Elongation, Solute Loss, Osmotic Potential Changes, and Respiration Rate Changes during the Treatment of Avena Coleoptile Segments with Superoptimal Auxin Concentrations

When treated with 100 μg/ml (0.57 mM) indole-3-acetic acid at pH 4.5, Avena sativa coleoptile segments elongate rapidly at first but begin to shrink after a few hours and eventually approach their initial length. Sufficient quantities of potassium and reducing sugars leak into the medium to reflect a significant change in osmotic potential of the tissues due to solute loss. Plasmometric measurements of subepidermal cell osmotic potentials reveal no alterations in that cell layer due to superoptimal auxin treatments: therefore other cells, presumably those of the epidermis, must be responsible for both the obvious loss of segment turgor and most of the solute loss. The relationship of the change in length to change in volume is the same for segments growing in no auxin, optimal auxin, and superoptimal auxin, indicating that cell swelling in other dimensions is not related to the differences in elongation. The respiration rate in superoptimal auxin falls several hours before the growth slows and stops. This result and the observation that auxin must be accumulated by segments to exert a growth inhibition suggest a site of inhibitory auxin action at or inside of the plasma membrane and not just upon the cell wall.