Effect of absorbable and non-absorbable solutes on the elongation of barley coleoptiles

Abstract

The effect of combinations of mannitol and KCl on the elongation of barley coleoptile sections was investigated. During initial phases KCl reduced the lengths of coleoptile sections but after a few hours, a stimulatory effect became progressively more evident and an optimum KCl concentration at which elongation was at a maximum was clearly indicated. In time, the position of the optimum was shifted toward higher KCl concentrations. Mannitol depressed the elongation rate at all times and it caused a shift of the optimum KCl concentration toward higher values. This response of elongating Coleoptiles to added osmotica is caused by a dual effect of absorbable solutes: (1) the external water potential is reduced and through it the turgor pressure and (2) after absorption of the solutes the vacuolar osmotic component of the water potential is reduced and consequently the turgor pressure is increased. During initial phases, only the depressing effect of an osmoticum is possible but as the consequences of solute uptake become effective, the stimulation becomes noticeable. For elongating tissues, continuous solute uptake is necessary to maintain favorable cellular water relations. The observed optimum is due to the hyperbolic nature of the KCl absorption isotherm. Since little mannitol enters the cells, only a depressing effect by this osmoticum is observed. The elongation of coleoptiles could reasonably be predicted from a model of cell elongation in which solute uptake was partly rate limiting. Solute uptake is essential for maintaining a small water potential gradient favorable for water uptake which itself is necessary to maintain turgor pressure and cause elongation.