Long-distance vascular transport of endogenous hormones in plants and their role in source : sink regulation

Analyses of phloem and xylem saps from a wide variety of plants have demons trated the presence of the major endogenous plant hormones. The auxin 3-ind ole-acetic acid, a number of gibberellins, cytokinins, and abscisic acid ha ve been found in one or the other or both of these vascular saps, allowing the sites of hormonal synthesis and putative target tissues within the plan t to be deduced. Exogenously applied hormones can also be readily taken up into these vascular pathways and may be translocated over considerable dist ances from a point of application. Observations such as these indicate a po tential coordination system between root and shoot, regulated by the synthe sis and transport of endogenous hormones initially through the vascular sys tem followed by extravascular transport. The partitioning of assimilates be tween photosynthetic or storage source organs and utilizing sink organs is widely accepted to be regulated by endogenous plant hormones. The key inter mediate steps, phloem loading and unloading, are responsive to exogenously applied hormones. Vascular saps from different plants have been analyzed by various investigators using a range of physicochemical methods. Their obse rvations are evaluated and interpreted here in the light of current models of source:sink regulatory processes and the long-distance transport of endo genous hormones in higher plants.