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The goal of this study was to prove that aquaponic systems can produce lettuce of equal growth and quality compared to hydroponic lettuce production and to determine the stocking density of fish required for plant growth. Aquaponics is the integration of recirculating aquaculture and hydroponic plant production. The project had four objectives. The first objective was to determine the biomass of fish required for plant growth to develop a fish to plant density ratio. The second objective was to compare lettuce grown with aquaponic water and a hydroponic solution under the same environmental conditions. The third objective was to compare the quality of lettuce grown with aquaponics water plus nutrient supplementation with a hydroponic solution. The fourth objective was to determine the nitrogen dynamics in the aquaponic system and to compare the nutrient composition of lettuce grown with aquaponics water with nutrient supplementation and hydroponic solution. It was determined that under the specified environmental conditions 5 kg m⁻³ of Nile tilapia (O. niloticus) fed 2% of their body weight daily yields on average 4.7 kg m⁻² of lettuce (L. sativa cv. Rex) in 35 days. There was no significant difference (p ≤ 0.05) in biomass or chlorophyll concentration index in lettuce (L. sativa cv. Rex) grown with aquaponics water and nutrient supplements versus a hydroponic solution. The aquaponics solution generated equal biomass and chlorophyll concentration indexes compared to the hydroponic solution. Aquaponics water plus supplementation can yield L. sativa cv. Rex with equal biomass accumulation and chlorophyll concentration indexes compared to hydroponics lettuce. Nutrients added to the aquaponics system consisted of iron, manganese, and zinc. These nutrient concentrations became depleted in the aquaponics water over time and were not replenished via the fish feed. Dolomite was added to the aquaponics system every two weeks to increase the buffering capacity of the water and maintain optimal pH levels. Aquaponics lettuce had similar nutrient composition to hydroponic lettuce. One head of L. sativa cv. Rex (176.75 ± 31.03) will assimilate approximately 5.96 grams of nitrogen (3.38% per dry gram lettuce). One kilogram of fish will yield 6.4 lettuce heads (1,128 grams) and fixate 38.13 grams of nitrogen.

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In this study, RAPD (Randomly Amplified Polymorphic DNA) and SCAR (Sequence Amplified Characterized Region) markers found within 5 centiMorgans of known disease resistance loci in L. sativa were tested for their potential use in MAS. Out of thirty RAPD and SCAR markers evaluated, ten were found to be reliable predictors of disease resistance or susceptibility across a wide range of commercial and reference cultivars. Direct sequencing of seven selected markers did not reveal any significant similarity with known sequences. Three SNPs (Single Nucleotide Polymorphism) associated with two markers found in close proximity to corky root (cor) and Lettuce mosaic virus resistance (mo12) genes were identified. This information was used in the development of a non-electrophoresis PCR-based assay called FRET (Fluorescence Resonance Energy Transfer) hybridization probes assay.

Little is known about the mechanism of virus disease resistance in plants. The aim of the work presented here was to answer whether disease resistance is offered within the cell or at the level of intercellular movement of the virus. The protoplast system was used for this purpose. Conditions were optimized to isolate viable protoplasts from the leaves of Lactuca sativa cultivars. Protoplasts and leaves from resistant and susceptible Lactuca sativa cultivars were inoculated separately with turnip mosaic virus (TuMV) and lettuce mosaic virus (LMV), Virus multiplication was examined over time using enzyme-linked immunosorbent assay. Resistant cv. Kordaat did not support TuMV multiplication in protoplasts as well as in leaves. The results indicated that resistance to TuMV is available within the cell. The results ruled out the possibility of involvement of cell to cell movement and resistance to TuMV seems to be constitutive. On the other hand, protoplasts and leaves from both resistant and susceptible lettuce cultivars supported LMV multiplication. This suggested that resistance to LMV may not be offered within the cell. The results also indicated that the resistance to LMV was partly due to a hypersensitive response though virus was still able to spread systemically. To contribute towards mapping of the Tu resistance gene, the genotype of F$ sb2$ individuals was determined by screening an F$ sb3$ population from 71 F$ sb2$ individuals of a cross between cv. Calmar and cv. Kordaat for TuMV-infection. These data were useful for the production of bulks around the Tu locus to facilitate the search for new molecular markers linked to the Tu gene.

Soil salinity is a common abiotic stress for plants, that is having an increasing impact on international food production. A practical strategy to help mitigate the adverse effects of salinity stress on crop productivity is to increase salt tolerance of crop plants. It has been shown that exogenous application of L-proline and L-glutamate is capable of reducing the severity of salinity stress on seed germination and early seedling growth of brassica and cucumber, respectively. The main aim of the present study was to investigate the potential of all 20 common protein amino acids to alleviate salinity stress in lettuce (Lactuca sativa L., variety ‘Great Lakes’) during and immediately following germination. Sowing lettuce seeds in different concentrations of sodium chloride (NaCl) adversely affected germination and early seedling growth in a dose-dependent manner. After 48 hours of sowing lettuce seeds in 1 mM of any of the 20 exogenous amino acids in the absence of NaCl, it was found that the amino acids also inhibited seedling growth, particularly root elongation. However, in direct treatment experiments involving addition of seven amino acids singly (L-asparagine, L-isoleucine, L-leucine, L-proline, L-phenylalanine, L-tyrosine and L-valine) to an inhibitory concentration (60mM) of NaCl, it was found that lettuce seedling growth was protected from the salt stress. Additionally, seeds pre-treated for 8 hours before germination with L-arginine, L-glycine, L-histidine, L-methionine and L-phenylalanine, showed significant growth recovery after a further 40 hours growth exposed to 60mM NaCl. The measurements of cell size in root maturation zone and mitotic index at the root tip of lettuce seedlings after 48 hours from sowing seeds suggested that it might be possible that some amino acid treatments could affect cell elongation and / or cell division. However, further in-depth investigations are required and warranted to elucidate the mechanism(s) whereby exogenous amino acids could play a role in alleviation of salt stress in lettuce. It is concluded that several L-amino acids have the potential in pre-sowing seed treatment (seed priming technology) to enhance salt tolerance for crop stand establishment in soils with salinity issues.

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Ready-to-eat vegetable salads containing lettuce as a main ingredient have become popular food items in recent years. Microorganisms associated with these products determine their shelf-life, sensory appeal and safety. This thesis investigates the bacterial ecology of lettuce, aspects of their pre-harvest contamination with microorganisms, and the presence of antimicrobial constituents in such produce. Commercial pesticides (insecticides, herbicides, fungicides), used during lettuce cultivation were examined as potential sources of microbial contaminants. None of the pesticide concentrates contained viable microorganisms. After reconstitution in water, two of the pesticides supported growth of inoculated species of Pseudomonas, Salmonella and Escherichia coli. Pesticides reconstituted in agricultural waters (bore, dam and river) supported the growth of microorganisms (e.g. Pseudomonas, Acinetobacter, Aeromonas spp. and coliforms) naturally present in these waters. Unless properly managed, pesticide application could contribute microbial contaminants to vegetable produce, thereby affecting their quality. Bacterial species associated with retail samples of lettuce were examined by plate culture on Tryptone Soy Agar and PCR-DGGE analysis. Macerates and rinses of lettuce sub-samples with and without addition of Tween 80 were examined to maximize bacterial recoveries. Predominant bacteria isolated by agar culture included species of Pseudomonas, Agrobacterium, Curtobacterium and Burkholderia, at populations of 103-106 cfu/g. PCR-DGGE was unable to recover the same incidence of species as agar culture and failed to detect bacteria in many samples. In some samples, PCR-DGGE detected species of Bacillus, Pseudomonas, Serratia and Acinetobacter, not found by culture. Failure of the PCR-DGGE analyses was attributed to interference by plant chloroplast DNA. Preparative agarose gel electrophoresis of lettuce macerates was necessary to remove chloroplast DNA before application of PCR-DGGE analysis. Thirty percent of lettuce samples contained Acinetobacter species at 101-104 cfu/g when examined after culture on minimal salts agar or enrichment in Baumann enrichment medium. Other Acinetobacter media failed to give reliable isolation of these species from lettuce and salad vegetables. Lettuce could be an environmental source of Acinetobacter nosocomial infections. Juices, solvent extracts and supercritical fluid carbon dioxide extracts of lettuce and capsicum samples did not exhibit antimicrobial action against a range of food spoilage and pathogenic bacteria.

Typescript (photocopy) / iv, 206 leaves, [4] leaves of plates : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Entomology, 1980