81 Polyploidy Induction of Dendrobium formosum by Colchicine Treatment In Vitro S. Yenchon and S. Te-chato Department of Plant Science Faculty of Natural Resources Prince of Songkla University Songkhla Thailand Keywords: tetraploid plants, flow cytometry, protocorm-like bodies, MS medium, phytagel Abstract In vitro polyploidy induction using protocorm-like bodies (PLBs) of Dendrobium formosum was studied. PLBs were treated with colchicine at different concentrations at 0, 0.05, 0.10, 0.15 and 0.20% in liquid MS medium for 24 or 48 h. The PLBs were then transferred to basal MS medium solidified with 0.2% phytagel and subcultured at six weeks interval for six months. The results revealed that colchicine at concentration of 0.2% for 48 h gave the highest percentage of tetraploid plantlets. Morphological and physiological characters of those putative tetraploid plants were clearly different with the diploid plants. Tetraploid plants had thick and dark green leaves and large stem size. The stomata size, both length and width, of tetraploid plants were significantly larger than those of diploid plants, while their density was lower. Chromosome observation revealed that tetraploid plants had chromosome number 2n=4x=76 whereas diploid plants had 2n=2x=38. This evidence was also confirmed by flow cytometry. INTRODUCTION Orchids are well known as important ornamental plants, Dendrobium formosanum Roxb. is a member of the Orchidaceae genus Dendrobium which is considered the largest plant family. An in vitro polyploidy induction has been an important method for improvement of orchids to create increasing interest and demand in the floriculture market. Colchicine, a compound that effectively arrests mitosis at the anaphase stage, has been found to have a significant effect on polyploid induction. The concentration of colchicine as well as the duration of treatment are important factors that should be determined for each type of explants and plant species. Higher concentration of colchicine or prolonged treatments may be lethal to sensitive plant tissue (Silva et al., 2000). In breeding programs, it is important to determine the ploidy level in a quick and simple way in various stages of plant development. In many plant species, there is correlation between ploidy level and physiological characteristics such as chloroplast number in guard cell, size of stomata cell, stomata density and pollen grain diameter (Omidbaigi et al., 2010). Flow cytometry (FCM) is a fast and accurate method for the estimation of the nuclear DNA content of different plant species (Thiem and Sliwinska, 2003; Bennett and Leith, 1995). Thus, in this study we report the induction of tetraploid of Dendrobium formosum Roxb. using in vitro treatment of PLBs with colchicine and identified the polyploidy plants by using morphological, physiological characteristics and also confirmed those plants by flow cytometry and chromosome counting in mitotic cell of root tip. MATERIALS AND METHODS Plant Materials Mature pod of Dendrobium formosum was cleaned by washing with running tap water for a few minutes, subsequently soaking in 95% ethanol and flaming. The pod was Proc. IS on Orchids and Ornamental Plants Eds.: N. Chomchalow et al. Acta Hort. 1025, ISHS 2014