Vitex L. is a genus of ≈250 species distributed throughout the world. Vitex has recently been moved taxonomically from the Verbenaceae to the Lamiaceae (Judd et al., 2002). Introduced to the United States in 1570, Vitex agnus-castus is used ornamentally as a shrub border or as a specimen planting (Gilman and Watson, 1994). Vitex rotundifolia L.f., native to East Asia, Australia, Pacific Islands, and Hawaii (Wagner et al., 1999), was originally introduced to the United States by the J.C. Raulston Arboretum at North Carolina State University, Raleigh, NC, in the 1980s. It has been used for dune stabilization and as an ornamental.
Phenotypic plasticity, or environmentally dependent phenotypic expression, is an important consideration in a plant breeding program because it can confound selection schemes and greatly reduce selection effectiveness. Developmental patterns, reproductive timing, and breeding systems are all affected by the environment (Sultan, 2000). Reproductive traits are, in general, less plastic than vegetative traits (Bradshaw, 1965; Frazee and Marquis, 1994); however, earlier flowering was a response to stress in Arabidopsis L. (Westerman and Lawrence, 1970), and water stress decreased flower production in Clarkia unguiculata Lindl. (Smith-Heurta and Vasek, 1987), Lavandula stoechas L. (Herrera, 1991), and species of Phlox (Schlichting, 1986).
Breeders generally attempt to improve a number of traits simultaneously. If these traits are positively correlated, the response to selection will be more rapid than for characteristics selected separately. Conversely, attempting to select for traits with negative genetic correlations can slow their rate of simultaneous improvement (Antonovics, 1976; Lande, 1982). Trait incorporation may even be delayed for many generations if several important traits are negatively correlated (Lande, 1980). Harding et al. (1981, 1987, 1990, 1991) have studied heritability and correlation of traits in the cut flower Gerbera hybrida. Traits with high heritability generally had high correlation values with each other. Traits having low heritability and low correlation values were typically inflorescence traits resulting in the lowest efficiency of selection (Harding and Huang, 1998; Huang et al., 1990).
New ornamental cultivars must display horticultural superiority when grown both in containers and in the field. However, limited research has been conducted to determine whether initial evaluations would be more effective in the container or field. Numerous studies involving agronomic crops have addressed the issue of trait stability in varying environments (EdSouza, 1993; Fukai and Cooper, 1995; Kang, 1997; Patil and Deshmukh, 2000), but very few have evaluated variation between field-grown and container-grown plant material. One such study (Booker et al., 2005) examined seed yield between field-grown and container-grown soybean plants. Researchers found that values for most reproductive traits as well as stem biomass were less in containers than in their field-grown counterparts. Increased variability in characteristics of container-grown plants may be the result of higher fluctuations in root zone temperatures when compared with field-grown plants (Fiscus et al., 2007; Ingram et al., 1993; Ruter, 1993).
Ornamental research that has compared field-grown with container-grown plant material has focused primarily on the differences in growth rate and rooting characteristics and not on the effect on ornamental traits. Ruter (1993) evaluated growth differences in above-ground container-grown and in-ground pot-in-pot production. The pot-in-pot treatment increased the root dry weight in Lagerstroemia and Magnolia and increased the root–shoot ratio of Lagerstroemia. In a similar study, Magnolia grandiflora ‘St. Mary's’ plants were taller in the pot-in-pot treatment (Ruter, 1995). In Myrtus communis L. (common myrtle), however, Miralles et al. (2009) observed greater plant height and shoot dry weight in the above-ground container treatment compared with those grown in the pot-in-pot treatment.
Woody ornamentals are typically sold in containers for eventual use in the field. Container performance is important to growers and retailers, because growth and appearance in a container influence sales and profitability. Ultimately, however, field performance is critical to end-user satisfaction. Therefore, it is important to determine trait expression in both environments. No research has been conducted on field versus container variation in Vitex. The objectives of this study were to evaluate parents and segregating populations of Vitex in both containers and in the field to determine whether quantitative traits of breeding interest were expressed similarly in the two environments and to determine trait correlations in each environment.
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