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The expression, accumulation and kinetic properties of 1 -aminocyclopropane- 1 -carboxylic acid (ACC) oxidase (ACO), the enzyme which catalyses the final step in the ACC-dependent pathway of ethylene biosynthesis in plants, is examined. The investigation is divided into three sections: (i) identification of two ACO genes in apple leaf tissue, designated MD-ACO2 and MD-ACO3, (ii) kinetic analyses of each of the three isoforms of ACO in apple (MD-ACO1, MD-ACO2 and MD-ACO3) expressed in E. coli, and (iii) temporal and developmental expression in vivo of each of the ACO genes and accumulation of the corresponding gene products in leaf and fruit tissue. The coding regions of putative ACO gene transcripts were generated from leaf tissue using RT-PCR. Sequence alignments and interrogation of the expressed sequence tags (ESTs) database indicated that the entire open reading frame (ORF) sequences were encoded by two distinct genes, and these are designated MD-ACO2 and MD-ACO3. A third ACO gene had been identified in apple by other research workers previously, and designated MD-ACO1. Differences are obvious in the number of base-pairs (bp) constituting the entire ORF of MD-ACO1 (942 bp), MD-ACO2 (990 bp) and MD-ACO3 (966 bp). MD-ACO1 and MD-ACO2 share a close nucleotide sequence identity of 93.9 % in the ORF but diverge in the 3' untranslated regions (3' -UTR) (69.5 %). In contrast, MD-ACO3 shares a lower sequence identity with both MD-ACO1 (78.5 %) and MD-ACO2 (77.8 %) in the ORF, and in the 3'-UTR (MD-ACO1, 68.4 %; MD-ACO2, 71 %). A comparison of the gene structures show that the endonuclease restriction sites are unique to each individual MD-ACO sequence. Genomic Southern analysis, using probes spanning the 3'-UTR and the 3'-end of the coding region confirmed that MD-ACO3 is encoded by a distinct gene. However, while the distinction between MD-ACO1 and MD-ACO2 is not as definitive, different gene expression patterns adds credence to their distinctiveness. Each of the three

The aim of this thesis was to investigate some aspects of breeding for quantitative traits in apple. First, this study explored the measurement and inheritance of a complex quantitative trait (fruit shape). Fourier analysis was used to mathematically describe apple fruit shape in an objective manner and principal component analysis grouped the Fourier descriptors into meaningful shape traits. Heritabilities were estimated to determine genetically inherited shape traits, and genotype by environment interactions were estimated to determine the stability of the trait expression across environments. Fruit aspect ratio accounted for over 76% of the phenotypic variation in shape, fruit conicity for 6% and fruit squareness for only 2%. These traits had moderate to high narrow-sense heritabilities (0.79, 0.38 and 0.38 respectively), which indicated that individual selection would be efficient. High between-site genetic correlation (rA > 0.8), indicated low genotype-by-environment interaction and suggests breeding based at one site would be efficient for altering shape at other sites. A shape chart was constructed to enable these three apple shape traits to be evaluated quickly and accurately in the field, avoiding the need for image capture and Fourier analysis. Second, heritabilities and genetic correlations were estimated for a range of apple traits from a genetically broad-based apple breeding population of 71 families. Narrow-sense heritability was estimated at each of two sites and across sites for fruit acidity (0.17-0.22), fruit sugars (0.25-0.26), and tree growth habit (0.19-0.41). Higher heritabilities were estimated for fruit squareness (0.32-0.43), fruit conicity (0.32-0.46), powdery mildew incidence (0.40), vigour (0.28-0.62), fruit firmness (0.44-0.53), harvest time (0.66-0.82), leafing (0.60-0.83), fruit aspect ratio (0.74-0.89), flowering (0.92) and fruit size (0.90-1.01). Traits with a high narrow-sense heritability could be selected for efficiently with ind