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

Abstract

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.

Sugar Retrieval by Coats of Developing Seeds of Phaseolus vulgaris L. and Vicia faba L.

“…The magnitude of these effluxes can be computed making several assumptions. These are that sucrose influx and efflux by the symporter share similar kinetic values (see Table 1) and the transmembrane differences in sucrose concentration are approximately 10 and 30 mM for seed coats of Phaseolus and Vicia, respectively (Patrick 1994). This approach yields estimates of sucrose effluxes by facilitated diffusion of »2.7±0.04 mmol g -1 (FW) h -1 for Phaseolus and »4.6±0.08 mmol g -1 (FW) h -1 for Vicia seed coats.…”

“…Patrick 1994). Under these conditions, the symporter would function as a sucrose facilitator (see earlier discussion).…”

“…Under these conditions, the symporter would function as a sucrose facilitator (see earlier discussion). Indeed as a facilitator, it would contribute to sucrose efflux from the "release" cells as concentration differences across their plasma membranes are positive in direction of the seed apoplasm (Patrick 1994). The magnitude of these effluxes can be computed making several assumptions.…”

“…This approach yields estimates of sucrose effluxes by facilitated diffusion of »2.7±0.04 mmol g -1 (FW) h -1 for Phaseolus and »4.6±0.08 mmol g -1 (FW) h -1 for Vicia seed coats. These fluxes account for about 25 and 60% of the total net efflux of sucrose from Phaseolus and Vicia seed coats (Patrick 1994) and for the observed pCMBS-insensitive component of sucrose efflux Patrick 1993, Walker et al 1995). In addition, functioning of the sucrose symporter, as an efflux device in the "release" cells is consistent with the finding that rates of radiolabeled photoassimilate unloading from "empty" seed coats are unaltered when sucrose is added to the bath solution (Patrick 1984, Minchin andThorpe 1990).…”

Rapid Response of the Yield Threshold and Turgor Regulation during Adjustment of Root Growth to Water Stress in Zea mays

“…It is tempting to argue that pressure-sensitive transport processes, particularly for solutes, may be involved. Turgor-sensitive processes have been proposed for Suc transport (Oparka and Wright, 1988) in potato tubers as well as for assimilate transport across the seed coat of beans (Wolswinkel, 1990;Patrick, 1994). Apparently, pressure-sensitive regulation of symplastic solute transport through plasmodesmata (Robards and Lucas, 1990) may not explain completely the observed high ERs of roots (Bret-Harte and Silk, 1994).…”