An Investigation of the Relay Hypothesis of Phloem Transport in Ricinus communis L.

To test for the existence of an apoplastic unloading/reloadingstep in phloem translocation, as envisaged in the relay hypothesisof phloem transport, isotopic trapping experiments were performedon Ricinus comunis L. var. Gibsonii. A CO₂ buffer system wasused to supply ¹⁴CO₂ at constant partial pressure and constantspecific activity to a photosynthesizing leaf. The subtendinginternode was perfused with solutions of sugars or of mannitoland transolation of ¹⁴C past the perfused zone was monitoredby the collection of phloem exudate. Trapping of activity inthe perfusate was enhanced by the presence of sugars as wouldbe expected with an unloading/reloading process. However, therewas no evidence that introduction of the unlabelled sugars tothe apoplast also reduced activity in the phloem exudate. Moreover,the rate of loss of activity to the perfusing solutions representedonly 1% of the rate of appearance in the exudation. It is suggestedthat the trapping results may reflect an unloading of tracerfrom the phloem associated with a subsequent reloading by adjacenttissues rather than by the sieve tubes. To investigate the length of sieve tube continuity in Ricinus,a horizontal incision was made to the bark and the rate of exudationof phloem sap was monitored. Successive circumferential cutswere made above the exuding incision and progressively closerto it. In general, a girdling incision produced a transientdecrease in the rate of change of exudation rate (i.e. the firstderivative became more negative/less positive). The magnitudeof this response rose with exudation rate and fell with thedistance at which a girdling cut was made. Fitting an appropriatemodel yielded an estimate for contributory length of 69 ±6 cm. This was comparable with the distance of the initial tangentialincision from the stem apex, suggesting a continuous sieve tubesystem in Ricinus. A similar investigation on the petiole yieldedan estimate of around 7.0 cm. This lower estimate for contributorylength is believed to reflect a rapid sealing process that limitsthe distance of propagation of turgor-release rather than alimited length of sieve tube continuity. The results of this investigation do not support the relay hypothesisof phloem transport. Rather they suggest a continuous sievetube system which has a distributed capacity to load and unloadsolutes, and which may exhibit a sealing response when injured. Key words: Ricinus communis L, phloem transport, phleom unloading