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Ji Gang Kim, Sun Tay Choi, and Chai Il Lim

Modified atmosphere (MA) packaging has been used to maintain quality of fresh-cut lettuce, but O2 depletion and excessive CO2 accumulation can be injurious. This study was conducted to evaluate the effect of delayed MA packaging using three different films on quality maintenance of fresh-cut iceberg lettuce. Iceberg lettuce leaves were sliced, washed, dried, and placed for 0, 5, 10, and 15 hours under air atmosphere at 5 °C. Samples were packaged with 2.9 and 16.6 pmol·kg·Pa·s, oxygen transmission rate (OTR) films and microperforated film (P-plus), and stored for 12 days at 5 °C. Results indicated that delayed packaging affected gas composition (O2 and CO2), fermentative volatile production, off-odor, color, and tissue electrolyte leakage. With 2.9 OTR film, longer delayed time maintained quality by inhibiting off-odor development and the accumulation of ethanol. However, delayed packaging was not effective in maintaining quality of samples packaged with 16.6 OTR film and P-plus film. Regardless of delayed time, all 16.6 OTR-packaged samples had severe discoloration after 7 days of storage. All delayed treatments packaged with P-plus film attained the desired O2 (3–5 kPa) and CO2 levels (10–12 kPa) from day 9 to the end of storage. A 15-hour delayed packaging into packages prepared from 2.9 OTR film and samples packaged with P-plus film maintained quality by inhibiting off-odor development. The results indicate that delayed packaging could be an alternative method when fresh-cut lettuce is packaged with low OTR films.

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M.A. Ritenour, M.E. Saltveit, and M.J. Ahrens

Russet spotting (RS) is an important postharvest disorder in Iceberg lettuce (Lactuca sativa L.). Previous studies showed that RS is induced by exposure to ∼5 ppm (ul/l) ethylene at ∼5C for 3 days and is characterized by the appearance of 1 to 2 mm diam. oval, brown sunken spots along the midrib. Increases in phenylalanine ammonia-lyase (PAL) activity and phenolic content are highly correlated with RS development. Ethylene-induced PAL activity is much less at higher (12C) or lower (0C) temperatures. In this study isolated whole leaves were exposed to a log series of ethylene concentrations from 0.1 to 10 ppm at temperatures from 0.0C to 20C for up to 8 days. Tissue was transferred among these various treatments to investigate the kinetics of PAL induction, activity and deactivation, phenolic accumulation, and RS development. A subjective evaluation was then made of RS development using a 1 to 9 scoring system in which 1 was no RS, and then PAL activity and phenolic content were measured. Preliminary results indicate that ethylene-induced PAL activity was decreased more rapidly upon transfer to temperatures above 10C than to 0.0C. Accumulation of phenolic compounds and development of RS paralleled each other, and were positively related to PAL activity. Practical implications of these results will be discussed.

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M.A. Ritenour, M.J. Ahrens, and M.E. Saltveit

Experiments were conducted to study the effect of temperature on the ethylene inducibility of phenylalanine ammonia lyase (PAL, EC 4.3.1.5) activity and the development of russet spotting (RS) in Iceberg lettuce (Lactucu sutiva L.). Tissue exposed to ethylene and held at 15 or 20C showed earlier hut lower peaks in PAL activity than tissue held at 5C. Accumulation of total soluble phenolic compounds, and the development of RS paralleled increases in PAL activity after a short lag period. Lettuce tissue transferred from 5 to 15C after 1 to 3 days showed later hut higher peaks in PAL activity than tissue continuously held at 15C. Transferring ethylene-exposed tissue to 0C before RS symptoms developed greatly limited further rise in PAL activity and RS development. Discontinuing ethylene exposure after 4 days resulted in a rapid decrease in PAL activity. Lettuce tissue exposed to ethylene for 4 days and then transferred from 5 to 0C after the development of moderate RS symptoms showed no difference in RS after 8 days compared to tissue stored continuously at 5C.

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M. Murshidul Hoque, Husein Ajwa, Mona Othman, Richard Smith, and Michael Cahn

planting and iceberg lettuce was harvested 83 d after planting. A total of 30 plants was sampled from the center four seed rows by selecting every eighth marketable plant in a row. Whole-plant weight from a composite sample of 10 plants was determined

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Ryan J. Hayes, Karunakaran Maruthachalam, Gary E. Vallad, Steven J. Klosterman, Ivan Simko, Yaguang Luo, and Krishna V. Subbarao

.J. Subbarao, K.V. 2006 Release of iceberg lettuce germplasm with resistance to verticillium wilt Agricultural Research Service, U.S. Dept. of Agriculture Hayes, R.J. Vallad, G.E. Qin, Q.-M. Grube, R.C. Subbarao, K.V. 2007 Variation for resistance to

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Lee F. Johnson, Michael Cahn, Frank Martin, Forrest Melton, Sharon Benzen, Barry Farrara, and Kirk Post

replicated trials. Relationships between applied water and crop yields were evaluated. Crops were grown using standard regional production, cultivation, and harvest practices. Materials and Methods Replicated field trials in iceberg lettuce ( Lactuca sativa

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

fields are surrounded by other fields planted with various low annual crops such as cotton and processing tomato. The crop lysimeter and field were planted with broccoli in Fall 2002, iceberg lettuce in Fall 2004, bell pepper in spring to Summer 2005, 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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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