SUCROSE CONCENTRATION GRADIENTS ALONG THE POST-PHLOEM TRANSPORT PATHWAY IN THE MATERNAL TISSUES OF DEVELOPING WHEAT GRAINS

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 li near concentration gradient along the pathway, with a difference betwe en the ends of the pathway of about 180 mM. This indicates an unusuall y low resistance for cell-to-cell transport, due perhaps to the large size-exclusion limit for the pathway. However, the existence of concen tration gradients raises presently unresolvable questions about the re lative contributions of diffusion versus bulk flow to transport within the symplast. The concentration gradient disappeared when sucrose mov ement ceased (i.e. in excised grains or when endosperm cavities of att ached grains were perfused with p-chloromercuribenzene sulfonate [PCMB S] 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 bloc k movement across the chalaza. Sieve element/companion cell unloading appears to be an important control point for assimilate import. The su crose concentration gradient and, probably, turgor and osmotic gradien ts are extremely steep there. PCMBS blocked import without affecting t he 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.