O trabalho teve como objetivos determinar os efeitos de inundações do sistema radicular em diferentes estádios de desenvolvimento sobre a produtividade das culturas da batata e da alface, identificar a velocidade mínima de rebaixamento do nível freático capaz de evitar prejuízos a estas culturas e obter relações entre a produtividade relativa das mesmas e o índice diário de estresse por excesso de água no solo. Para tal foram conduzidos dois experimentos em lisímetros construídos a partir de tubos de concreto, considerados como parcelas, de julho a outubro de 2001 com a cultura da batata, e de dezembro de 2001 a janeiro de 2002 com a cultura da alface. O delineamento experi-mental adotado foi inteiramente casualizado, arranjado em esquema fatorial [(3 x 4) + 1], com quatro repetições para a cultura da batata e três repetições para a cultura da alfa-ce. Para ambas as culturas os tratamentos consistiram de 3 períodos de aplicação do en-charcamento, 4 velocidades de rebaixamento do nível freático (30 cm em 24, 48, 72 e 96 horas), mais uma testemunha na qual não foi aplicado estresse por elevação do nível freático. A análise dos resultados revelou que dentre os três estádios nos quais o estresse foi aplicado, o que ocasionou maiores perdas para a cultura da batata foi o terceiro (apli-cado do 85o ao 88o dia após o plantio), enquanto que para a cultura da alface o estádio mais crítico foi o primeiro (aplicado do 12o ao 15o dia após o transplantio) . Não foi pos-sível determinar a velocidade de rebaixamento do nível freático mais adequada, pois mesmo com a maior velocidade testada (30 cm em 24 horas), houve decréscimos de cer-ca de 50% da produtividade, para ambas as culturas. Foram obtidas correlações lineares negativas entre as produtividades relativas de ambas as culturas e o índice diário de es-tresse por excesso de água no solo. / This work had as objectivies determinating the effects of root system flooding in three stages of growing of potato and lettuce crops, identifying the minimum water-table drawdown velocity necessary to avoid yield reductions and obtaining relations between relative yield and stress day index caused by excess of water in the soil. For these purposes two experiments were conducted in lisimeters, buit with concrete pipe, considered as parcels, from july to october of 2001, with potato crop, and from decem-ber of 2001 to january of 2002, with lettuce crop. The statistical experimental design was a complete randomizer in a factorial arrangement [(3 x 4) + 1], with four replicati-ons for potato crop and three replications for lettuce crop. For both crops the treatment consisted in 3 growing stages of water-logging application, 4 drawdown velocities (30 cm during 24, 48, 72 and 96 hours) and a control in wich was not applied the stress of rising of the water-table. The results analysis showed that among the three stages where the water-logging were applied, the one who caused bigger yield reduction was the third (applied from the 85th to the 88th day after planting), for the potato crop and the first (ap-plied from the 12th to the 15th day after re-planting), for the lettuce crop. It was not pos-sible to identify the more adequate water-table drawdown velocity because even using the biggest velocity (30 cm during 24 horas) there were a 50% yield reduction for both crops. The potato and the lettuce relative yields showed a negative linear relation with the stress day index.

Master of Science / Food Science Program / Thomas J. Herald / For the last decade in the U.S., consumers have demanded fresh, healthier convenience-type foods namely, fresh-cut vegetables. Globalization has played a major role in rapid growth of the fresh-cut industry sector. Thus, consumers may purchase their favorite seasonal vegetables in local grocery stores year-round. However, the convenience of year-round produce availability brings the potential of foodborne outbreaks. Thirty-two states reported 190 produce-associated outbreaks, 16,058 reported illnesses, 598 hospitalizations, and eight deaths from 1973 to 1997. Pathogenic bacteria contaminate raw agricultural commodities through various pathways such as irrigation with untreated water, use of noncomposted animal manure as fertilizer, and wash water systems. The increasing number of produce-related outbreaks has raised awareness to interventions that remove human pathogens on fresh produce. Washing solely with tap water cannot be relied upon to completely remove pathogens. Chlorinated water is the most frequently used sanitizer, however, reductions are less than 2.63-log CFU/g on leafy and salad vegetables. Such reductions, although significant, are not sufficient to assure the microbial safety of minimally processed vegetables. The efficacy of several other chemical agents such as chlorine dioxide, ozone, electrolyzed water, hydrogen peroxide, organic acids, and other commercial products have been evaluated as potential alternatives to chlorine.

Equidistant, uniform seed placement is important in lettuce production as seeds are densely planted within the row, typically only about 5 cm apart. When seeds are sown too close together, it is time consuming to thin seedlings to the desired plant spacing of 20 to 30 cm by hand and very difficult to do mechanically. The overall goal of the project was to improve lettuce seed placement accuracy and reduce the percentage of seeds spaced closely together. Specific objectives were: 1) to compare vacuum and belt-type planters to determine which style of planter provides better planting performance, 2) to evaluate belt planter seeding performance with different types of furrow openers, and 3) to develop practical modifications for vacuum planters to improve lettuce seed placement accuracy. Three vacuum planter configurations, an unmodified and two reduced seed drop height designs and a belt planter equipped with two types of furrow openers were tested in situ with pelleted lettuce seed at four travel speeds ranging from 1.6 to 4.0 kph. Belt planter performance was significantly better than that of the vacuum planter. Vacuum and belt planters both provided acceptable levels of performance at speeds below 2.4 kph, but at higher speeds, seed placement accuracy declined rapidly. No differences in planter performance were found between the two belt planter configurations tested. The mid-level drop height vacuum planter configuration had significantly better seed placement precision and fewer closely spaced seed spacings as compared to the unmodified vacuum planter. These findings illustrate the significant effect planter type, travel speed, and drop height have on planter performance. They also contradict the common perception that vacuum planters deliver seed more precisely and with fewer closely-spaced seeds as compared to belt planters. Further study is needed to determine the extent to which planter performance affects hand thinning labor costs, automated machine performance, final plant stand uniformity, and crop yield.

Lettuce is becoming an increasingly important vegetable, both as a fresh market product and a ready-to use vegetable, especially in urban areas of South Africa. Nutrient solution concentration is one of the most practical and effective ways of controlling and improving the yield and nutritional quality of crops for human consumption. However, optimal fertilizer concentration for leafy vegetables also depends on the prevailing environmental conditions. This study was carried out to determine the effects of different nutrient solution concentrations and growing seasons on growth, yield and quality of leafy lettuce. The trial was conducted in a black and white shade net structure and the nutrient concentration treatments were 1.0, 2.0, 3.0, and 4.0 mS.cm-1. Measurements taken included: leaf number, leaf area, fresh leaf mass, dry leaf mass, dry root mass, as well as chlorophyll content. The sensory evaluation procedure was only done on plant samples grown during summer and winter seasons. The results showed that growth was less affected by nutrient concentration than by growing season. Regardless of the nutrient concentration, plants grown in summer reached maturity quicker as compared to plants grown in winter. Generally, leaf number, leaf area, leaf area index, fresh leaf mass, dry leaf mass and dry root mass did not significantly increase with increasing nutrient concentrations and therefore, yield was not influenced by nutrient concentrations. Quality was influenced by nutrient concentrations during the summer-autumn seasons where increasing nutrient concentration induced increased chlorophyll content of the leaves. During the winter-spring seasons this phenomenon was not significant. The study demonstrated that growth, yield and quality of lettuce were not significantly influenced by nutrient solution concentrations of 1.0, 2.0, 3.0 and 4.0 mS.cm-1. The sensory evaluation also showed no significant differences on the colour (quality) and flavor of the lettuce samples grown during summer and winter seasons and unfortunately it was not done during autumn and spring seasons. Irrespective of the nutrient solution concentration, growth was influenced by growing season because plants grown during summer reached maturity quicker as compared to plants grown during the other seasons. / Dissertation (MInstAgrar)--University of Pretoria, 2012. / Plant Production and Soil Science / unrestricted

Many studies have investigated the issue of feeding the world’s growing population from different perspectives. Conventional agricultural methods usually have small production yields while requiring large amounts of scarce and unevenly distributed resources such as farmland and water. Furthermore, although produced food may meet the demands, it is still inefficiently delivered among different regions. Urban agriculture has been proposed to produce food inside urban areas with higher yields and less resource consumption. Hydroponics is one of the urban farming methods that needs further research before being applied on large scales. This study aims to investigate the potential lettuce production using hydroponic systems to grow lettuce domestically in urban areas in Sweden to lower its lettuce import and motivate local food production to become self-sufficient. The study is performed using a literature review, theoretical design of a system, and scenario-based assessment of hydroponic lettuce production. The detailed analysis is performed via a case study of lettuce production in Uppsala municipality and two scenarios considering the reduction of lettuce import to Sweden and related challenges and opportunities of the designed system. The analyses demonstrate that the system paves the way to decrease lettuce import and dependence on trade. Therefore, applying the system increases self-sufficiency and decreases vulnerability to shocks. Nevertheless, the study does not necessarily address issues related to transportation and food miles. Moreover, finding suitable places to establish the system needs further studies.

Tomato and lettuce are amongst the most important fresh vegetables used in South Africa. However, growth, yield and quality of tomato and lettuce are constrained by water shortage and poor productivity of sandy soil. In South Africa, large parts of the agricultural land are in a semi-arid region and water is becoming scarcer and more costly. Recognizing the fundamental importance of water-holding amendments like gel-polymers to enhance water use efficiency and soil physical properties, this study was carried out to investigate the effects of pure gel-polymer and fertiliser-fused gel-polymer soil amendments across five irrigation intervals on growth, yield and quality of tomato (Lycopersicon esculentum Mill.) and lettuce (Lactuca sativa L.). The response of tomato growth, yield and quality to irrigation interval and gel-polymer soil amendments (pure gel-polymer and fertiliser fused gel-polymer) was conducted in a tunnel. The gel-polymer treatments were: control (sandy soil), two pure gel-polymer levels (8 and 16 g-20 L-1 sandy soil, equivalent to 400 g and 800 g-m-3) and two fertiliser fused gel-polymer levels (20 and 40 g-20 L-1 sandy soil, equivalent to 1 kg and 2 kg-m-3). Irrigation was either applied once daily or every second, third, fourth or fifth day, equivalent to 0.8, 1.25, 1.45, 1.88 and 2.29 L of water per 20 L bag of sand. Fruit mass, fruit diameter, fruit number, plant height, stem diameter, number of trusses, root fresh and dry mass, total soluble solids, fruit juice pH and titratable acidity were determined. Neither irrigation interval nor gel-polymer amendments had an influence on tomato quality (total soluble solids, pH and titratable acidity). Generally, plant yield, height, stem diameter, number of trusses, and root fresh and dry mass were increased with gel-polymer amendments compared to pure sandy soil. Regardless of irrigation interval, both fertilizer-fused gel-polymer levels appeared to be effective in improving plant growth and yield compared to pure gel-polymer, which gave good results only at the higher level of application. The study revealed that gel-polymer amendments increased productivity of tomato on a sandy soil. Similarly, the response of lettuce growth, yield and quality to gel-polymers and irrigation intervals was investigated under a tunnel conditions. The gel-polymer treatments were: control, two pure gel-polymer levels (4 and 8 g-10 L-1 sandy soil, equivalent to 400 g and 800 g-m-3) and two fertilizer-fused gel-polymer levels (10 and 20 g-20 L-1 sandy soil, equivalent to 1 kg and 2 kg-m-3). Irrigation was either applied daily or every second, third, fourth or fifth day, equivalent to 0.63, 0.83, 1.04, 1.25 and 1.46 L per 10 L plastic bags. Measurements were made of fresh head mass, head height, head circumference, head diameter, stem diameter, fresh root mass, dry root mass and dry head mass. The dried head samples were analysed for percentage tissue calcium and nitrogen. Lettuce grown on sandy soil amended with higher level of pure gel-polymer (Stock 8) and both fertiliser fused gel-polymer levels (Aqua 10 and 20) resulted in significantly higher fresh and dry head mass, head circumference, head diameter, head height, stem diameter, and fresh and dry root mass as compared to low level of pure gel-polymer (Stock 4) and sandy soil without gel-polymer (control). All irrigation intervals did not have an effect on growth, yield and quality of lettuce except at irrigation interval of every third day, which significantly lowered head circumference. Gel-polymer did not have a significant effect on percentage calcium and nitrogen concentration in the leaf tissue. Growing lettuce in soil amended with higher pure gel-polymer (Stock 8) level and both fertiliser-fused gel-polymer (Aqua 20 and Aqua 40) would likely be economically advantageous for a grower due to improved growth and higher yield of good quality lettuce. / Dissertation (MSc (Horticulture))--University of Pretoria, 2005. / Plant Production and Soil Science / unrestricted

This study was undertaken to determine the effect of four different combinations of treatments and storage; i.e., controlled atmosphere (2.5 percent O2 and 2.5 percent CO2), controlled atmosphere in combination with phaltan (1,000 ppm), controlled atmosphere with packaging in polyethylene bags, controlled atmosphere with phaltan and packaging in polyethylene bags, as well as conventional refrigeration, on the chemical composition of lettuce heads (Cultivar "Great Lakes") during 75 days of storage. Analyses of total sugars, starch, reducing sugars, total organic acids, free amino acids, soluble proteins, pH, titratable acidity, and total carotenes were made on the fifteenth, thirtieth, forty-fifth, sixtieth, and seventy-fifth days of storage at 35 F. There were no significant differences in total organic acids and free amino acids between the conventional refrigeration (35 F) and other treatments during the early stage of storage. They were, however, higher after 45 days of storage with phaltan treatments, although still not changed in controlled atmosphere and controlled atmosphere in combination with packaging. The soluble proteins and the reducing sugars were lower in the controlled atmosphere lettuce than in the conventional refrigeration lettuce. The lettuce treated with phaltan or phaltan in combination with polyethylene packaging had higher amounts of soluble proteins. Although pH of the lettuce heads was not changed throughout 75 days of storage, the titratable acidity was higher in all treatments during the storage.

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