Micropropagation of Arbutus unedo, A. andrachne, and their natural hybrid, A. x andrachnoides from seedling explants

Abstract

SummaryMicropropagation of Arbutus unedo, A. andrachne, and A. x andrachnoides could facilitate their use as ornamental, forestry and medicinal plants, while the use of in vitro-grown seedlings as stock material could solve contamination and seasonality problems. Seeds germinated at high rates (87 – 99%) at 15ºC on half-strength Murashige and Skoog (MS) medium after 24 – 30 d, depending on the species. Nodal explants were excised from 2-month-old seedlings grown on MS medium or on woody plant medium (WPM) salts supplemented with 100 mg l–1 myo-inositol, 1.0 mg l–1 thiamine, 0.5 mg l–1 pyridoxine, and 0.5 mg l–1 nicotinic acid, containing 1.0 mg l–1 indole-3-butyric acid (IBA). Multiple shoots were produced from each initial culture, and from subsequent sub-cultures on MS medium or on WPM containing 2.5 or 5.0 mg l–1 zeatin (ZEA). The use of thidiazuron (TDZ) at 0.2 or 2.0 mg l–1, instead of ZEA, resulted in a reduced response and fewer shoots per explant. In addition, these TDZ-treated shoots had malformations and were very short, making them unsuitable for multiplication. The rates of multiplication on both media containing 2.5 or 5.0 mg l–1 ZEA were six- to seven-fold higher than on TDZ-containing or hormone-free media. MS medium resulted in higher rates of shoot production than WPM (4.4 – 7.8 vs. 2.2 – 5.3 shoots per explant, respectively, depending on the species and concentration of plant growth regulator) for cultures with a higher shoot proliferation potential. In contrast, WPM induced higher rooting percentages and more roots per micro-shoot (82 – 97% and 4.5 – 8.5, respectively, depending on the species and IBA concentration) compared to MS medium (42 – 64% and 3.1 – 4.1, respectively). A. x andrachnoides had a proliferation potential (4.6 shoots per explant on average on both media and ZEA concentrations) that was between the two parental species (5.9 and 3.4 shoots per explant for A. andrachne and A. unedo, respectively). Ex vitro acclimatisation and survival of plantlets in a 1:1 (v/v) mix of peat:perlite was highly successful (92 – 100%).