Head Lettuce Research Articles

Agent‐based simulation of cross‐contamination of Escherichia coli O157:H7 on lettuce during processing and temperature fluctuations during storage in a produce facility. Part 2: Model implementation

AbstractAn agent‐based simulation was developed to represent the spatial and temporal Escherichia coli O157:H7 cross‐contamination dynamics in a processing facility for fresh‐cut romaine lettuces using NetLogo. An extension was added to the model to predict E. coli O157:H7 growth due to temperature abuses in a cold storage facility. A user‐friendly interface was created to follow variations in model outputs. The number of contaminated bags and lettuce contamination levels were computed and visualized. Experimental data of cross‐contamination from literature was used to describe the facility and validate the model. The key factor affecting cross‐contamination in the process is the free chlorine (FC) concentration dose rate. The number of contaminated bags per batch decreases nonlinearly as FC is added to the flume tank. The contamination level in the shredded lettuces increases linearly with initial microbial load or the initial contamination probability on lettuce heads. Both the level and probability of contamination in the facility's environment alter the number of contaminated bags. Batch size influences the number of contaminated bags when the first incoming lettuce batch is contaminated. The smaller the batch size the smaller number of contaminated bags. Storage room temperature fluctuations (13 and 19°C and 8 and 12°C) show the importance of real‐time monitoring to avoid pathogen growth and number of contaminated bags. This work provides insight on applications of agent‐based modeling approach using NetLogo to visualize cross‐contamination in fresh‐cut leafy green processing operations. It analyzes cross‐contamination and its impact on processing performance by studying the effect of mitigation strategies.Practical applicationsTo support ready‐to‐eat leafy greens postharvest operations, this work investigated the benefits of using visualization to develop strategies to prevent produce cross‐contamination along processing and the effect of temperature abuse during storage. Output from the model can be used in Quantitative Management Risk Assessment (QMRA) applications. By developing a simulation‐based decision support system, the safety of processing and storage steps of fresh‐cut leafy greens are investigated. Pathogen cross‐contamination and growth models are integrated to observe impacts of varying contamination levels in incoming produce and equipment surfaces, concentrations of free chlorine in the wash water, and storage temperatures on the number of contamination bags over time. Computational experiments study a pilot plant of Romaine lettuces contaminated with E. coli O157:H7. Results indicate that the key factor affecting cross‐contamination is the chlorination concentration dose rate in the wash water. Other factors include the level of contamination on the incoming lettuce heads or in the facility environment (equipment). Storage room temperature fluctuations showed the importance of real‐time monitoring to avoid microorganism growth and thus prevent an increase in the number of contaminated bags.

English

The effect of Phosgreen fertilizer on the growth, development and biometric traits of lettuce was determined in comparison to superphosphate on two selected horticultural media. The aim of the experiment was to evaluate the effect of Phosgreen on the basis of the reaction of tested plants. Lettuce was grown on two horticultural media: mineral soil and deacidified peat. This study determined phosphorus content and uptake as a phosphorus fertilizer effect on different horticultural media. Lettuce developed and grew better on deacidified peat. Based on this study, it can be concluded that struvite is a phosphorus fertilizer with great potential and warrants further testing. A significant increase in lettuce head mass, number of leaves and rosette width under the influence of struvite was found compared to control conditions. The increase in rosette mass compared to the control was approximately 54%. The width of the rosette increased by circa 32% due to the use of Phosgreen compared to the control and by 3% compared to superphosphate. Both P content and uptake by lettuce under Phosgreen was comparable to that under superphosphate. Cu, Zn, Pb and Cd content in lettuce leaves under Phosgreen fertilization was comparable to that under superphosphate. © 2022 Friends Science Publishers

Mechanical properties of lettuce (Lactuca sativa L.) for horticultural machinery design

Currently, there is a growing need to develop machines that replace human work efficiently and effectively in horticulture with the same sensibility of the human hand, since horticultural foods are notably very fragile to handle and process, especially considering machinery and systems. This work aimed to determine the mechanical properties of lettuce (Lactuca sativa L.) variety Crispa. For that, two methodologies were used in the field of material engineering: one specific for polymers and composites – the standard ASTM D3039 – Standard test method for tensile properties of polymer matrix composite materials (ASTM, 2002a) and the other specific for plastic films – the ASTM 882 – Standard test method for tensile properties of thin plastic sheeting (ASTM, 2002b). The tests were adapted for lettuce samples. The mechanical properties for the leaf and stem of head lettuce (var. Crispa) were obtained with appropriate statistical rigor, which can be considered valid initial estimates for dimensioning mechanisms and systems of machines for horticultural works specialized in lettuce. These properties provide fundamental engineering parameters to design machine elements that interact with biological materials, allowing to develop devices that generate minor damage to biological structures in lettuce.

Artificial Shading Can Adversely Affect Heat-tolerant Lettuce Growth and Taste, with Concomitant Changes in Gene Expression

Shading has been used to produce high-quality lettuce (Lactuca sativa) in locations where production conditions are not optimal for this cool-season crop. To learn what additional benefits shading provides if heat-tolerant cultivars are used and to understand the effects of shading on growth, sensory quality, chemical content, and transcriptome profile on heat-tolerant lettuce, we grew two romaine lettuce cultivars with and without shading using 50% black shadecloth in 2018 and 2019. Shading reduced plant leaf temperatures, lettuce head fresh weights, glucose and total sugars content, and sweetness, but not bitterness, whereas it increased lettuce chlorophyll b content compared with unshaded controls. Transcriptome analyses identified genes predominantly involved in chlorophyll biosynthesis, photosynthesis, and carbohydrate metabolism as upregulated in unshaded controls compared with shaded treatments. For the tested cultivars, which were bred to withstand high growing temperatures, it may be preferable to grow them under unshaded conditions to avoid increased infrastructure costs and obtain lettuce deemed sweeter than if shaded.

Quality and Yield of Lettuce in an Open-Air Rooftop Hydroponic System

In this study, the yield and growth performance of lettuce in an open-air rooftop hydroponic system were investigated. Lettuce was grown in a closed recirculating nutrient film technique (NFT) unit using a standard nutrient solution (NS). Yield, fresh weight, and nutrient content in the leaf tissue of the harvested lettuce were measured. The results were compared with the results obtained in indoor hydroponic lettuce growth with artificial lightning. Despite strong winds during the growth period, 25% of the total lettuce heads weighed twice the marketable weight; however, 25% of the total lettuce heads were below the marketable weight. A more efficient nutrient uptake was indicated by the lettuces in the rooftop system compared with the uptake in the indoor system. Foliar analysis revealed a higher content of all nutrients in the leaves of rooftop hydroponic lettuce compared with indoor hydroponic lettuce. This study suggests that hydroponic rooftop-grown lettuce can be competitive with their indoor counterparts if the rooftop hydroponic system is protected from extreme weather conditions.

Selenium Enrichment of Green and Red Lettuce and the Induction of Radical Scavenging Potential

Selenium (Se)-enriched vegetables are promising dietary sources of Se, which provides beneficial biological effects in humans. In this study, we investigated the effects of foliar application of Se on hydroponically grown multi-leaf green (V1) and red (V2) lettuce plants. Three selenate (SeIV) amendment levels were evaluated for their influence on plant growth, elemental composition and radical scavenging capacity. Lettuce heads biofortified with 0.598 mg Se plant−1 accumulated 19.6–23.6 and 14.9–17.6 μg Se g−1 DM in the multi-leaf green (V1) and red (V2) lettuce plants, respectively. The accumulated Se levels can contribute significantly to the recommended dietary allowance of 70 µg day−1 for adult men and 60 µg day−1 for adult women. Accordingly, both V1 and V2 lettuce cultivars grown under the Se3 foliar application condition can cover the daily requirement for adult men by approximately 100% and 85% to 100%, respectively, by consuming 75–90 g or 100 g fresh weight from V1 or V2, respectively. The ABTS radical scavenging potential of green lettuce was induced at Se2 and Se3 foliar application levels, where the IC50 was 1.124 ± 0.09 μg mL−1 at Se0 and improved to 0.795 ± 0.03 and 0.697 ± 0.01 μg mL−1, respectively. There was no cytotoxicity against Vero kidney cells among all treated lettuce plants at the highest concentration tested of 1 mg/mL. Finally, a further focused investigation of the metabolic profile of lettuce plants under varied Se levels needs to be investigated in future studies.

Evaluation of glove type on survival and transfer of Escherichia coli in model systems and during hand harvesting of lettuce.

BackgroundBoth reusable and single‐use gloves can be employed during hand harvesting of lettuce and leafy greens. The impact of glove type on survival and transfer of Escherichia coli was evaluated using agar or lettuce in a laboratory setting and during simulated lettuce harvesting in the field.ResultsTextured and smooth reusable latex and smooth disposable latex gloves inoculated with E. coli were sequentially touched to 10 or 20 agar plates or 20 lettuce leaves (n = 6; laboratory) or used to sequentially harvest 20 heads of lettuce (n = 6; field). E. coli was recovered by enrichment from significantly fewer leaves (46%; 55 of 120) or heads (26%; 31 of 120) of lettuce when inoculated reusable textured gloves were used compared with disposable gloves (leaves: 98%; 118 of 120, or heads: 74%; 89 of 120). In contrast, when a single head of lettuce was the point source for glove contamination, there was no significant difference in the number of E. coli‐positive lettuce heads harvested with reusable textured (71%; 85 of 120) or disposable gloves (75%; 90 of 120). In either field‐contamination scenario, at the 20th head of lettuce harvested with a single glove (final sample point), E. coli was recovered from one to five of six lettuce heads across experimental trials.ConclusionContamination of a glove from a single point source can lead to subsequent contamination of multiple heads of lettuce during hand harvesting, showing the importance of policies to manage hand hygiene and glove use for harvest crews.

First report of Pythium aphanidermatum causing root rot of head lettuce in China.

Head lettuce (Lactuca sativa L.) is an important crop for fresh consumption in China. In Shandong Province, head lettuce is planted in spring and in autumn each year. Because of the on-and-off rain for three weeks, head lettuce plants planted directly into the field in Jiyang City, in July 2017, 20% of the plants rapidly showed symptoms of rotting, water-soaked lesions on roots and stem bases, and then death. The diseased plants first appeared in low-lying areas prone to water accumulation. One-millimeter pieces were excised from water-soaked roots and stem bases, dipped in a 0.2% calcium hypochlorite solution for 10 min, then placed on V8 medium, and incubated in the dark at 28°C for 5 d. Two Pythium-like strains were isolated from the roots and stems. The isolates transferred to CMA and grown for 7 d, and the morphological characteristics of the two isolates on corn meal agar (CMA) were white with dense, cottony, aerial and well-branched mycelia. The two isolates produced sporangia, oogonia, antheridia and oospores. Most of the sporangia were lobate. The oogonia were smooth, nearly globose and terminal. Oospores were globose, smooth and aplerotic. The average dimensions of 50 oogonia and oospores respectively ranged from 19.5 to 25.2 (av. 23.1) µm and 17.8 to 22.3 (av. 19.9) µm. The antheridia were broadly sac-shaped. The isolates morphological characteristics were consistent with P. aphanidermatum (van der Plaats-Niterink, 1981). The COI gene and ITS region of the rDNA were amplified and sequenced using primers FM55/FM52R (Long et al. 2012) and ITS1/ITS4 (White et al. 1990), respectively. The two aligned COI sequences were identical for both isolates, as were the two ITS sequences. BLASTn analysis of the 1,133-bp COI sequence (accession no. MT952703) resulted in a 100% identity with accession number AY129164 from Lactuca sativa, which belongs to P. aphanidermatum, and the 808-bp ITS sequence (accession no. MT921597) showed a 99% identity with Genbank accession number HQ643442 belonging to P. aphanidermatum. Koch's postulates were conducted by first soaking corn kernels for 24 h in water, and then autoclaving for 2 h at 121˚C. Isolate SDHL-1 was grown on CMA for 10 days, after which agar plugs were transferred to the sterilized corn kernels and incubated at 28℃ for approximately 15 d, until the corn kernels were covered in white hyphae. Ten healthy head lettuce plants were transplanted into a sterilized loam potting soil artificially infested with the corn inoculum (3 g inoculum per 100 g loam mixture). Inoculated plants and noninoculated controls were maintained in a greenhouse at 28°C and 100% relative humidity with a 12-h photoperiod; the experiment was repeated once. All twenty inoculated plants exhibited symptoms within one week similar to those observed. Pythium aphanidermatum was recovered only from the water-soaked roots and stem bases of inoculated plants and the re-isolated cultures again identified based on morphological characteristics and sequencing of the ITS and COI genes. No symptoms were observed on the control plants. Sclerotinia sclerotiorum is reported to cause stem base rot of L. sativa in China (Zhou et al. 2011). To our knowledge, however, this is the first report of root rot of head lettuce caused by Pythium aphanidermatum. Identification of the pathogen will assist in devising strategies to reduce yield loss.

Variety testing of new head lettuce hybrids

The growing demand of the country’s population for head lettuce is met, according to various authors’s estimates, by no more than 30-35%. The mains suppliers from Iran, Israel, and Uzbekistan are actively involved in the import of green crops in Russian Federation. One of the existing levers of increasing the production of head lettuce is the introduction of new high-yielding and complex-resistant hybrids into production and the development of varietal agricultural techniques for their cultivation. The article presents an agro-technology evaluation of new hybrids of head lettuce F1 Serena and F1 LE 10585 of the company “Sakata” according to the main economically valuable indications (productivity, yield, biochemical analysis) in the conditions of the Anapa-Taman zone.

Disposición a pagar y preferencias por lechugas orgánicas (Lactuca sativa L)

Global production of organic products has grown significantly due to increased environmental awareness and healthier lifestyles. However, vegetable growers often lack relevant information about people’s preferences and willingness to pay (WTP) for such products. This difficulty is even more pronounced in developing countries, as Chile, mainly due to the low demand increase for organic products. This research aims to analyze preferences and WTP for Chilean organic lettuce. To obtain the WTP, the contingent valuation (CV) method and conjoint analysis (CA) was applied. The data were obtained from a cross-sectional survey using a representative sample of 416 subjects from the Maule region, Chile, stratified by income level. The results showed that subjects were willing to pay a premium of 26.72 % and 52.83 % for an organic head of lettuce compared to conventionally grown products, according to CA and CV methods, respectively. These results show a strong preference for organic products. Furthermore, the CA method indicated that the organic variety of Lactuca sativa L. var. crispa L., at its lowest observed price, was the consumer’s product of choice. Finally, the main reasons to consume this type of product are, firstly, its higher quality, followed by its taste or smell, and finally, it has better nutritional contents. Therefore, lettuce growers can use the estimated premium price as a reference when making decisions about production and pricing.

In-Field Evaluation of Drone-Released Lacewings for Aphid Control in California Organic Lettuce.

Aphids are one of the most economically important pests to California's Central Coast lettuce industry. Aphids vector the lettuce mosaic virus and are crop contaminants in the packaging of the product. Lettuce aphid, Nasonovia ribisnigri (Mosley) (Hemiptera: Aphididae), is one of the predominant aphid species in lettuce, and it poses unique management challenges forming colonies inside the lettuce head. Current management practices rely on repeated foliar insecticide applications to reduce aphid densities per plant. Some organic growers have explored the release of laboratory-reared beneficial insects to manage aphids in their commercial fields. This project sought to document the effects of drone-released lacewing eggs on lettuce aphid densities in organic romaine lettuce fields. Commercially reared lacewing eggs were released at a rate of 74,131 eggs/ha and organic-certified insecticides were sprayed following their respective label recommendations. Our results could be interpreted as preliminary evidence that drone release of lacewings could reduce aphid densities (15.6-150.0 aphids/lettuce head) when compared to the untreated plots (32.1-257.9 aphids/lettuce head). Aphid densities were also decreased after the application of foliar organic-certified insecticides (11.77-143.5). Traditionally, the cost of labor has limited the use of beneficials in the lettuce production system, but the use of drones for these releases may make this strategy more attractive. Based on our calculations, spraying an organic-certified insecticide is less expensive (direct operational costs of U.S. $ 116.36/ha) than releasing beneficial insects by hand ($ 185.40) or by drone ($ 176.00) when mimicking the conditions of this study.

Evaluation of Potential Impacts of Free Chlorine during Washing of Fresh-Cut Leafy Greens on Escherichia coli O157:H7 Cross-Contamination and Risk of Illness.

Addition of chlorine‐based antimicrobial substances to fresh‐cut leafy green wash water is done to minimize microbial cross‐contamination during processing. We developed the FDA Leafy Green Risk Assessment Model (FDA‐LGRAM) to quantify the impact of free chlorine concentration in wash water during fresh‐cut lettuce processing on the extent of water‐mediated cross‐contamination between shredded lettuce and the associated risk of illness due to exposure to Escherichia coli O157:H7. At different contamination prevalence and levels of E. coli O157:H7 on incoming lettuce heads, the model compared the predicted prevalence of contaminated fresh‐cut lettuce packages and the risk of illness per serving between: (1) a scenario where fresh‐cut lettuce was packaged without washing; and (2) scenarios involving washing fresh‐cut lettuce with different levels of free chlorine (0 ppm, 5 ppm, 10 ppm, 15 ppm, and 20 ppm) prior to packaging. Our results indicate that the free chlorine level in wash water has a substantial impact on the predicted prevalence of contaminated fresh‐cut lettuce packages and the risk of illness associated with E. coli O157:H7 in fresh‐cut lettuce. Results showed that the required level of free chlorine that can minimize water‐mediated cross‐contamination and reduce the corresponding risk of illness depended on contamination prevalence and levels of E. coli O157:H7 on incoming lettuce heads. Our model also indicated that the pathogen inactivation rate in wash water via free chlorine was a key model parameter that had a significant impact on the extent of cross‐contamination during washing and the predicted associated risk of illness.

Hydroponic Lettuce Cultivation Using Organic Nutrient Solution from Aerobic Digested Aquacultural Sludge

The aim of this study was to demonstrate how aquacultural sludge can be processed and utilized as an organic nutrient solution (ONS) for hydroponic lettuce production. By using a previous developed method, approximately 80% of the processed sludge was reclaimed as a clear, nutrient-rich solution. The performance of the recovered nutrient solution on lettuce growth was assessed in a nutrient film hydroponic system. The results were compared to the results obtained using a conventional nutrient solution (CNS). Yield, fresh weight, water consumption, and nutrient and heavy metal content in leaf tissue were measured. In spite of a 16% lower average fresh weight obtained in ONS compared to the weight obtained in CNS, there was no statistical difference of the yield of lettuce among the two nutrient solutions. After the cultivation period, 90% of the lettuce heads grown in ONS exceeded the marked weight of 150 g. Foliar analysis revealed a similar or higher content of all nutrients, except of magnesium and molybdenum in the leaves of lettuce grown in the ONS compared to lettuce grown in the CNS. This study shows that nutrients recovered from aquacultural sludge can be utilized as fertilizer, thereby reducing the dependency on mineral fertilizer in hydroponic and aquaponic systems.

Remediation of heavy metal-polluted alkaline vegetable soil using mercapto-grafted palygorskite: effects of field-scale application and soil environmental quality.

Remediation materials are the most critical factors for in situ immobilization of soil contaminated by heavy metals. In this study, in order to improve the performance of palygorskite (Pal), a new remediation material, mercapto-grafted palygorskite (MPal) was synthesized by grafting mercapto groups onto the surface of Pal. The results of field application in northern China showed that at a dosage of 0.12-0.23 kg m-2, MPal significantly reduced the available concentrations of Cd, Pb, and Cr in the soil by 52.2%, 29.9%, and 46.2%, respectively. Concurrently, Cd, Pb, and Cr concentrations in the shoots of head lettuce also decreased significantly, with the highest reduction being 44.0%, 61.5%, and 50.0%, respectively. At the same dosage, MPal had a better immobilization effect than Pal. There was no significant change in the pH of the vegetable soil, while the zeta potential decreased significantly, indicating that the MPal did not immobilize the heavy metals by increasing the pH, making it suitable for alkaline farmland soil. In addition, soil environmental quality was improved overall. MPal increased the activities of urease, β-glucosidase, cellulase, and catalase by 15.4%, 56.5%, 7.8%, and 14.9%, respectively. It increased the number of fungi and actinomycetes by 4.5% and 23.1%, respectively. MPal, as a new remediation material for soil contaminated by heavy metals, could achieve efficient remediation effects when applied in small doses. Compared with Pal, it is environmentally friendly, is low cost, and is more suitable for the treatment of heavy metal pollution in large areas of farmland.

Monetizing environmental impact of integrated aquaponic farming compared to separate systems

Aquaponics is an emerging industry promoted as a sustainable agricultural practice. Economic sustainability of aquaponics is challenging, partly because some of the benefits are external to the grower, necessitating public intervention to support the industry. We used life cycle assessment to estimate the environmental impact of a proposed aquaponic system and applied a set of economic valuation methods to assess the costs of identified impact factors. We found that the system, planned to produce 60,000 ornamental fish and 108,000 lettuce heads per year would impact the environment with a cost of 10,700 EUR annually, about half the environmental cost of separate production of the same produce. Most of the external cost can be attributed to the industrial processes that prepare products used for aquaponic production. Although this method provides only a rough estimate of actual system impact, it can potentially be used to assess the cost-effectiveness of aquaponics from an environmental perspective.

Effects of Hydrogen Peroxide on Organically Fertilized Hydroponic Lettuce (Lactuca sativa L.)

Hydroponic production typically uses conventional fertilizers, but information is lacking on the use of organic hydroponic fertilizers. Development of microbial communities and biofilm that can reduce dissolved oxygen availability is a difficulty with organic hydroponics. One potential solution is the use of hydrogen peroxide (H2O2) which can reduce microbial populations and decompose to form oxygen. However, information is lacking on the impact of hydrogen peroxide on hydroponic crop performance. The aim of this study was to determine the effects of H2O2 concentrations in deep water culture hydroponics by assessing how it affects plant size and yield in lettuce (Lactuca sativa L.) “Rouxai”. In this experiment, three H2O2 treatments, namely the application of 0, 37.5 or 75 mg/L H2O2 to 4 L aerated hydroponic containers with either conventional or organic fertilizer, were compared. The containers had either fish-based organic fertilizer (4-4-1, N-P2O5-K2O) or inorganic mineral based conventional nutrient solution (21-5-20, N-P2O5-K2O), both applied at 150 mg/L N. Three replicates of each H2O2 treatment–fertilizer combination were prepared resulting in a total of eighteen mini hydroponic containers each with one head of lettuce. There were two growth cycles: fall 2018 and spring 2019. When added to conventional fertilizers, both 37.5 mg/L and 75 mg/L of H2O2 led to stunted growth or death of lettuce plants. However, when 37.5 mg/L of H2O2 was applied to organic fertilizers, the lettuce yield nearly matched that of the conventionally fertilized control, demonstrating that the application of H2O2 has the potential to make organic hydroponic fertilization a more viable method in the future.

Antimicrobial Mitigation via Saponin Intervention on Escherichia coli and Growth and Development of Hydroponic Lettuce

Various saponins have demonstrated allelochemical effects such as bactericidal impacts as well as antimycotic activity against some plant pathogenic fungi, thereby acting to benefit plant growth and development. A commercial saponin solution was evaluated for bactericidal effects against Escherichia coli and growth of lettuce (Lactuca sativa) in a hydroponic system. E. coli (P4, P13, and P68) inoculum at final concentration of 108 colony-forming units (cfu)/mL was added to 130 L of a fertilized solution recirculating in a nutrient film technique (NFT) system used to grow ‘Rex’ lettuce. After 5 weeks in the NFT system, E. coli populations were lowest in the inoculated treatment that did not contain any saponin addition (0.89 log cfu/mL) when compared with all other inoculated treatments (P < 0.001). The treatment containing 100 µg·mL−1 saponin extract had an E. coli population of 4.61 log cfu/mL after 5 weeks that was higher than treatments containing 25 µg·mL−1 or less (P < 0.0001). Thus, higher E. coli populations were observed at higher saponin concentrations. Plant growth was also inhibited by increasing saponin concentrations. Fresh and dry shoot weight were both higher in the inoculated and uninoculated treatments without the saponin addition after 5 weeks in the NFT system (P < 0.0001). Lettuce head diameter was smaller when exposed to saponin treatments with concentrations of 50 and 100 µg·mL−1 (P < 0.0001). Lettuce leaves were also tested for the potential of E. coli to travel systemically to the edible portions of the plant. No E. coli was found to travel in this manner. It was concluded that steroidal saponins extracted from mojave yucca (Yucca schidigera) are not an acceptable compound for use in mitigation of E. coli in hydroponic fertilizer solution due to its ineffectiveness as a bactericide and its negative impact on lettuce growth.

Lighting Direction Affects Leaf Morphology, Stomatal Characteristics, and Physiology of Head Lettuce (Lactuca sativa L.).

Plants are exposed to numerous biotic and abiotic stresses, and light is one of the most important factors that influences the plant morphology. This study was carried out to examine how the lighting direction affected the plant morphology by investigating the growth parameters, epidermal cell elongation, stomatal properties, and physiological changes. Seedlings of two head lettuce (Lactuca sativa L.) cultivars, Caesar Green and Polla, were subjected to a 12 h photoperiod with a 300 μmol·m−2·s−1 photosynthetic photon flux density (PPFD) provided by light emitting diodes (LEDs) from three directions: the top, side, and bottom, relative to the plants. Compared with the top and side lighting, the bottom lighting increased the leaf angle and canopy by stimulating the epidermal cell elongation in leaf midrib, reduced the leaf number and root biomass, and induced large stomata with a low density, which is associated with reduced stomatal conductance and carbohydrate contents. However, the proline content and quantum yield exhibited no significant differences with the different lighting directions in both cultivars, which implies that the plants were under normal physiological conditions. In a conclusion, the lighting direction had a profound effect on the morphological characteristics of lettuce, where the plants adapted to the changing lighting environments.

DESEMPENHO AGRONÔMICO DE ALFACE ORGÂNICA EM FUNÇÃO DA COBERTURA DO SOLO

O uso de cobertura do solo na cultura da alface garante a qualidade e incrementa a produção ao melhorar as condições de cultivo, por meio da redução da evapotranspiração, aumento da taxa de infiltração de água no solo, por manter a temperatura do solo estável e reduzir matocompetição na área. Neste sentido, o presente estudo teve por objetivo avaliar o desempenho produtivo da alface do tipo crespa da variedade Simpson sob diferentes tipos de cobertura do solo. O delineamento experimental empregado foi em blocos ao acaso, com 4 repetições e 5 tratamentos, sendo estes a testemunha (sem cobertura), a lona plástica dupla face, o papel kraft, a palhada de amendoim forrageiro e a palhada de braquiária. Aos 38 dias após o transplantio as plantas foram avaliadas quanto a biomassa fresca e seca (g planta-1), altura de planta (cm planta-1), diâmetro da cabeça (cm planta-1) e número de folhas (folhas planta-1). A cobertura que proporcionou melhores resultados para as variáveis estudadas foi a lona de dupla face, sendo significativamente melhor que os demais tratamentos. As outras coberturas avaliadas foram significativamente iguais a testemunha, ou seja, não exerceram influência na melhoria das condições do solo. Concluiu-se que a cobertura que proporcionou melhor desempenho produtivo da alface crespa variedade Simpson para as condições climáticas de Ituiutaba (MG) foi a lona dupla face.

Miniature Head Lettuce Yield and Anthocyanin Concentration under High Tunnels and the Field in Georgia

This study evaluated the yield of eight miniature lettuce (Lactuca sativa) cultivars (i.e., mini-lettuce) grown under organically managed high tunnels compared with a field system during two spring seasons in Georgia. Mini-lettuce required an average of 36 to 40 days to harvest in both systems with a 86% to 97% marketability rate. The high tunnels provided a heat gain on the coldest days, decreased leaf wetness, and resulted in a lower daily light integral compared with the field. In 2015, mini-lettuce yields were similar between the high tunnel and field, but in 2016, yields were greater under the high tunnels. In 2016 only, there was a significant system by cultivar interaction for yield, suggesting that the high tunnels provided a yield increase for ‘Baby Green Oakleaf’ and ‘Spretnak’ mini-lettuce. Differences in the daily light integral between the high tunnels and field appeared to affect the accumulation of anthocyanins in red-pigmented mini-lettuce. Anthocyanin concentrations were 26% to 194% greater in mini-lettuce grown in the field compared with under high tunnels. The cultivar Rhazes had the greatest anthocyanin concentrations of all red-pigmented mini-lettuce evaluated but also lower yields.