Analysis of the driving forces of phloem transport in Ricinus seedlings: sucrose export and volume flow are determined by the source

The aim of this study was to reveal the factors determining sucrose export and volume flow through the sieve tubes in Ricinus communis L. seedlings. The cotyledons take up sucrose from the apoplasm in vivo, and export most of it to the growing sinks, hypocotyl and root. This simple source-sink system allowed sucrose uptake and export to be studied under controlled conditions with respect to apoplasmic sucrose concentrations. From the additional knowledge of the sucrose concentrations in the mesophyll and the sieve tubes, transmembrane concentration differences were calculated. The volume flow rate along the sieve tubes could be calculated from the export rate and the sucrose concentration in the sieve tubes. While the export rate exhibited saturation kinetics, the volume flow rate decreased at high external sucrose concentrations. The export rate correlated with the sucrose uptake rate, the volume flow rate correlated with the sucrose concentration (osmotic pressure) difference across the sieve-tube plasma membrane, the driving force for transmembrane water flux. From these data it can be concluded that sucrose export and the volume flow through the sieve tubes are determined by activities of the source. Export out of Ricinus cotyledons was considerably higher than export out of green source leaves of different species. The concomitant comparatively low sucrose concentration in the sieve-tube sap of the seedlings can thus be attributed to a very high water flux into and along the sieve tubes associated with the high sucrose flux. Received: 28 November 1997 / Accepted: 4 April 1998