An Investigation of the Relay Hypothesis of Phloem Transport inRicinus communisL.

Abstract

To test for the existence of an apoplastic unloading/reloading step in phloem translocation, as envisaged in the relay hypothesis of phloem transport, isotopic trapping experiments were performed on Ricinus communis L. var. Gibsonii. A C02 buffer system was used to supply 14C02 at constant partial pressure and constant specific activity to a photosynthesizing leaf. The subtending internode was perfused with solutions of sugars or of mannitol and transloation of 14C past the perfused zone was monitored by the collection of phloem exudate. Trapping of activity in the perfusate was enhanced by the presence of sugars as would be expected with an unloading/reloading process. However, there was no evidence that introduction of the unlabelled sugars to the apoplast also reduced activity in the phloem exudate. Moreover, the rate of loss of activity to the perfusing solutions represented only 1 % of the rate of appearance in the exudation. It is suggested that the trapping results may reflect an unloading of tracer from the phloem associated with a subsequent reloading by adjacent tissues 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 exudation of phloem sap was monitored. Successive circumferential cuts were made above the exuding incision and progressively closer to it. In general, a girdling incision produced a transient decrease in the rate of change of exudation rate (i.e. the first derivative became more negative/less positive). The magnitude of this response rose with exudation rate and fell with the distance at which a girdling cut was made. Fitting an appropriate model yielded an estimate for contributory length of 69 ± 6 cm. This was comparable with the distance of the initial tangential incision from the stem apex, suggesting a continuous sieve tube system in Ricinus. A similar investigation on the petiole yielded an estimate of around 7 0 cm. This lower estimate for contributory length is believed to reflect a rapid sealing process that limits the distance of propagation of turgor-release rather than a limited length of sieve tube continuity. The results of this investigation do not support the relay hypothesis of phloem transport. Rather they suggest a continuous sieve tube system which has a distributed capacity to load and unload solutes, and which may exhibit a sealing response when injured.