Callus induction and browning suppression in tree peony Paeonia ostii ‘Fengdan’

Callus induction is an important stage in micropropagation. In this study, embryos, cotyledons, and hypocotyls of tree peony (Paeonia ostii ‘Fengdan’) were used as explants to induce callus formation. Callus induction was largely influenced by the medium, plant growth regulator, and explant. All combinations of the medium and plant growth regulator were conducive to callus induction from zygotic embryos, where the ratio of explants with callus induction was very high in all of the combinations. The greatest ratio of explants with callus induction from the cotyledon explants was found on Woody Plant Medium (WPM) or Murashige and Skoog (MS) medium supplemented with both 0.5 mg L−1 thidiazuron (TDZ) and either 0.5 mg L−1 2,4-dichlorophenoxyacetic acid (2,4-D) or 0.5 mg L−1 1-naphthaleneacetic acid (NAA), and on the WPM containing 0.5 mg L−1 NAA and 0.5 mg L−1 6-benzylaminopurine (BA). The ratio of explants with callus induced from the hypocotyl explants was the greatest on either MS medium or WPM supplemented with both 0.5 mg L−1 2,4-D and 0.5 mg L−1 TDZ. In contrast, a combination of 0.5 mg L−1 2,4-D and 0.5 mg L−1 TDZ, or 0.5 mg L−1 NAA and 0.5 mg L−1 TDZ was highly likely to cause a browning problem during culture. For browning suppression, calcium chloride (CaCl2), polyvinyl pyrrolidone (PVP), gallic acid, and caffeic acid were much more effective than other reagents. The efficacy on browning suppression was concentration-dependent, and the best results were obtained at a concentration of 4.0 mg L−1 CaCl2, 1.0–2.0 g L−1 PVP, 0.5–1.0 mg L−1 gallic acid, and 0.5–1.0 mg L−1 caffeic acid, respectively. In summary, callus formation was successfully induced from the zygotic embryo, cotyledon, and hypocotyl explants, and callus browning was effectively suppressed by caffeic acid (0.5–1.0 mg L−1), gallic acid (1.0 mg L−1), CaCl2 (4.0 mg L−1), and PVP (1.0 g L−1).