Sugar Selectivity and Other Characteristics of Phloem Loading in Beta vulgaris L.

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Abstract
The rate of phloem loading, its selectivity and the disposition of labeled C were studied following application of 14C-labeled sugars to the free space of source leaves of sugar beet (B. vulgaris L.). Buffered 10 mM solutions of 14C-labeled sucrose, fructuse, stachyose, mannitol, 3-0-methyl glucose or L-glucose were applied to the abraded epidermis of source leaves held in the dark. Distributuion of the labeled C from sugar taken up from the free space was studied y microdensitometry of autoradiographs. Uptake of labeled sugar from the free space, partition between mesophyll and minor veins, metabolic conversions, export and respiration were followed during the 3 h time course studies. Rates of sugar uptake into the minor veins, flux rates through the sieve element-companion cell complex membrane and concentration ratios between free space and the interior of the minor vein phloem cells were comapred for the 6 sugars studied for evidence of active uptake. The composition of the free space solution in leave photosynthesizing in 14CO2 was studied by vacuum infiltration of the source leaf air spaces and removal of the solution by centrifugation. Labeled compounds in this solution were compared to those in an aqueous ethanol extract of the same leaf pieces. The results in sugar beet source leaves support the concept of direct, active uptake of sucrose from free space into minor veins. This is not the case for fructose,3-0-methyl glucose, mannitol or stachyose. The latter 2 sugars, which are translocated in some plants, are not loaded into the minor veins at a rate sufficient to make them a significant component of the material translocated. The rate of phloem loading is controlled in part by mesophyll metabolism, especially as its affects the availability of sucrose to the free space. Both the rate and selectivity of export are controlled by uptake from the free space into the sieve element-companion cell complex of the minor veins.