Transport of nutrients into developing seeds: a review of physiological mechanisms

Abstract

After synthesis in the vegetative parts of the plant, assimilates are translocated to fruits through xylem and phloem. Research on factors controlling nutrient transport into developing seeds via the phloem has been stimulated by the development of the empty seed coat technique.There is a consensus that as assimilates are transported from maternal tissues to filial tissues they are delivered to the extracellular space (the apoplast) separating the two generations, prior to uptake from the apoplast into the tissues of the embryo or endosperm. The empty seed coat technique has been used for the study of several aspects of nutrient transport into seeds, e.g. metabolic control and turgorsensitive transport. The osmotic environment of seed tissues has a strong effect on assimilate transport into empty seeds. Several lines of evidence suggest that one of the main 'secrets' of the high sink strength of developing seeds, at least in many taxonomic groups of dicotyledons, is that the sink end of the phloem pathway is 'bathed' in an apoplast solution with a high concentration of osmotically active solutes. Data on maize do not fit this pattern. A turgor homeostat mechanism may help to maintain high solute concentrations in the seed apoplast. The apoplast environment of seed tissues may also stimulate synthesis of storage proteins and be involved in the prevention of precocious germination. In addition to the osmotic environment, other factors influencing sink strength are discussed. Some aspects of solute transformation during transport through seed tissues are described.

Sucrose Release into the Endosperm Cavity of Wheat Grains Apparently Occurs by Facilitated Diffusion across the Nucellar Cell Membranes

“…A11 of these characteristics differ from the situation in wheat grains. In addition, efflux in legumes is sensitive to externa1 osmotic pressure (Wolswinkel, 1992), whereas in wheat it is not. As in wheat, solute release from corn pedicels is insensitive to osmotica and to metabolic inhibitors (although not to PCMBS; Porter et al, 1987).…”

“…The presence of this step, necessarily involving the release of solutes from the maternal symplast, has been used to considerable advantage in studies of phloem unloading and post-phloem transport in developing seeds (see reviews by Thorne, 1985;Patrick, 1990;Wolswinkel, 1992). The site of solute release in Vicia seeds (Offler and Patrick, 1993) and in wheat (Triticum aestivum L.) grains (Wang and Fisher, 1994b) is a layer of transfer cells in the maternal tissues facing the embryo or endosperm, respectively.…”

Monitoring Phloem Unloading and Post-Phloem Transport by Microperfusion of Attached Wheat Grains

“…to about -1 MPa; Wolswinkel, 1992), contrasts with the response of legume seed coats (Patrick, 1990;Wolswinkel, 1990Wolswinkel, , 1992Ellis et al, 1992) and com pedicels (Porter et al, 1987). Although those systems react differently to increasing osmotic concentration (solute release from seed coats increases, whereas that from pedicels decreases), both appear to be responding to osmotically induced turgor changes in the matemal tissues.…”

“…Results from this approach have implicated transport, within matemal tissues as a controlling factor for the rate of seed growth (Wolswinkel, 1992). Assimilate import into the ovules of legumes, the most extensively studied species, increases with increasing apoplastic osmotic concentration.…”

“…104,1994 action of even nonpermeant inhibitors such as PCMBS (Minchin and Thorpe, 1990). Matemal pool sizes and rates of phloem transport often (Gifford and Thome, 1986;Ellis and Spanswick, 1987;Minchin and Thorpe, 1989;Ellis et al, 1992), but not always (Wolswinkel, 1992), decline with time. These uncertainties complicate the interpretation of treatment effects, especially in distinguishing between phloem unloading in the stict sense (i.e.…”