Date of Award

2016

Rights

Access is available to all users

Document Type

Thesis

Degree Name

Master of Science (MS) in Biology

Department

Biology

Abstract

"Germplasm repositories are an invaluable resource for the collection and preservation of important living genetic material, and provide a multitude of research opportunities for crop improvement. Advances in genomics research have created an urgent demand for phenotypic data in order to assist researchers in identifying relationships between genetic makeup and phenotypic variability. The peach, Prunus persica, is an important economic crop species that is heavily bred and cultivated. In peach, specific tree growth habits are used in breeding to select for tree architectures that may provide growers with more options for orchard design and cultural practices. The presence of extrafloral nectaries on leaf petioles has been found to confer beneficial associations and is linked to specific disease resistant qualities. The canopy volume plays an important role in capturing photosynthetically active radiation and can help growers predict several important economic outcomes, and shoot growth characteristics, like absolute growth rate and branch extension rates affect fruit development as well as overall growth habit, thus playing an important role in determining fruit yield. The goals of this project were to assess five phenotypic traits among 364 peach genotypes in 9 distinct taxa that consist of cultivars, breeder's selections, and wild relatives, held at the National Clonal Germplasm Repository in Davis, CA. The recent repropagation of these accessions provided a unique opportunity to establish baseline data on a large set of uniformly aged trees. The growth habit of all accessions was primarily standard (55%), while open, upright, and compact forms were less common (36%, 6%, and 3% respectively). The wild type accessions were slightly more upright (10% higher) than cultivated accessions, while the cultivated accessions were slightly more open (5% higher) than wild-type accessions. Extrafloral nectaries were present in 97% of accessions, likely due to the strong influence of breeding programs that have sought to eliminate eglandular phenotypes. Mean canopy volumes, absolute branch growth rates and branch extension rates differed significantly among the 9 taxa and between wild and cultivated genotypes, but were not significantly different within all taxa. The branch absolute growth rates, branch lengths (of pruned branches over 0.5 cm at base), branch extension rates and canopy volumes were consistently higher in two wild-type accessions, P. davidiana and P. mira, than in the persica group. The P. persica accessions had significantly different branch growth and extension rates, and canopy volume, while both P. persica var. persica and P. persica var. nucipersica did not have significantly different branch growth and extension rates, and canopy volumes. The wild-type species, along with the hybrids and P. spp, had greater variation among branch growth characteristics (length and extension rates, and canopy volume). The first two principal components explained 76% of the total observed phenotypic variability using 8 variables. A hierarchical cluster dendrogram with 4 groupings placed most cultivated accessions (P.persica, P. persica var. persica, P. persica var. nucipersica, and P. spp) in one group along with a wild-type P. ferganensis accession. Another grouping included P. davidiana and P. hybrids, while the final two taxa, P. mira and P. kansuensis were each grouped individually. The PCA and cluster analysis both primarily grouped wild accessions separately from cultivated accessions (with the exception of a P.ferganensis accession), and combined P. persica varieties with P. persica. The groupings created by both analyses suggest P. persica var. persica and P. persica var. nucipersica are phenotypically more similar to P.persica, while the wild relatives differ substantially from one another and the persica group. The results suggest there is substantial phenotypic variability within the repository collection and our characterization of that variation will be invaluable to growers and breeders seeking information on specific growth characteristics. The variation predominantly reflects the historical objectives of breeding and cultivation, and serves as a valuable tool for the development of new cultivars through the use of wild and cultivated genotypes"--Leaves iii-v.

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