Lettuce produced in the desert typically shows large yield responses to N fertilization. However, concern about the potential threat of nitrate-N to ground water has prompted additional studies aimed at developing improved N management practices. Field experiments were conducted between 1992 and 1994 to evaluate the response of iceberg lettuce to N rate and N management practice. The use of controlled release N sources (CR19 were compared to a soluble N fertilizer applied preplant (PP), and a soluble N fertilizer applied in split-sidedress applications (SD). Rates of N fertilizer application ranged from 0 to 300 kg ha⁻¹. Lettuce generally showed significant responses to N rate and N management practice. However, response to management practice varied by site-season. When conditions for N losses were high, SD and CRN management strategies were superior. However, in other site seasons SD management sometimes resulted in inferior head quality and marketable yield when compared other management strategies. Data averaged over six -site seasons shows improved yield and quality to CRN management strategies compared to PP and SD strategies. Although the controlled-release fertilizers used in these experiments cost three times more the conventional soluble sources used, preliminary analysis shows the use of CRN strategies would sometimes be economically favorable.

Les activités humaines agricoles, urbaines et industrielles, sans cesse croissantes, sont à l'origine d'une contamination de notre environnement par les métaux lourds. Alors que de nombreuses molécules organiques peuvent être dégradées, les métaux lourds ne le peuvent pas et leur concentration augmente régulièrement dans les sols et les eaux. Ceci expose les plantes à des concentrations croissantes de métaux lourds. L'accumulation de métaux lourds dans les plantes présente un risque toxique pour l'Homme, car les plantes cultivées sont le point d'entrée dans la chaîne alimentaire. Par comparaison aux autres espèces, la laitue «Lactuca sativa» a tendance à présenter des teneurs élevées en cadmium, un métal lourd très toxique dont la concentration croît régulièrement dans les sols cultivés pour des raisons environnementales. En termes de sécurité alimentaire, il est donc important de produire des variétés accumulant des teneurs en cadmium réduites, et la laitue est un bon modèle pour débuter ce type d'approche. L'objectif de notre thèse était de caractériser aux niveaux physiologique, génétique, et moléculaire, les déterminants majeurs contrôlant l'accumulation du cadmium chez la laitue. Notre objectif a été décliné en différentes opérations. Tout d'abord une analyse de diversité nous a permis de décrire la capacité d'accumulation de cadmium de 18 génotypes de laitue et de sélectionner des génotypes présentant des performances extrêmes vis-à-vis du cadmium. Ainsi, des génotypes présentant des performances extrêmes du point de vue de la tolérance au cadmium, de l'accumulation de cadmium et de la capacité de translocation du cadmium des racines vers les parties aériennes ont été sélectionnés. Dans une deuxième étape, nos variétés extrêmes pour les caractères d'accumulation de cadmium et de translocation de cadmium des racines vers la partie aérienne ont été utilisées pour développer une approche physiologique. L'objectif de cette approche était de montrer l'origine de leur variabilité afin de comprendre les déterminants physiologiques contrôlant l'accumulation du cadmium chez la laitue. Dans le cadre de cette approche, des expériences d'efflux faites en utilisant du cadmium radioactif (109Cd) ont montré que la variété la moins accumulatrice de cadmium se caractérise par un efflux de cadmium marqué plus important par comparaison aux autres variétés. Cette variété possède vraisemblablement un mécanisme d'efflux plus efficace par comparaison aux autres variétés qui pourraient être le déterminant majeur permettant d'obtenir une plus faible accumulation de cadmium chez cette variété. Notre étude a permis de montrer une forte corrélation positive entre teneur en cadmium et teneur en zinc. Ce qui a permis d'émettre l'hypothèse d'un transport très associé entre le cadmium et le zinc. / Cadmium is an extremely toxic pollutant causing a great number of diseases. It is a widespread metal which concentration rises in cultivated soils, thus exposing food or feed crops to it and ultimately causing potential major sanitary problems as a consequence of its entry in the food chain. Among the vegetable species used for food, lettuce shows one of the highest capacities of accumulating cadmium. Our objectives are to characterize mechanisms controlling cadmium accumulation in this species and identify the corresponding genetic determinants, ultimately aiming at breeding lettuce for cadmium under-accumulation.A phenotypical analysis of cadmium accumulation was carried out on 18 lettuce genotypes representing the genetic diversity of the species, revealing a great variability of response, both with respect to cadmium accumulation and cadmium translocation from roots to shoot. The lettuce genotypes displayed independent variations for both traits, and also between these traits and cadmium tolerance. In contrast, a very strong positive correlation linked cadmium and zinc accumulation. Increasing calcium and iron concentration in the culture medium had a protective effect against cadmium accumulation, however this characteristic did not discriminate the lettuce genotypes presenting extreme cadmium accumulation phenotypes. Interestingly, experiments measuring 109Cd influx in roots, 109Cd efflux from roots and 109Cd translocation from roots to shoots revealed that the genotype displaying the least cadmium accumulation could be discriminated from the genotype displaying the highest one by its markedly increased ability to efflux cadmium from the roots to the culture medium. Progeny analysis from crosses between the genotypes displaying extreme performances for cadmium accumulation, cadmium translocation from roots to shoots and cadmium tolerance revealed that none of these traits was supported by a single genetic determinism. QTL mapping is under development. Surprisingly, the genetic analysis demonstrated that the ability to limit cadmium accumulation (and thus probably to increase cadmium efflux from the roots) was recessive.In conclusion, phenotypic analysis of cadmium accumulation in a large set of lettuce genotypes revealed that a major determinant limiting cadmium accumulation in these species is the ability to efflux cadmium from the root to the culture medium.

Thesis submitted in fulfilment of the requirements for the degree Master of Technology: Horticulture in the Faculty of Applied Sciences at the Cape Peninsula University of Technology, 2012 / This article based thesis includes two individual studies evaluating the role of silicon (Si) in mitigating the negative effects that are associated with sodium chloride (NaCl) induced toxicity in lettuce (Lactuca sative L. 'Eish!').

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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.

Comparisons between subsurface trickle-(drip) and furrow-irrigated leaf lettuce, scheduled by AZSCHED and by tensiometers were made to evaluate their effects on crop coefficient, crop water use, nitrogen uptake and nitrogen leaching. A field experiment with four treatments and five replications was conducted during the fall-winter 1994-95 growing season at The University of Arizona's Campus Agricultural Center. Results showed that there was no significant difference at 95% confidence level among treatments, with respect to crop coefficients. A Fourier series was fitted to represent the Growing Degree Days (GDD)--Crop coefficient (Kc) relationship that can be recommended for irrigation scheduling of leaf lettuce at any geographical situation and planting date. A table of Kc for "days after thinning" was derived for Tucson-AZ. Marketable yield averaged 25.0 Mg ha⁻¹ with crop water use efficiency of 9.8 Kg of marketable yield per cubic meter of water (including rainfall). The 1994-95 growing season was not typical for Tucson-AZ. There was 181 mm of rainfall compared to the long term average of 91 mm. Based on the studied condition we could also conclude that, for supplementary irrigation, that is typical of humid areas, there is no difference if one irrigate leaf lettuce by short-end-closed furrow or subsurface - trickle irrigation, scheduled either by tension of 20 kPa or AZSCHED software, concerning yield and nitrogen uptake. The risk of nitrogen leaching is higher for furrow irrigation systems than for drip.

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This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project.

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No description available.