A proportion (20ndash;30) of sugars is washed out from sugar beet leaf plates during a 30-min period of tissue rinsing with water. It is assumed that these sugars are compartmentalized in the free space (FS) of cells and that their leaching is not directly controlled by cell membranes. This fraction comprises sucrose, glucose, and fructose in ratios close to those characteristic for given cells.FS sugars interact with those compartmentalized in the cells. When mesophyll or conducting bundle tissues washed free of mobile sugars are placed for 15ndash;30ensp;min into a 0.002ensp;Msolution of14C-sugars, the latter enter tissue FS and accumulate inside the cells. Conducting tissue cells show a much greater capacity for sugar absorption from FS than do mesophyll cells.When sugars are removed from the mesophyll FS. it is refilled with sugars from cells in 60ensp;min. On repeated leachings of FS, about half of the sugars found in mesophyll cells may pass into FS. When conducting bundle tissues are leached, sugars can be removed only once from their FS. Keeping the leached bundles for an hour or more does not lead to their resaturation with sugars from the conducting cells. It may therefore be concluded, that in a whole leaf, the FS of conducting tissues is filled with sugars coming from mesophyll cells.After a 5-min photosynthesis in mesophyll disks in the presence of14CO2, labeled assimilates are detectable in the FS of this tissue after as early as 10ensp;min, while after 120ensp;min the labeled products in the FS constitute over 40 of the newly-formed14C-assimilates.It is suggested that in a whole sugar beet plant, assimilate outflow from leaf plates may occur with the participation of tissue ldquo;free space,rdquo; from which sugars are absorbed by a network of conducting bundles, thereby causing more amounts of sugars to come from photosynthesizing mesophyll cells.
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