Soybean Seed Water Relations during in Situ and in Vitro Growth and Maturation

Abstract

Water, osmotic, and pressure potentials of soybean (Glycine max [L.] Merrill) embryos and related matemal tissues were measured during periods of seed growth and maturation to test the involvement of embryo water relations in seed maturation. Seeds were matured in situ or in an in vitro liquid culture medium in detached pods or as isolated seeds. Changes in water relations of embryo tissues were independent of maternal tissues. During seed maturation in situ, water and osmotic potentials in both embryo and maternal tissues declined sharply near the time of maximum dry weight. During in vitro seed culture with and without pods, water and osmotic potentials in axis and cotyledon tissues declined continuously during growth. Water and osmotic potentials of the seed coat, which was present only during in vitro seed culture with pods, changed little during the culture period. Positive turgor in the embryo was maintained beyond maximum dry weight and the loss of green color during in vitro culture but declined to zero at maturity in situ. The osmotic potential in embryo tissues declined from -1.1 megapascals at early pod fill to between -1.65 and -2.2 megapascals at maximum seed dry weight across all maturation environments. It is suggested that the decreasing osmotic potential in the growing soybean embryo reaches a threshold level that is associated with cessation of growth and onset of seed maturation.Seed growth and transport to embryos have been studied extensively (3,13,18,23), but regulation of seed maturation, and in particular, the role of seed water relations in growth cessation is not understood.In several crop species, water relations of embryo and endosperm tissues were independent of water relations of maternal tissues and of external environmental factors during seed development. In developing bean (Phaseolus vulgaris L.) seeds, water and osmotic potentials ofembryo tissues changed independently of maternal tissues (25). Maize (Zea mays L.) kernel water relations were unaffected by imposed water deficits that caused significant declines in water and osmotic potentials of leaves (12,22 and osmotic potentials declined during late growth and maturation in both wheat (1) and maize (21).Increasingly, evidence is pointing to the regulation of solute availability to developing embryos and endosperm by water relations of the embryo itself and the surrounding medium (2, 13, 18, 19, 23) (and others). Although solute unloading rates from seed coats varied considerably with the osmolality of the receiving medium in P. vulgaris L. (13), faba bean (Vicia faba L.), and pea (Pisum sativum L.) (23), sucrose unloading rates from attached soybean seed coats were reduced only at trap solution concentrations over 500 mM (18).Assimilate uptake also may be regulated by water relations inside the embryo. Guldan and Brun (2) reported that rates of asparagine uptake into excised cotyledons of field grown soybean (Glycine max [L.] Merrill) were lowered when the tissue osmotic potential was reduced to -1.8 MPa and below. On...

A Role for the Surrounding Fruit Tissues in Preventing the Germination of Tomato (Lycopersicon esculentum) Seeds

“…1), and yet the seeds appear to be the least sensitive to this growth regulator at this time. A low sensitivity to ABA during the late stages of seed maturation has been observed in other species also (14,27,28 Although the water potential gradient, as measured for thee different parts of the developing fruit, would appear to favor the flow of water to the seed, there may be restrictions to this that could not be detected experimentally (23). Studies using isolated seeds on hydrated solutions of osmoticum or growth regulators can only approximate the conditions within the developing fruit.…”

“…The ABA concentration in the sheath tissue immediately surrounding the seed increases with time of development, whereas that of the locule declines. The water contents of the seed and fruit tissues are similar during early development (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30), but decline in the seed tissues between 30 and 40 DAP. The water potential and the osmotic potential of the embryo are lower than that of the locular tissue after 35 DAP also.…”

“…3), two adjacent tissues with no obvious barriers to water transport between them. Isolated or in vitro embryos exhibit different sensitivities to applied osmotica from intact embryos and/or differences in osmotic potential (6,23 (Fig. 3).…”

Soluble Carbohydrates in Soybean

“…Shrinkage of the seed from the pod wall is the most reliable indicator of the cessation of soybean seed growth on plants (Crookston & Hill, 1978). At the time seed growth ceases, the osmotic potential of embryo tissues is about -1.8 MPa (Saab & Obendorf, 1989;Egli, 1990;Slawinska & Obendorf, 1991). Axis tissues turn yellow and cease growth before cotyledons (Obendorf et al, 1984(Obendorf et al, & 1998b.…”

“…myo-Inositol may undergo chemical isomerization after heating under specific conditions (Sasaki et al, 1988;Taguchi et al, 1997). Therefore, seed culture media are sterilized by ultafiltration (Saab & Obendorf, 1989;Obendorf et al, 1990;1998a;1998b;Wettlaufer & Obendorf, 1991). The filtrate of the aqueous ethanolic extracts of seeds or seed parts may be evaporated under a stream of nitrogen gas at room temperature leaving a dry residue for liquid chromatography or for derivatization in preparation for analysis by gas chromatography.…”