extension involves two steps, each of which requires turgor pressure. Covalent bonds which render the wall rigid are broken only when the wall is under tension and when auxin is present in the tissue. Extension of the wall then requires that hydrogen bonding between polymers be broken by a TP in excess of Pc.