Turgor-dependent unloading of photosynthates from coats of developing seed of Phaseolus vulgaris and Vicia faba. Turgor homeostasis and set points

Key physiological characteristics of turgor-dependent efflux of photosynthates were examined using excised coats and cotyledons of developing Phaseolus vulgaris (cv. Redland Poineer) and Vicia faba (cv. Coles Prolific) seed during the linear phase of seed fill. Exposure to solutions of high osmotic potential inhibited net uptake of ¹⁴C]sucrose by cotyledons at developmental stages less than 60% of their final dry weight. The effect could not be fully reversed by transferring cotyledons to solutions set at lower osmotic potentials. The inhibition became apparent at osmotic potentials that were higher than those that caused stimulation of efflux from seed coats. Net ¹⁴C]sucrose uptake by cotyledons at more advanced stages of development was unaffected by external osmotic potential. Specified tissue layers were removed from seed coats by pretreatment with pectinase. Efflux studies with the pectinase-modified coats of Phaseolus and Vicia seed demonstrated that the cellular site of turgordependent efflux was the ground parenchyma and thin-wall parenchyma transfer cells, respectively. Coats subjected to long-term (hours) incubations, under hypo-osmotic conditions, exhibited the capacity for turgor regulation. This was mediated by turgor-dependent efflux of solutes. The solutes exchanged were of nutritional significance to the developing embryo. The relationship between efflux and coat turgor was characterised by a turgor-independent phase at low turgors. Once turgor exceeded a minimal value (set point), efflux increased in proportion to the magnitude of the turgor deviation (error signal) from the set point. For coats of sink-limited seed of Vicia and Phaseolus, efflux exhibited apparent saturation at turgors above 0.25 and 0.5 MPa respectively. The putative turgor set point and slope of the turgor-dependent component of efflux varied with seed development, the prevailing source/sink ratio and genetic differences in seed growth rate. The nature of these kinetic variations was compatible with the competitive ability of the seed. A turgor homeostat model is proposed that incorporates the observed functional attributes of turgor-dependent efflux. Operationally, the model provides a mechanistic basis for the integration of assimilate demand by the cotyledons with assimilate import into and unloading from the seed coat.