Adventitious rooting: examining the role of auxin in an easy- and a difficult-to-root plant

The rooting responses of cuttings of difficult-to-root lilac (Syringa vulgaris) and easy-to-root forsythia (Forsythia x intermedia) were compared. The rooting ability of lilac cuttings declined over the growing season (May-June). There was also a decline in the initial concentration of free IAA at the base of the cuttings, but there was not a tight relationship between basal IAA concentration and rooting ability. Polar auxin transport ability was measured in lilac and forsythia during the period of maximum growth by [(3)H]IAA application to stem internodal tissue. Transport ability declined in lilac over this time period, particularly in terms of transport intensity and percentage of [(3)H]IAA transported. In contrast the changes in polar auxin transport ability in forsythia were less marked. This difference between species was maintained in winter hardwood cuttings, with forsythia tissue showing greater polar auxin transport ability than lilac. The importance of polar auxin transport for adventitious rooting was demonstrated in both lilac and forsythia softwood cuttings by use of the polar transport inhibitor 2,3,5-triiodobenzoic acid (TIBA). Overall the results indicate that differences in polar auxin transport ability between lilac and forsythia contribute to differences in rooting ability.