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David R. Bryla, Thomas J. Trout, and James E. Ayars

broccoli, iceberg (head) lettuce, bell pepper, and garlic. LYSIMETER CONSTRUCTION AND DESIGN Each lysimeter consists of a 2 m × 2 m × 2.25-m deep steel soil tank positioned on a mechanical tank scale (Model FS-4; Cardinal Scale Manufacturing Co., Webb City

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Thomas G. Bottoms, Richard F. Smith, Michael D. Cahn, and Timothy K. Hartz

fertilization rates and to bring N loading from fertilizer and irrigation water into approximate balance with crop N uptake. In this region, lettuce N uptake has been reported to average 130 kg·ha −1 for iceberg and 107 kg·ha −1 for romaine ( Breschini and

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C.A. Sanchez and N. Obeker

Approximately 30,000 ha of iceberg lettuce (Lactuca sativa L.) are produced in the low desert region of the southwestern United States during the fall–winter–spring period each year. During this period, soil temperatures in lettuce beds range from 10 to 30°C. During the cooler part of the growing season, growers typically use nitrate-N sources because they believe they are generally more available for plant uptake. However, limited experimental evidence exists to support this practice. Three field studies were conducted during the 1994–1995 growing season to evaluate the response of iceberg lettuce to N rate and N source. The N sources urea, ammonium sulfate, ammonium nitrate, and calcium nitrate were applied at rates ranging from 0 to 300 kg N/ha. Although lettuce growth, N accumulation, and marketable yield significantly increased by N rate, there were generally no differences due to N source.

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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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Ryan Hayes

Tipburn (TB) is a physiological disorder that results in necrosis along the margins of lettuce leaves. The disorder is objectionable to consumers and reduces the shelf life of whole and minimally processed lettuce. The objectives were to 1) determine the variation for tipburn resistance in iceberg, romaine, green leaf, and red leaf cultivars; and 2) determine the genotype × location interaction for tipburn resistance. Tipburn incidence was recorded on 10 plants in each of three repetitions in Salinas, Calif., and Yuma, Ariz., trials with 20 iceberg, 21 romaine, 11 green leaf, and six red leaf cultivars. Data were analyzed using analysis of variance type statistics of ranked data. Variation for TB resistance was found in all lettuce types at both locations, although iceberg cultivars (average percentage of TB = 31% Salinas; 77% Yuma) had significantly (P < 0.01) higher levels of resistance at both location than romaine (58% Salinas, 81% Yuma), green leaf (52% Salinas; 88% Yuma), and red leaf (43% Salinas, 89% Yuma). The Yuma, Ariz., trial was more conducive for TB, and had less variation (range of percentage of TB = 33% to 100% Yuma, 0% to 100% Salinas). Four iceberg, one green leaf, and one red leaf genotype with industry acceptable levels of TB (<5%) were identified in the Salinas environment. Genotype × location interaction was present (P < 0.01), and included rank order changes within all lettuce types. The correlation between the locations was low, 0.26, but significant (P = 0.045). A need exists for romaine, green leaf, and red leaf germplasm with improved TB resistance, particularly in the Yuma environment. The presence of variation within each lettuce type and genotype × location indicates that genetic improvement should be possible using within type crosses followed by selection in the Yuma or Salinas target environment.

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Edward J. Ryder and Bert J. Robinson

We thank Dick Lindsey, Nunes Vegetables, for providing land for many field trials. Part of this research was supported by a grant from the California Iceberg Lettuce Research Program. The cost of publishing this paper was defrayed in part by

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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

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 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

David S. Milligan provided technical help. This research was partiallv supported by the California iceberg Lettuce Research Board. The Costof publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this