Phloem loading--not metaphysical, only complex: towards a unified model of phloem loading.

Phloem loading comprises the entire pathway of phloem-mobile solutes from their place of generation (or delivery) to the sieve tubes in a sequence of transport steps across or passing by several different cell types. Each of these steps can be classified as symplastic or apoplastic. The detailed anatomical-cytological work in the past ten years made clear that the symplastic continuity from mesophyll to sieve tubes may be very different for different plant species or even in different vein orders.

Phloem loading in Ricinus cotyledons: sucrose pathways via the mesophyll and the apoplasm.

Ricinus communis cv. Carmencita seedlings with their cotyledons incubated in sucrose solution and their hypocotyls cut to induce exudation of phloem sap, constitute a system of sucrose fluxes into and out of the cotyledons. This system was characterized with respect to quasi-steady-state conditions of sucrose uptake and export and then used to investigate the pathways of sucrose during phloem loading.

Microautoradiographic studies of the role of mesophyll and bundle tissues of the Ricinus cotyledon in sucrose uptake.

The aim of the study was to show which tissues and cell types of the cotyledon of Ricinus communis L. are responsible for uptake of sucrose by H(+)-sucrose symport. The cotyledons were incubated in labelled sucrose for up to 20 min and then the amount of radioactivity in each cell type of the cotyledon was assessed by microautoradiography.

Sucrose transport into the phloem of Ricinus communis L. seedlings as measured by the analysis of sieve-tube sap.

Careful cutting of the hypocotyl of Ricinus communis L. seedlings led to the exudation of pure sieve-tube sap for 2-3 h. This offered the possibility of testing the phloem-loading system qualitatively and quantitatively by incubating the cotyledons with different solutes of various concentrations to determine whether or not these solutes were loaded into the sieve tubes.