Roses, which are admired for their diversity of floral and vegetative characteristics, are members of Rosaceae, the most important horticultural family in the world. They are the world’s most important ornamental crop, with an estimated annual production of 18 billion cut stems, 60 to 80 million potted roses, and 220 million roses for landscape uses (Blom and Tsujita, 2003; Pemberton et al., 2003; Roberts et al., 2003). In the United States, 36.6 million garden rose bushes are produced, generating sales of $203.5 million (U.S. Department of Agriculture, National Agricultural Statistics Service, 2015)
There are four subgenera under the genus Rosa, which has more than 100 species, ranging from diploid to decaploid. There are more than 30,000 cultivars (mainly diploid, triploid, and tetraploid), with a wide interspecific and intraspecific cross-compatibility (Blechert and Debener, 2005; Byrne and Crane, 2003; Cairns, 2000; Jian et al., 2010; Ueckert et al., 2015; Zlesak, 2009). The role roses play in landscapes is extensive, with their use adorning roadsides, public parks, commercial properties, and residential areas. Garden roses provide aesthetic value throughout the growing cycle for both their vegetative and reproductive organs (De Vries and Dubois, 1996).
For ornamental crops like roses, plant architecture is the key factor determining the appearance of the plant and its commercial value. In recent years, researchers have constructed models that describe the plant architecture of rose bushes according to growth and branching processes. These traits can be characterized morphologically by their number, length, and diameter; by the way they are connected; and by their positioning in space (i.e., shoot angles).
Roses have a wide diversity of plant architectures, ranging from prostrate to compact, highly branched forms to large, spreading to erect bushes to those that climb. Among garden roses, many of these growth forms are useful for specific garden situations, whereas for cut-flower roses, a growth form that produces long stems (50 cm or greater), good basal breaking, and little side shoot formation is required.
The Texas A&M University Rose Breeding and Genetics program has the objective of developing well-adapted (resistant to black spot, Cercospora, and rose rosette disease; tolerant to extreme summer heat), compact, highly floriferous bushes suitable for garden use. Thus, the program is very interested in how a rosebush grows and produces flowers not just during its first flush and growth phase during the spring and early-summer months, but also during the second growth phase in mid- to late-summer and fall months.
The broad-sense heritability of rose architectural traits has been estimated using a set of eight diploid rose cultivars grown in pots in a greenhouse (Crespel et al., 2013, 2014), a small (98 individuals) F1 diploid population planted in the field (Kawamura et al., 2011, 2015), and a biparental F1 tetraploid cut flower population that was planted under greenhouse conditions in the Netherlands and Kenya (Gitonga et al., 2014). In general, researchers found that the architectural traits they measured during the first flush of growth after dormancy (garden-rose situation) or pruning (cut-rose situation) showed a moderate to high broad-sense heritability. Important genotype-by-environment and genotype-by-year interactions were also identified and attributed to differences in total radiation and temperatures that affected total growth rate and budbreak along the stem (Crespel et al., 2014; Gitonga et al., 2014). Kawamura et al. (2011) identified various quantitative trait loci (QTLs) associated with the flower inflorescence structure and identified candidate genes involved in auxin and gibberellin metabolism that controlled node production, internode elongation, and axillary bud branching. Unfortunately, none of these studies analyzed the second growth phase of a garden rose that occurs after the first flush. In Texas, this second growth phase begins in mid to late summer and continues until fall temperatures put the plant into dormancy.
A garden rose needs to flower well not only during this first flush after dormancy, but also throughout the summer and fall months. As the Texas A&M Rose Breeding and Genetics program has the objective of developing roses that bloom continuously throughout the growing cycle and that establish a compact growth form, our experiment was designed to compare the growth of a set of breeding populations at first flush and later at the end of the year to understand the architectural components important in flower productivity and bush shape. The objective of this research was to evaluate the genetic variation and heritability of six plant architectural components within 13 diploid rose populations.
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