Vascular transport of auxins and cytokinins in Ricinus

Analyses of vascular saps supplying source and sink organs have demonstrated the presence of major endogenous hormones and/or their precursors. Indol-3yl-acetic acid, a number of gibberellins, cytokinins and abscisic acid, as well as the precursor for ethylene production have been found in these vascular saps, allowing the sites of hormonal synthesis and putative target tissues to be deduced. Exogenously applied hormones are also readily loaded into these vascular pathways and may be translocated over considerable distances from a point of application. Observations such as these indicate a possible co-ordination system between source and sink regulated by the synthesis and transport of endogenous hormones. It is widely accepted that the partitioning of assimilates between photosynthetic source organs and utilising sink organs is regulated by endogenous plant hormones. The key intermediate steps involved in assimilate transport, such as phloem loading and unloading, have been shown to be responsive to applied hormones, although the role of endogenous hormones in these processes remains essentially unresolved. Results of the analyses of vascular saps from Ricinus communis, which have been obtained using a range of physicochemical methods, are compared and contrasted with those obtained by the application of exogenous hormones or their precursors. These results are evaluated critically and interpreted in the light of current models of source:sink regulatory processes and the long-distance transport of auxins and cytokinins in higher plants.