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Edward J. Ryder

This work was partially supported by the California Iceberg Lettuce Research Board. Thanks to David J. Milligan for technical help. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal

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Edward J. Ryder

I thank Bert Robinson and David Milligan, agricultural research technicians, for helping with field planting and LMV testing, respectively. Part of this research was supported by the California Iceberg Lettuce Research Program. The cost of

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Edward J. Ryder and William Waycott

1 Current address: Petoseed Co., 650 Leanna Drive, Arroyo Grande, CA 93420. The research was partially funded by the California Iceberg Lettuce Research Board. The cost of publishing this paper was defrayed in

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Edward J. Ryder, William Waycott, and James D. McCreight

supported by a grant from the California Iceberg Lettuce Research Program. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to

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Edward J. Ryder and Bert J. Robinson

assistance with the greenhouse work. Part of this research was supported by the California Iceberg Lettuce Research Board. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore

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Ariena H.C. van Bruggen, Philip R. Brown, and Art Greathead

tissue and soil samples. We are grateful to R.G. Grogan and Doug O'Brien for reviewing the manuscript. This research was sponsored by the California Iceberg Lettuce Research Advisory Board, Salinas, Calif. The cost of publishing this paper was defrayed in

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F.A. Tomás-Barberdán, J. Loaiza-Velarde, and M.E. Saltveit

Mechanical wounding and exposure to ethylene induces an increase in phenylpropanoid metabolism in lettuce and an increase in the concentration of several soluble phenolic compounds that are easily oxidized to brown substances by polyphenol oxidase. To study the early response of lettuce to wounding and ethylene, leaves of iceberg, butter leaf, and Romaine lettuces were either wounded or exposed to ethylene at 10 μL·L–1 in flows of humidified air at 5 or 10°C. Soluble phenolic compounds were extracted at intervals up to 72 hours and were analyzed by HPLC. After 72 hours, wounded leaves of all three lettuce types showed elevated levels of caffeoyl tartaric acid, Chlorogenic acid, dicaffeoyl tartanc acid, and 3,5-dicaffcoyl quinic acid at both temperatures. In contrast, there were no significant increases in soluble phenolic compounds in iceberg lettuce exposed to ethylene at 10°C. At 5°C for iceberg, and at both temperatures for the other two types, there was the same pattern for ethylene treated and wounded leaf tissue. The kinetics of wound and ethylene-induced phenolic metabolism are different and will be discussed in relation to phenolics produced and browning susceptibility.

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C.A. Sanchez, M. Wilcox, J.L. Aguiar, and K.S. Mayberry

Twenty field experiments were conducted to evaluate the response of iceberg lettuce (Lactuca sativa L.) to N and evaluate various diagnostic technologies as tools for assessing the N nutritional status of lettuce. Lettuce yields showed a curvilinear response to N in most experiments. Generally, the dry midrib nitrate-N test and the sap nitrate-N test appear to be sensitive indicators of the N nutritional status of lettuce after the folding stage of growth. The chlorophyll meter was not a sensitive indicator of the N nutritional status of lettuce. Preliminary data also show that canopy reflectance, including digital analysis of aerial photographs, is correlated to N nutritional status of lettuce. However, reflectance technologies do not readily distinguish between N deficiencies and other factors (insects, diseases, water stress, etc.) that affect plant biomass and color. Because plant tests do not appear to be sensitive indicators of N nutrition during early growth stages (before folding), a post-thinning (and pre-sidedress) soil nitrate-N test is currently being evaluated.

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A. Galadima, C.A. Sanchez, J. Palumbo, B. Tickes, M. Matheron, and M. McGiffen

Experiments were conducted during 1998–99 seasons to evaluate the potential for organic vegetable production in the low desert of the southwestern United States. The experimental design included three summer management options [fallow, cowpea (Vigna sinensis), and sudangrass (Sorghum vulgare)] in factorial combination with alternative production systems, which included organic and conventional systems. The crops cultivated were iceberg lettuce (Lactuca sativa L) during the fall–winter period and melons (Cucumis melo Reticulatus Group) during the spring. The organic plots were managed with strict adherence to California Certified Organic Farmers (CCOF) guidelines. Summer cover crop management seemed to influence the early growth and N uptake of lettuce, but had no final effect on yield and quality. The organic production system resulted in lower yields and inferior product quality compared to the conventional system. Generally, disease and weeds were not limiting factors, although labor costs for weed control would be slightly higher in organic plots. Insects, primarily aphids (various types) and thrips (Frankliniella Occidentalis Perancle), and fertility, primarily N, were factors limiting yield and quality in organic systems. Control of whiteflies (Bemisia argentifoli) was the limiting factor for melons. Studies during 1999–2000 are focused on overcoming the challenges of the insect and fertility management in organic systems.

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Gloria Lopez-Galvez, Mikal Saltveit, and Marita Cantwel

Phenylalanine ammonia-lyase (PAL) activity in iceberg lettuce (Lactuca sativa L.) is increased in response to several kinds of stress, including wounding, exposure to ethylene, and fungal infection. Ethylene-induced PAL activity is correlated with the discoloration and shelf-life of fresh cut lettuce (Couture et al. 1993. HortScience 28:723). The objective of this research was to further characterize the kinetics of wound-induced PAL in fresh cut lettuce. Leaves of different cvs were cut into salad pieces (1.5 × 3 cm), rinsed in chlorinated water, centrifuged manually, and placed into containers at 5 or 15C through which humidified air flowed. Samples were evaluated for overall visual quality and specific types of discoloration. Midrib tissue was also finely cut (1 × 0.5 cm) for enhanced wound-induction of PAL, which was assayed spectrophotometrically. The kinetics of PAL in midrib tissue of fine cut and salad cut lettuce were similar, with maximum activities obtained within 12-16 h at 15C and within 40-60 h at 5C. Maximum PAL levels in the fine cut lettuce were 1.5-2.0 times those observed in the salad cut pieces, and similar to those induced by ethylene. The usefulness of PAL as a predictor of the storage life of fresh cut lettuce depends on simplifying and expediting the PAL assay.