The research described in this thesis focussed on the potential effect of ethylene produced under severe water deficit conditions (SWD) on the concentration of phenolic compounds in fruit and leaves of commercial strawberry (Fragaria x ananassa Duch.). An increased ethylene production rate was detected in green fruit and leaves at midday of the second day of wilting in response to SWD. The stress-induced rise in ethylene production was successfully inhibited by applications of aminoethoxyvinylglycine (AVG) and subsequently re-instated by 1-aminocyclopropane- 1-carboxylic acid (ACC) or replaced by abscisic acid (ABA) in fruits. Initially, fruit total ellagic acid concentration increased along with ethylene production but conclusive evidence for the causal role of ethylene in the accumulation of individual phenolics in fruit from the inhibition, re-instatement and replacement of the ethylene signal was not obtained. The quantitative expression of selected genes in leaves was investigated, and a significant reduction in the relative expression of FaACS1 (coding for ACC synthase), an increase in FaACO1, FaACO4 (coding for ACC oxidase) expression and no change on the expression FaPAL (coding for Phenylalanine ammonia lyase) was observed. Increased leaf ethylene production rate was likely due to increased expression of genes regulating ACC oxidase activity. However, no evidence was found from these genetic studies to confirm the role of stress-induced ethylene production in accumulation of phenolics in fruit or leaves.

The cultivated strawberry (Fragaria × ananassa Duch.) is the third most economically important fruit crop. In recent years the withdrawal of many fungicides and soil fumigants have made the sustainability and profitability of this crop more challenging. To overcome these challenges, plant breeders aim to improve upon existing cultivars and to release new ones with higher yield, better fruit quality and more disease resistance. Through Quantitative Trait Mapping, markers linked to genetic variants associated with traits of economic and agronomic importance can be identified through and molecular markers such as simple sequence repeats (SSRs) and single nucleotide polymorphisms (SNPs), can be used to improve plant breeding efficiency at the molecular level, which significantly reduces the breeding time and cost of phenotyping. In this thesis the following work is described: a correlation analysis of plant characteristics and fruit quality traits; the saturation of an existing SSR-based linkage map; the development of a high density consensus SNP-based octoploid strawberry linkage map, and the identification of quantitative trait loci (QTL) linked to two key plant attributes, fruit quality and powdery mildew resistance. In addition, the most closely linked SSR markers were identified and validated in a wider strawberry germplasm using firmness as an example study. Moreover, the physical locations of expansin genes and SNP markers associated with firmness QTLs were investigated. The purpose of this analysis was to find out if QTLs associated with fruit firmness overlapped the positions of expansins, genes known to be important in controlling fruit firmness.

Date Palm (Phoenix dactylifera L. Family: Arecaceae; sub-family: Poaceae) is thought to be the oldest fruit tree grown in the Sultanate of Oman and is a major crop in terms of the number of trees and their distribution. Dates are widely considered to be a strategic source of food security as well as an important cash crop, and have always been looked upon as a key source of stability for oasis agro-ecosystems. Several insect and mite pests and plant pathogens attack Date Palm trees. The Dubas bug, Ommatissus lybicus DeBergevin (Homoptera: Tropiduchidae), has been considered the key pest of Date Palm plantations in Oman for 40 years due to the total area infested, the severity of infestations and the scale of the crop losses. Current control programmes for Dubas bug are reliant on chemical pesticides, applied by air and from the ground over an area ranging from 12,000-20,000 ha annually. These pesticides are currently the only feasible control measure against this pest, although research is underway on bio-control technologies. The cropping environment resulting from traditional Date Palm cultivation - high planting density, high temperature and humidity, topography of the plantations, intercropping and variability in cultivars and plant height - has an influence on pest and disease incidence and also affects the control options available. The present research attempts to determine the effects of technical, environmental and biological factors on the control of Dubas Bug in order to develop a more integrated control strategy. Data have emerged from this study on: 1) optimal spray droplet spectrum and application rates for improved coverage and placement of spray deposits, including under leaf deposition; 2) information on how Dubas bug daily movement can influence control efficacy; 3) on the selective use of insecticides against Dubas bug at economic threshold levels as a supplement to other IPM methods, including cultural practices such as thinning to maintain optimal distances between trees, irrigation management, the removal of old leaves including pruning cuts and surrounding tissues that may support and harbour Dubas bug.

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The coconut palm (Cocos nucifera L.) is a major crop plant which does not develop axillary shoots and must therefore be propagated from seed. As it is a heterozygous outbreeder, seedlings are variable and only after a long juvenile stage (4-7 years) can their performance be assessed. Thus the establishment of a reliable micropropagation method for elite palms is required. Much research has been devoted to this, but the recalcitrant character of the coconut palm in vitro has limited progress. The induction of embryogenic callus is the first stage in such a procedure and is the subject of this thesis. Immature inflorescence has been much used as a source of explants, but the rates of embryogenic callus production have been low. Values not exceeding 0.1% with low reproducibility were obtained in this investigation and therefore alternative sources of tissue have been explored. Experiments with zygotic embryos showed that while mature embryos were unsatisfactory for the culture of embryogenic callus, immature zygotic embryos yielded callus in considerable amounts. Isolation of plumular tissue from mature zygotic embryos gave embryogenic callus in around 80% of explants. This success level exceeded other systems reported to date. Development of clonal plantlets from plumular callus further indicated the significance of this as a useful tissue for further research. The synthetic auxin 2,4-D is normally used for the initiation of callus, but other synthetic auxins with related chemical structures were also examined. The highest yields of embryogenic callus (85 %) were obtained from plumules incubated with picloram at 6 x 10"* M in media containing activated charcoal (AC). Levels up to 2 X 10^ M picloram were also tolerated. The effect of AC in reducing the availability of regulators in media has been examined, using radio-labelled fluroxypyr acid, clopyralid and 2,4-D. Traces of suspended AC in media held a significant proportion of the auxin in solution. The release of adsorbed auxins from AC in culture media was achieved by the addition of certain organic acids. The monoclonal antibody ICS which hybridised with a 50 Kda protein in early embryogenic cultures of various monocotyledonous species was found to hybridise to a 50 Kda protein in coconut. Whether the presence of this protein is related to the embryogenicity of coconut cultures remains to be determined.

Nectria-canker of apple, caused by the fungus Nectria galligena Bres. (anamorph Cylindrocarpon heteronemum (Berk. And Broome) Wollenw.) produces serious losses to both young trees and stored fruit but the severity of the disease is highly variable between orchards and regions. This thesis describes experiments which were undertaken to identify mechanisms underlying the predisposing factors in Nectria canker expression, and thus to provide information on control strategies. A three-years study was carried out to determine if trees which became infected with N. galligena during propagation in the nursery expressed symptoms of canker as a result of water stress after lifting from nursery due of 13, 29, 43 and 56 days in cold store before planting. The experiment was subsequently extended to compare the susceptibility of these trees subjected to inoculum applied in the second year. The effect of nitrogen fertiliser on susceptibility was also tested. At the time of planting all of the trees appeared to be healthy and free of any symptom of infection. However in May, it was evident that some terminal buds on some of the trees held 29 days or more in store were dying. In the ensuing weeks the affected areas extended down the shoots resulting in obvious die-back. A number of fungal species were isolated from samples taken from these shoots, predominantly Aureobasidium sp. However N. galligena was not detected by direct isolation and diagnostic PCR tests also proved negative. Furthermore, no symptoms indicative of a natural infection by N. galligena were detected during the second growing season. The percentage of leaf-scars which became infected following inoculation with 50 and 100 conidia increased significantly with increasing time in store prior to planting. At concentrations greater than 500 conidia per leaf scar the effect of the delayed planting treatment was apparent only for trees held for more than 29 days. Therefore, it was demonstrated that even two years after planting trees which had been subjected to severe water stress in store were more susceptible to infection by artificial inoculation. The calculated EC50 values indicated that in the absence of nitrogen supplements 2-4 times as many conidia were required to infect leaf scars on trees planted ABSTRACT immediately on receipt as those on trees held in cold store 43 days or more. The application of N-fertiliser in the spring and early summer generally increased the percentage of leaf scars which became infected following inoculation at the next autumn leaf-fall. These differences were significant (P<0.05) for N. galligena isolates at the lowest inoculum dose but not at concentrations of conidia greater than 100 per scar. Experiments made to determine the distance that conidia could be carried within the transpiration stream of shoots of apple at different stages of development within the 1997 season indicated that the results varied widely with respect to the isolate of N. galligena used in the test. In each of the four months that the experiment was repeated, N. galligena isolate W74 was recovered significantly higher up the shoots of all cultivars than isolate W69, which was not detected in Bramley's Seedling at any time. The greatest distance of upward transport of W74 was recorded for Gala (18.4 cm) whereas the corresponding distance for Bramley's Seedling was less than half of that (8.8 cm). Approximately 94% of 178 xylem vessels of Bramley's Seedling had a crosssectional area of 400 |im or less, by contrast 75% of 172 vessels of Gala were 600 pm or greater. Therefore, the relative diameter of the xylem vessels could be a factor determining colonisation by this pathogen. The EC50 test enabled cultivars to be ranked in order of their known field resistance; Bramley > Cox's Orange Pippin > Discovery > Gala. The EC50 for Bramley was lOX greater than for the other cultivars. The same test indicated that the mean EC50 value for leaf scar infection for isolates obtained from fruit rots was approximately three times greater than for isolates obtained from stem cankers. Tests for their ability to rot ftuit revealed that few stem isolates could initiate progressive rots, but most fruit isolates caused extensive rotting. Significant negative correlations (f<0.001) were found between the volume of fruit tissue rotted and protease production for N. galligena isolates of UK and Chile. Protease elicits the antifungal compound benzoic acid in fruit, and the possibility that low protease producing types are selected by fruit infection from a segregating population of the pathogen in orchards is discussed.

No description available.