PLANT PHYSIOLOGY , Vol 109, Issue 2 587-592, Copyright © 1995 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
Sucrose Concentration Gradients along the Post-Phloem Transport Pathway in the Maternal Tissues of Developing Wheat Grains
D. B. Fisher and N. Wang
Department of Botany, Washington State University, Pullman, Washington 99164-4238
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.
