David MYCOCK, Maria Paula WATT, Muhammad NAKHOODA

The choice of auxin analogue for in vitro root induction infuences post-induction root development in Eucalyptus grandis

Previous studies on in vitro rooting for improved micropropagation of eucalypts indicated that root graviperception and post-acclimatisation architecture are determined by the relative exogenous auxin analogue and its stability, supplied during the pre-rootingculture stages. Te specifc roles of indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) in the rooting medium on the in vitro rootmorphological processes were explored using a good-rooting clone. In vitro rooting percentage was signifcantly reduced when either ofthe auxin inhibitors 2,3,5-triiodobenzoic acid (TIBA) and ρ-chlorophenoxyisobutyric acid (PCIB) or the auxin antagonist kinetin wassupplied at rooting, with or without exogenous auxin. For all treatments, at the time of root induction, shoots did not possess a vascularcambium, only procambial tissue, from where adventitious roots formed. However, when the inhibitors or the antagonist were suppliedto the roots 3 days afer root induction, they afected root growth and graviperception. Kinetin and PCIB signifcantly reduced the meanroot diameter from 552.8 µm (control) to 129.2 µm and 278.6 µm, respectively, over 3 weeks. While the PCIB treatment resulted in asignifcant increase in Δ root length over this period, the TIBA treatment signifcantly decreased ∆ root length and increased mean rootdiameter to 833.4 µm. Restricting IAA transport with TIBA further altered root vascular patterning and, as with PCIB, resulted in thecollapse of the columella region. Nevertheless, only a disruption in IAA transport and subsequent auxin distribution by TIBA treatmentresulted in altered root graviperception. Te results suggest the necessary inclusion of IAA in eucalypt micropropagation protocols toensure good quality roots.

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