Chao et al.: Leafy spurge dormancy and growth • 59 Weed Science, 54:59–68. 2006 Sugars, hormones, and environment affect the dormancy status in underground adventitious buds of leafy spurge (Euphorbia esula) Wun S. Chao Corresponding author. USDA–Agricultural Research Service, Plant Science Research, 1605 Albrecht Blvd., Fargo, ND 58105-5674; chaow@fargo.ars.usda.gov Marcelo D. Serpe Department of Biology, Boise State University, 1910 University Drive, Boise, ID 83725 James V. Anderson USDA–Agricultural Research Service, Plant Science Research, 1605 Albrecht Boulevard, Fargo, ND 58105-5674 Russ W. Gesch USDA–Agricultural Research Service, North Central Soil Conservation Research Laboratory, 803 Iowa Avenue, Morris, MN 56267 David P. Horvath USDA–Agricultural Research Service, Plant Science Research, 1605 Albrecht Boulevard, Fargo, ND 58105-5674 Signals from both leaves and apical or axillary meristems of leafy spurge are known to inhibit root bud growth. To test the hypothesis that carbohydrates and growth regulators affect root bud growth, decapitated leafy spurge plants were hydroponically treated with glucose, sucrose, gibberellic acid (GA), abscisic acid (ABA), 1-naphthal- eneacetic acid (NAA), 6-benzylaminopurine (BA), and a GA biosynthesis inhibitor, paclobutrazol. Both glucose and sucrose caused suppression of root bud growth at concentrations of 30 mM. The inhibitory effect of sucrose was counteracted by GA at 15 M. In contrast, BA, ABA, NAA, and paclobutrazol inhibited root bud growth at concentrations as low as 1, 2, 1, and 16 M, respectively. Sugar and starch levels were also determined in root buds at various times after decapitation. Buds of intact plants contained the highest level of sucrose compared with buds harvested 1, 3, and 5 d after decapitation. To determine how seasonal changes affect root bud dormancy, growth from root buds of field-grown plants was monitored for several years. Root buds of field-grown leafy spurge had the highest level of innate dormancy from October to November, which persisted until a prolonged period of freezing occurred in November or early December. Our data support the hypothesis that carbohydrates may be involved in regulating dormancy status in root buds of leafy spurge. Nomenclature: Leafy spurge, Euphorbia esula L. EPHES. Key words: Carbohydrates, dormancy. Leafy spurge (Euphorbia esula L.) is an invasive perennial weed that is estimated to cause annual economic losses of $130 million in the northern Great Plains (Leitch et al. 1996). Leafy spurge develops root buds less than 2 mo after seed germination, and additional buds form throughout the life of the plant (Coupland et al. 1955). Vegetative propa- gation through growth of underground adventitious buds on the root and crown, i.e., root and crown buds, is the primary means of reproduction and maintenance of its pe- rennial nature. These buds are maintained in various states of dormancy throughout the seasonal growth cycle but will develop into new shoots if top growth is damaged or sepa- rated from the roots under environmental conditions con- ducive to growth. Dormancy has been described as the temporary suspen- sion of visible growth of any plant structure containing a meristem (Lang et al. 1987). Dormancy is subdivided into three categories: (1) ecodormancy—growth cessation con- trolled by external environmental factors, (2) paradormancy (correlative inhibition)—growth cessation controlled by physiological factors external to the affected structure, and (3) endodormancy (innate dormancy)—growth cessation controlled by internal physiological factors. In leafy spurge, paradormancy inhibits buds from developing into new shoots through signals generated from the actively growing aerial portion of the plant. In comparison, endodormancy can be triggered by short-day length and cold temperature (Nooden and Weber 1978). Various factors including phytohormones, nutrients, wa- ter status, and temperature affect root bud dormancy in leafy spurge (Harvey and Nowierski 1988; McIntyre 1972; Nissen and Foley 1987a, 1987b). Since the late 1950s, gib- berellic acid (GA) has been known to overcome root bud dormancy in leafy spurge (Shafer and Monson 1958). More recently, Metzger (1994) suggested that sucrose inhibits root bud growth. However, the link between these two substanc- es was not developed until recently. Physiological studies on the responses of root buds to growing apical or axillary-bud meristems and leaves identified two signals, one from mature leaves and one from meristems, causing correlative inhibi- tion (Horvath 1999). The presence of either leaves or grow- ing axillary buds was sufficient to inhibit root bud growth; however, the leaf-derived signal required photosynthesis for its production or transport and could be overcome by the addition of GA. Consequently, it was hypothesized that sug- ar was the basis for the leaf-derived signal. However, pho- tosynthesis was not required for the signal from growing axillary buds, but auxin transport inhibitors blocked this signal. Current models suggest that the leaf-derived signal is responsible for inhibiting the G1/S-phase transition, and the meristem-derived signal is responsible for inhibition of cell division after the S-phase (Horvath et al. 2002). Little is known about the signaling mechanisms of leafy spurge that regulate the transition from correlative inhibi- tion to innate dormancy. Some early studies suggested that postsenescence and flowering may induce innate dormancy