Sucrose Concentration Gradients along the Post-Phloem Transport Pathway in the Maternal Tissues of Developing Wheat Grains

Abstract

Sucrose concentrations were measured in seria1 frozen sections of the post-phloem transport pathway i n developing wheat (Triticum aestivum 1.) grains. I n normally importing grains, there was an approximately linear concentration gradient along the pathway, with a difference between the ends of the pathway of about 180 mM. This indicates an unusually low resistance for cell-to-cell transport, due perhaps to the large size-exclusion limit for the pathway. However, the existence of concentration gradients raises presently unresolvable questions about the relative contributions of diffusion versus bulk flow to transport within the symplast. The concentration gradient disappeared when sucrose movement ceased (i.e. in excised grains or when endosperm cavities of attached grains were perfused with pchloromercuribenzene sulfonate [PCMBS] or with 1660 mOsm sorbitol). PCMBS appeared to block solute release into the endosperm cavity, whereas the sorbitol treatment, previously shown to cause localized plasmolysis in the chalaza, appeared to block movement across the chalaza. Sieve element/companion cell unloading appears to be an important control point for assimilate import. l h e sucrose concentration gradient and, probably, turgor and osmotic gradients are extremely steep there. PCMBS blocked import without affecting the sucrose concentration in the vascular parenchyma around the phloem. Thus, blockage of unloading was more complex than a simple "backing up" of solutes in the vascular parench yma.Assimilate import by a sink is a multistep process involving distinct types of transport through severa1 different tissues. Because this complexity occurs on a reduced physical scale, it is difficult to analyze the individual transport processes involved and the extent to which each might contribute to the control of import (Oparka, 1990). As noted more fully elsewhere (Wang and Fisher, 1994a), developing wheat (Triticum aestivum L.) grains provide some distinct advantages for this purpose. Transport steps are relatively separate anatomically, and solute concentrations and other transport-related parameters can be obtained readily from the sieve tubes and endosperm cavity. Based largely on the constancy of Suc import in the face of widely varying SUC and osmotic concentrations in the endosperm cavity, we have concluded that control of import is exerted by trans-

Gradients in Water Potential and Turgor Pressure along the Translocation Pathway during Grain Filling in Normally Watered and Water-Stressed Wheat Plants

“…As noted previously for normally watered plants (Fisher, 1995), probably the most striking aspects of the SE/CC unloading step in grains from droughted plants are the low turgor of the vascular parenchyma cells and the large pressure differential, more than 1 MPa, to drive transport at this step. As with normally watered plants, there was a considerable range in this turgor differential.…”

Review: Nutrient loading of developing seeds

“…Assuming a sucrose/H + stoichiometry of one, the following derivative of the Nernst equation predicts the intracellular (C i ) and external (C o ) sucrose concentration differences at which sucrose/H + symport will Fisher and Wang (1995). B Wang et al (1995).…”

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

“…The general similarity of the in vivo and in vitro efflux rates suggests that transport across the nucellar cell membranes could be involved in the control of assimilate import into the grain. Other evidence, however, strongly implicates movement out of the SE/CC complex (Fisher and Wang, 1995) as an important control point.…”

“…After the third post-PCMBS wash, the grains were sliced longitudinally parallel to the nucellar surface (see fig. 1B of Fisher and Wang, 1995), exposing about 75% of the length of the nucellus. The remaindix of the nucellus, at the embryo end where the cavity usually bends, was exposed by pulling off the overlying aleurone with forceps.…”

“…The "crease tissues" (i.e. a sample consisting of the vascular strand, chalaza, nucellus, and surrounding pericarp; for anatomy, see Wang and Fisher, 1994b;Fisher and Wang, 1995) were dissected from each grain by making parallel longitudinal slices along the pericarp on either side of the crease tissues. SUC, Glc, and Fru were assayed enzymatically (Jones et al, 1977).…”