Effect of the Osmotic Environment on K+and Mg2+Release from the Seed Coat and Cotyledons of Developing Seeds ofVicia fabaandPisum sativum. Evidence for a Stimulation of Efflux from the Vacuole at High Cell Turgor

Abstract

After removal of the embryo from developing seeds of Vicia faba L. and Pisum sativum L., the 'empty' ovules were filled with a substitute medium (pH 5-5) and the effect of the osmolality of the medium on K+ and Mg2 + release from the seed coat was examined. In long-term experiments (12 h or longer), with both attached and detached seed coats, the rate of K+ and Mg2 + release from seed coats filled with a solution without osmoticum was enhanced, in comparison with release from seed coats filled with a solution containing 400 mol m3 mannitol. In relatively short experiments (2-5 h) with excised seed-coat halves and cotyledons of pea, the stimulatory effect of a low osmolality of the medium on K+ and Mg2+ efflux into the bathing medium could also be observed in cotyledons. Quantitatively, efflux stimulation was most important in experiments with seed coats and cotyledons from ovules in an early stage of development. In experiments of 7 h, compartmental analysis was used for a more detailed analysis of efflux stimulation. Half-times for exchange (i}) of K+ and Mg2+ from the slowest exchanging compartment (the vacuole) were much lower in tissues incubated in a low-osmolality medium than in tissues incubated in a high-osmolality medium. There was no evidence for an effect on the permeability of the plasmalemma. An increase in the tonoplast permeability at high cell turgor, for all solutes measured (K+, Mg2 + , sucrose, amino acids), will help to maintain high solute concentrations in the seed apoplast.