Auxin concentrations in nodes and internodes of Impatiens sultani

Greater concentrations of auxin at nodes than in internodes, resulting from some nodal barrier to basipetal transport, have long been postulated as the cause of early differentiation of initially isolated xylem and cambium at the nodes. However, this study, using [¹⁴C] indole-3-acetic acid (IAA) applied apically and gas chromatography-mass spectrometry, found that in stems of Impatiens sultani the IAA concentrations (per unit f. wt) at nodes were similar to those in adjacent internodes, though a little greater at nodes if expressed per unit length of stem and a little less per unit d. wt. By contrast, in decapitated shoots and in stem explants of dicotyledons, loss of the apical source of basipetally flowing auxin can result in auxin drainage with some auxin retention in the uppermost remaining nodes. When [¹⁴C]IAA was applied apically to Shoots for 4 h and stem explants were excised, the explants had no nodal accumulation initially whereas comparable explants incubated for 20 h revealed significant nodal accumulation. If decapitation leads both to nodal auxin accumulation and to adventitious abscission just above the node, this fits the hypothesis that abscission sites are positioned where auxin concentration decreases locally in the apical direction. Difficulties in quantifying nodal auxin dynamics are discussed, and some crude estimates of metabolic rates and locations of the auxin are presented.