Chilling relieves corm dormancy in Calopogon tuberosus (Orchidaceae) from geographically distant populations

Many plant species require a chilling period to commence regrowth from overwintering structures such as buds, corms, tubers, and rhizomes. While the effects of chilling have been thoroughly studied in a horticultural context, little information exists regarding the relationship between ecotypic differentiation and chilling requirements. Effects of chilling storage organs on shoot emergence of widespread orchid species has not been examined, and ecotypic differentiation in the Orchidaceae has also received little attention. The effects of chilling on corm dormancy in Calopogon tuberosus, a widespread orchid of eastern North America, were studied. Seeds were collected from south Florida, north central Florida, South Carolina, and Michigan, and germinated in vitro to produce plants. After 20 weeks in vitro culture, corms were removed from seedlings and chilled for 0, 2, 4, 6, and 8 weeks. Corms were subsequently planted in a soilless potting mix and placed under ex vitro conditions in an environmental growth chamber. Shoot emergence was monitored bi-weekly for 16 weeks, and shoot length, leaf number, leaf width, root number, root length, and corm diameter were measured after 16 weeks. Longer chilling periods broke corm dormancy more effectively than shorter chilling treatments regardless of population. Shoots of all populations sprouted rapidly on corms after 6 and 8 weeks chilling. In addition, a higher percentage of shoots sprouted on corms after 8 weeks chilling. After 16 weeks, north central Florida and South Carolina plantlets were larger than Michigan and south Florida plantlets. Differing chilling requirements among C. tuberosus populations may reflect ecotypic differentiation resulting from varying environmental conditions at each site.