Mutations In Buddleia Induced by Gamma Ray Irradiation

Mutation induction by physical mutagens, such as gamma ray, X-ray, and fast neutron, has been proved to be very effective for plant improvement including many horticultural species. Buddleia davidii, known as butterfly bush, is commonly used for landscaping. Its heavy flowers and long bloom period are attractive to gardeners. However, this species has a few concerns such as lack of flower color, excessive growth, and invasiveness due to its prolific seed production. Attempts to improve Buddleia davidii using conventional breeding methods have resulted in limited success. Different ploidy levels of species in the genus Buddleia may impede the interspecific crosses; therefore, elite traits cannot be efficiently integrated into new plants. In this study, mutagenesis by gamma ray irradiation was employed to induce colorful, compact, and sterile phenotypes. In vitro shoot tips of two Buddleia cultivars, B. davidii ‘Potters Purple’ and Buddleia ‘Lochinch’, were exposed to 0-150 Gy gamma rays and then recovered in MS (Murashige and Skoog) medium supplemented with 1 µM benzyladenine (BA) for 1-2 month. Shoots that recovered from the gamma ray treatment were rooted in ½ MS medium with 0.5 µM naphthalene acetic acid (NAA) and grown in the greenhouse. The growth of shoot tips exposed to gamma rays significantly inhibited and subsequently in vitro rooting was reduced. An average of 55.1 % of shoots rooted when treated with 50 Gy gamma rays, whereas only 15.7 and 12.6 % of shoots rooted when exposed to 100 and 150 Gy gamma rays, respectively. All unrooted and a few rooted plants eventually died. Various phenotypic variations were observed in survived plants grown in the greenhouse. Variations included leaf shape and size, leaves with or without hairs, stem shape and size, internode length, plant structure (branching), and flower color, size, and number, etc.  Stability of these altered morphological traits is being evaluated.