Seedling emergence, when the dormant seed germinates and begins life as a seedling, is the most fragile of stages in the life cycle of annual plants. The timing or phenology of seedling emergence can strongly infl

American Journal of Botany 98(6): 975–985, 2011; http://www.amjbot.org/ © 2011 Botanical Society of America Seedling emergence, when the dormant seed germinates and begins life as a seedling, is the most fragile of stages in the life cycle of annual plants. The timing or phenology of seedling emergence can strongly infl uence plant fi tness, as well as the composition of a plant ’ s neighborhood, the environment which it experiences, its potential mates, and the maternal effects it imparts to its progeny. Many environmental factors that vary across a species ’ range, over time, and among microsites can infl uence emergence time ( Cruden, 1974 ; Kalisz, 1986 ). In some environments, seedling emergence is fi nely tuned to particular cues, such as the arrival of a limiting resource. For instance, spring monsoon rains can induce rapid emergence of desert annuals, while light fl ashes and nutrient pulses indicative of tillage stimulate seeds of agricultural weeds ( Venable, 1989 ; Dyer, 1995 ; Baskin and Baskin, 2001 ). These examples indicate some level of genetic control over emergence time, germination, and dormancy, all of which have been shown to be genetically variable traits ( Platenkamp and Shaw, 1993 ; Baskin and Baskin, 2001 ; Finch-Savage and Leubner-Metzger, 2006 ; Mercer et al., 2006 ; Leger et al., 2009 ). Biotic or abiotic conditions present during seed ripening may affect variation in seed germination, such that seeds from different maternal plants ( Roach and Wulff, 1987 ) or from samples within plants (e.g., Simons and Johnston, 2000) may differentially emerge. Effects of paternal variation have also been demonstrated, as in a study of percent emergence among seeds produced by crosses between plants of more and less distant provenances ( Waser et al., 2000 ). Thus, it is important to understand both the genetic and environmental factors that could affect variation in emergence timing and selection on this crucial trait. Emergence timing can have varying effects on plant survival, growth, and fecundity ( Verd ú and Traveset, 2005 ), as well as on patterns of genetic variation found in the standing population ( Cabin et al., 1997 ). Therefore, variation in emergence, whether genetically determined or not, may play an important role in directing the early life history and the ultimate fi tness of plants ( Roach and Wulff, 1987 ; Stratton, 1992 ; Geber and Griffen, 2003 ). Early, intermediate, and late emergence timings can all be benefi cial for particular species or conditions ( Baskin and Baskin, 2001 , p. 573), and a meta-analysis by Verd ú and Traveset (2005) found that early emergence benefi ted growth and fecundity more strongly than survival. Early-emerging seedlings may have an advantage because of (1) greater availability of resources, (2) protection from disease or predators, or (3) an indirect relationship between emergence time and seed 1 Manuscript received 13 October 2010; revision accepted 4 March 2011. We thank K. Sparks, M. Kost, S. Bodbyl-Roels, B. Begay, D. Finch, B. Roddy, J. Moody-Weis, as well as the University of Kansas Field Station staff, including B. Johanning and V. Salisbury, for help on fi eld work in Kansas, and P. Sweeney, B. Maxwell, N. Katema, S. Primer, and the Waterman Farm staff, including G.Mills, for help with crosses and laboratory work in Ohio. In addition, N. Slade, C. Holloman, and R. Shaw provided helpful statistical advice. The study was conducted at and supported by the University of Kansas Field Station, a research unit of the Kansas Biological Survey and the University of Kansas, and by a USDA Biotechnology Risk Assessment Grant (CSREES 2006-03681). This is publication number 909 of the University of Kansas Field Station. 5 Author for correspondence (e-mail: [email protected])