Characteristics of the phloem path: analysis and distribution of carbohydrates in the petiole of Cyclamen

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 14 CO 2 , 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 sucrose and planteose. The lateral 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 without 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 [ 14 C]glucose or [ 14 C]fructose. These data suggest that the phloem path is characterized by both source and sink like activity.