LONG-DISTANCE VASCULAR TRANSPORT OF ENDOGENOUS HORMONES IN PLANTS AND THEIR ROLE IN SOURCE:SINK REGULATION

Abstract

Analyses of phloem and xylem saps from a wide variety of plants have demonstrated the presence of the major endogenous plant hormones. The auxin 3-indole-aceticacid, a number of gibberellins, cytokinins, and abscisic acid have 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 plant to be deduced. Exogenously applied hormones can also be readily taken up into these vascular pathways and may be trans located over considerable distances from a point of application. Observations such as these indicate a potential coordination system between root and shoot, regulated by the synthesis and transport of endogenous hormones initially through the vascular system followed by extra vascular transport. The partitioning of assimilates between photosynthetic or storage source organs and utilizing sink organs is widely accepted to be regulated by endogenous plant hormones. The key intermediate 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 observations are evaluated and interpreted here in the light of current models of source: sink regulatory processes and the long-distance transport of endogenous hormones in higher plants.