Characteristics of the phloem path: analysis and distribution of carbohydrates in the petiole of Cyclamen |
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Authors: | Rothe K; Porzel A; Neumann S; Grimm E |
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Institution: | Martin-Luther-Universitat Halle-Wittenberg, Institut fur Pflanzen- und Zellphysiologie, Am Kirchtor 1, D-06108 Halle (Saale), Germany; Institute fur Pflanzenbiochemie, Weinberg 3, D-06120 Halle (Salle), Germany; Correspondence address; Fax: +49 345 552 7219; E-mail: grimm@landw.uni-halle.de |
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Abstract: | Compartmentation fluxes of carbohydrates along the phloem path were
analysed in the petiole of Cyclamen persicum (L.)
Mill. Sucrose represented the dominant fraction (58-75% of soluble
carbohydrates in the vascular symplast). Planteose (12-22%), glucose (3-8%)
and fructose (3-13%) occurred in lower amounts (data from liquid
chromatography, percentages of the total peak area). Starch was not
detectable. Upon feeding leaves with 14CO2, 98% and
90% of radiolabel was recovered as sucrose in the vascular symplast after 3
h and 24 h, respectively. Thus, sucrose appeared to be the exclusive
transport sugar in Cyclamen. Experiments with
asymmetrically labelled sucrose revealed that there was no metabolism of
translocated sucrose. Analysis of six consecutive petiole segments (each 2
cm in length) showed a homogeneous longitudinal distribution of these
sugars differed markedly. On average, the sucrose concentration amounted to
4.7 and 0.4 mg g-1 FM in the vascular apoplast and
petiole parenchyma, respectively. Sucrose was unloaded with out hydrolysis
and stored in the periphery of the phloem path. Planteose was identified as
another storage saccharide. Sucrose synthesis by sucrose phosphate synthase
occurred when isolated vascular bundles were incubated with
14C]glucose or
14C]fructose. These data suggest that the phloem
path is characterized by both source and sink like
activity. |
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