Abstract
A micropropagation system was developed for Acacia mearnsii De Wild., which is the principal source of the world's tanbark and an excellent firewood. Shoot tips 5-mm long from 3-wk-old seedlings germinated in vitro served as explants. The seeds were germinated on hormone-free MS medium and the shoot tips were cultured on three-fourth-strength MS medium supplemented with combinations of auxins [indole-3-butyric acid (IBA) and$\alpha-naphthaleneacetic$acid (NAA)] and cytokinins [kinetin and benzylaminopurine (BAP)]. Cultures were maintained at$25\textdegree \pm 5\textdegree C$and exposed to 12-h photoperiods of cool-white fluorescent light ($70 \mu Em^{-2} \cdot s^{-1}$). Multiple shoot formation was promoted by BAP at$2 mg \cdot liter^{-1}$($8.87 \mu M$) and higher combined with or without$0.01 mg \cdot liter^{-1}$($0.049 \mu M$) IBA. Cytokinins at concentrations of less than$1 mg \cdot liter^{-1}$combined with 0.01 to$0.1 mg \cdot liter^{-1}$auxin inhibited multiple shoot formation and promoted rooting of the shoot tip explants. Shoot multiplication cultures were maintained by transferring segments of multiple-shoot clusters onto a medium containing$2 mg \cdot liter^{-1}$BAP and$0.01 mg \cdot liter^{-1}$IBA. Although higher levels of BAP promoted more multiple shoot formation, this BAP level allowed shoot elongation as well as multiplication. In-vitro-produced shoots were induced to root on a range of NAA concentrations (0.0 to$0.8 mg \cdot liter^{-1}$[$4.3 \mu M$]) supplemented to half- or full-strength MS medium. The highest frequency of root proliferation was on half-strength MS medium supplemented with$0.6 mg \cdot liter^{-1}$($3.22 \mu M$) NAA. Plantlets survived in potting soil and exhibited normal growth under greenhouse conditions.