Magnolia virginiana var. virginiana (sweetbay magnolia) is a native component of the flora of the eastern U.S. coastal plain and mid-Atlantic states and is used as an ornamental plant in commercial and residential landscapes. The species is noteworthy for its large, fragrant white flowers produced over a long period, starting in mid-May and lasting until mid-July (Blake and Struve, 2007). This magnolia is one of the most adaptable for the midwestern United States, growing well on a range of soil pH levels and soil types and surviving winters in U.S. Department of Agriculture (USDA) plant hardiness zone 5 (Callaway, 1994; Nash and Graves, 1993; Spongeberg, 1976; Treseder, 1978). Additional ornamental traits include a late summer display of red follicles, semievergreen to evergreen foliage, aromatic leaves and bark, and use as a plant attractive to wildlife (Priester, 1990).
Although various cultivars of M. virginiana var. virginiana have been described, none plays a dominant role in the nursery industry; these selections are the result of happenstance rather than systematic improvement programs. We have defined two desirable ideotypes for M. virginiana: first, a compact shrub form 2 to 2.5 m tall and 2 to 2.5 m wide with abundant flowering; and second, a single-stemmed small tree with a mature size of 3 to 4 m tall and 2 to 3 m wide and with abundant flowering. Ideally forms would be evergreen, hardy to USDA plant hardiness zone 5, adaptable to various soil types, easily propagated by sexual or asexual means, and amenable to container and field nursery production systems.
Extensive phenotypic variation occurs in M. virginiana, and there exists opportunity for selection toward our proposed ideotypes (McDaniel, 1966, 1970, 1984; Treseder, 1978). Cultivar selections have been made on various characteristics such as evergreen foliage, quality of foliage coloration, precocious flowering, shrub and tree form habit, rapid growth, and hardiness; but none of these individual selection criteria fully embodies the ideal cultivar described by our ideotypes (Callaway, 1994; Gardiner, 2000; McDaniel, 1984; Treseder, 1978). Magnolia virginiana is comprised of two botanical varieties: The more southern M. virginiana var. australis and the northern M. virginiana var. virginiana each have some of the desired characteristics, but neither has the full combination of traits. For example, the northern botanical variety tends toward a multistemmed, shrub-like growth habit, whereas the tree form predominates in the southern botanical variety (Callaway, 1994). Magnolia virginiana is later flowering and tends to escape the late spring frosts that damage the flowers of the popular precocious flowering species of Magnolia (subgenus Yulania). We have noted that some individuals repeat bloom during the late summer, beginning mid-August, and can flower until the onset of frost in the Columbus, OH, area.
The limited breeding work that has occurred with M. virginiana has focused on the creation of interspecific hybrids with the goal of improved flower quality, flower color, or flowering period. There have been no reports of studies on other morphological characteristics of M. virginiana such as height, branching pattern, or flower abundance that could be incorporated into schemes of general species improvement for varied landscape applications or for development strategies for novel characteristics. We developed a diverse population suitable for evaluation of morphological and flowering characteristics and to assess whether a container production system could be used to accelerate genetic testing (Struve and McKeand, 1993). The objective was to determine the degree of genetic control over these traits when plants were grown in a container nursery and examine genetic variation in the population to help guide decisions regarding plant selection and cultivar development.
Azuma, H., Figlar, R.B., Del Tredici, P., Camelbeke, K., Palmerola-Bejerano, A. & Romanov, M.S. 2011 Intraspecific sequence variation of cpDNA shows two distinct groups within Magnolia virginiana L. of eastern North America and Cuba Castanea (in press)
Cline, M.G. & Harrington, C.A. 2007 Apical dominance and apical control in multiple flushing of temperate woody species Can. J. For. Res. 37 74 83
Cockerham, C.C. 1963 Estimation of genetic variances 53 93 Hanson W.D. & Robinson H.F. Statistical genetics and plant breeding. Natl. Acad. Sci Natural Resource Council Washington, DC
Dirr, M.A. 1998 Manual of woody landscape plants: Their identification, ornamental characteristics, culture, propagation, and uses Stipes Champaign, IL
Dudley, J.W. & Moll, R.H. 1969 Interpretation and use of estimates of heritability and genetic variances in plant breeding Crop Sci. 9 257 262
Figlar, R.B. 2000 Proleptic branch initiation in Michelia and Magnolia subgenus Yulania provides basis for combinations in subfamily Magnolia Proc. Intl. Symp. Family Magnoliaceae 2 14 25
Franklin, E.C. 1979 Model relating levels of genetic variance to stand development of four North American conifers Silvae Genet. 28 207 212
Herms, D.A. & Mattson, W.A. 1991 Does reproduction compromise defense in woody plants? 35 46 Baranchikov Y.N., Matison W.J., Hain F.P. & Payne T.L. Forest insect guilds: Patterns of interaction with host trees U.S. Dept. Agr. For. Serv. Gen. Tech. Rpt. NE-153
Ishida, K., Yoshimaru, H. & Hiroki, I. 2003 Effects of geitenogamy on seed set of Magnolia obovata Thunb. (Magnoliaceae) Intl. J. Plant Sci. 164 729 735
Luken, J.O. 1987 Interactions between reproduction and vegetative growth in staghorn sumac Rhus typhina L Bull. Torrey Bot. Club 114 247 251
Nash, L.J. & Graves, W.R. 1993 Drought and flood stress effects on plant development and leaf water relations of five taxa of trees native to bottomland habitats HortScience 118 845 850
Priester, D.S. 1990 Magnolia virginiana L. sweetbay 449 454 Burns R.M. & Honkala B.H. Silvics of North America Vol. 2 Hardwoods U.S. Dept. Agr. For. Serv. Agr. Hdbk. 654
Rweyongeza, D.M., Yeh, F.C., Dancik, B.P. & Dhir, N.K. 2003 Genetic variation in height, branch, and needle lengths of Pinus sylvestris L. from Siberia tested in Alberta, Canada Silvae Genet. 52 52 60
Vargas-Hernandez, J.J., Adams, W.T. & Joyce, D.G. 2003 Quantitative genetic structure of stem form and branching traits in douglas-fir seedlings and implications for early selection Silvae Genet. 52 36 44
Yong-Bi Fu, G., Clarke, P.Y., Namkoong, G. & Yanchuk, A.D. 1998 Incomplete block designs for genetic testing: Statistical efficiencies of estimating family means Can. J. For. Res. 28 977 986