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

Abstract

The rte of phloem loading, its selectivity, and the disposition of labeled carbon were studied following application of 14C-labeled sugars to the free space of source leaves of sugar beet (Beta vulgaris L.). Buffered 10 mm solutions of 14C-labeled sucrose, fructose, stachyose, mannitol, 3-0-methyl glucose or L-glucose were applied to the abraded epidermis of source leaves held in the dark. Distribution of the labeled carbon from sugar taken up from the free space was studied by 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-hr 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 compared for the six sugars studied for evidence of active uptake. The composition of the free space solution in leaves photosynthesizing in 14CO2 was studied by vacuum infiltration of the source leaf air spaces and removal of the solution by centgation. 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 two 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 it 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.Phloem loading in leaves of higher plants has an important role in the productivity of crops. Because the bulk of material *that is translocated must first undergo phloem loading, factors which affect this latter process are likely to play a significant part in determining the partitioning of assimilate between export and retention in source leaves. The major portion of material exported from the leaves of higher plants consists of a relatively few sugars and sugar alcohols (20). Data from previous studies (5,6,16) indicate that phloem loading involves active uptake of sugar from the apoplast. An understanding of selectivity and other characteristics of active uptake from the free space of the various exportable sugars seems to be an important starting point for studying the control of export from leaves. 2 Abbreviations: SA: specific activity; se-cc: sieve element-companion cell; 3-0-MG: 3-0-methyl glucose.Mechanisms which affect the availability of various sugars to the free space and their selective uptake into the phloem are likely to control export from the leav...

The Evidence for Symplastic Phloem Loading

“…The problem is exacerbated if sucrose enters the mesophyll and then travels to the minor veins during the course of the experiment (13,27). It is surprising, therefore, that so few studies have been conducted over very short time periods.…”

“…Mean concentrations in the apoplast of sugar beet leaves are in the micromolar range (13,16), well below the Km of the sucrose carrier and well below the apoplast concentration needed to sustain normal rates of export (approximately 20 mM; Ref. 13).…”

“…Mean concentrations in the apoplast of sugar beet leaves are in the micromolar range (13,16), well below the Km of the sucrose carrier and well below the apoplast concentration needed to sustain normal rates of export (approximately 20 mM; Ref. 13). This can be explained if "mobile" sugar destined for export is unloaded into the apoplast at, or near, the boundary of the SE-CCC where the local concentration could be much higher than in the mesophyll apoplast in general (13,16).…”

“…13). This can be explained if "mobile" sugar destined for export is unloaded into the apoplast at, or near, the boundary of the SE-CCC where the local concentration could be much higher than in the mesophyll apoplast in general (13,16).…”

Kinetics Analysis of the Plasma Membrane Sucrose-H+ Symporter from Sugar Beet (Beta vulgaris L.) Leaves

“…Fondy and Geiger (1977) estimated the concentration of Suc in the sugar beet (Beta vulgaris L.) leaf apoplast to be approximately 70 p~; the concentration in the metabolic space was approximately 100 times this value. These values were averages based on the surface area of the leaf and did not reflect cellular differences.…”