Abstract
Sucrose concentrations were measured in serial frozen sections of the post-phloem transport pathway in developing wheat (Triticum aestivum L.) grains. In 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 p-chloromercuribenzene 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. The 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 parenchyma.
Full Text
The Full Text of this article is available as a PDF (570.1 KB).
Selected References
These references are in PubMed. This may not be the complete list of references from this article.
- Fisher D. B., Gifford R. M. Accumulation and Conversion of Sugars by Developing Wheat Grains : VI. Gradients Along the Transport Pathway from the Peduncle to the Endosperm Cavity during Grain Filling. Plant Physiol. 1986 Dec;82(4):1024–1030. doi: 10.1104/pp.82.4.1024. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Fisher D. B., Gifford R. M. Accumulation and Conversion of Sugars by Developing Wheat Grains : VII. Effect of Changes in Sieve Tube and Endosperm Cavity Sap Concentrations on the Grain Filling Rate. Plant Physiol. 1987 Jun;84(2):341–347. doi: 10.1104/pp.84.2.341. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Fisher D. B. In situ measurement of plant water potentials by equilibration with microdroplets of polyethylene glycol 8000. Plant Physiol. 1985 Sep;79(1):270–273. doi: 10.1104/pp.79.1.270. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Fisher D. B. Measurement of Phloem transport rates by an indicator-dilution technique. Plant Physiol. 1990 Oct;94(2):455–462. doi: 10.1104/pp.94.2.455. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Fisher D. B., Wang N. A Kinetic and Microautoradiographic Analysis of [14C]Sucrose Import by Developing Wheat Grains. Plant Physiol. 1993 Feb;101(2):391–398. doi: 10.1104/pp.101.2.391. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Jones M. G., Outlaw W. H., Lowry O. H. Enzymic assay of 10 to 10 moles of sucrose in plant tissues. Plant Physiol. 1977 Sep;60(3):379–383. doi: 10.1104/pp.60.3.379. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Wang N., Fisher D. B. Monitoring Phloem Unloading and Post-Phloem Transport by Microperfusion of Attached Wheat Grains. Plant Physiol. 1994 Jan;104(1):7–16. doi: 10.1104/pp.104.1.7. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Wang N., Fisher D. B. Sucrose Release into the Endosperm Cavity of Wheat Grains Apparently Occurs by Facilitated Diffusion across the Nucellar Cell Membranes. Plant Physiol. 1995 Oct;109(2):579–585. doi: 10.1104/pp.109.2.579. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Wang N., Fisher D. B. The Use of Fluorescent Tracers to Characterize the Post-Phloem Transport Pathway in Maternal Tissues of Developing Wheat Grains. Plant Physiol. 1994 Jan;104(1):17–27. doi: 10.1104/pp.104.1.17. [DOI] [PMC free article] [PubMed] [Google Scholar]