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Beiquan Mou and Yong-Biao Liu

Leafminer (Liriomyza langei Frick) is a major insect pest of many important agricultural crops including lettuce (Lactuca sativa L.). The goals of this study were to evaluate lettuce genotypes for resistance to leafminer and to estimate heritabilities of three leafminer-resistant traits. Forty-six lettuce genotypes were evaluated in two tests in insect cages. Wild species (Lactuca serriola L., Lactuca saligna L., and Lactuca virosa L.) had significantly fewer leafminer stings than cultivated lettuce (L. sativa) in both tests. PI 509525 (L. saligna) had few leafminer stings and no flies emerged. Leaf (leaf and romaine) lettuce also showed significantly less stings than head (crisphead and butterhead) types, while differences between leaf and romaine lettuces, and between crisphead and butterhead types were nonsignificant. Broad-sense heritability for number of stings per unit leaf area was relatively high, averaging 65% over the two tests. Heritabilities for egg-hatching period and flies per plant were 10% and 15%, respectively. Stings per unit leaf area from the two tests were highly correlated (r = 0.828), suggesting that resistance was stable over different plant ages and against different pressures of leafminer. These results suggest that genetic improvement of cultivated lettuce for leafminer resistance is feasible.

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Milton E. McGiffen Jr., John Manthey, Aziz Baameur, Robert L. Greene, Benjamin A. Faber, A. James Downer, and Jose Aguiar

A 1992 article by Nonomura and Benson (Proc. Natl. Acad. Sci. 89:9794-979X) reported increased yield and drought tolerance in a wide range of C3 species following foliar applications of methanol. The article was widely reported in the trade and popular press, which created a huge grower demand for information on the use and efficacy of methanol. To test the validity of the reports, we applied methanol with and without nutrients to a wide range of crops across California following Nonomura and Benson's (1992) protocol. Crops included watermelon, creeping bentgrass, lemons, savoy cabbage, carrots, romaine lettuce, radish, wheat, corn and peas. Environments included the greenhouse and field tests in coastal, inland valley, and desert locations. To test whether methanol improved drought tolerance, the savoy cabbage and watermelon experiments included both reduced and full irrigation. In no case was yield increased or drought tolerance attributable to methanol treatment. In some cases, methanol caused significant injury and decreased yield.

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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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S.J. Breschini and T.K. Hartz

Trials were conducted in 15 commercial fields in the central coast region of California in 1999 and 2000 to evaluate the use of presidedress soil nitrate testing (PSNT) to determine sidedress N requirements for production of iceberg and romaine lettuce (Lactuca sativa L.). In each field a large plot (0.2-1.2 ha) was established in which sidedress N application was based on presidedress soil NO3-N concentration. Prior to each sidedress N application scheduled by the cooperating growers, a composite soil sample (top 30 cm) was collected and analyzed for NO3-N. No fertilizer was applied in the PSNT plot at that sidedressing if NO3-N was >20 mg·kg-1; if NO3-N was lower than that threshold, only enough N was applied to increase soil available N to ≈20 mg·kg-1. The productivity and N status of PSNT plots were compared to adjacent plots receiving the growers' standard N fertilization. Cooperating growers applied a seasonal average of 257 kg·ha-1 N, including one to three sidedressings containing 194 kg·ha-1 N. Sidedressing based on PSNT decreased total seasonal and sidedress N application by an average of 43% and 57%, respectively. The majority of the N savings achieved with PSNT occurred at the first sidedressing. There was no significant difference between PSNT and grower N management across fields in lettuce yield or postharvest quality, and only small differences in crop N uptake. At harvest, PSNT plots had on average 8 mg·kg-1 lower residual NO3-N in the top 90 cm of soil than the grower fertilization rate plots, indicating a substantial reduction in subsequent NO3-N leaching hazard. We conclude that PSNT is a reliable management tool that can substantially reduce unnecessary N fertilization in lettuce production.

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Robert K. Prange, John M. DeLong, Peter A. Harrison, Jerry C. Leyte, and Scott D. McLean

A new chlorophyll fluorescence (F) sensor system called FIRM (fluorescence interactive response monitor) was developed that measures F at low irradiance. This system can produce a theoretical estimate of Fo at zero irradiance for which we have coined a new fluorescence term, Fα. The ability of Fα to detect fruit and vegetable low-O2 stress was tested in short-term (4-day) studies on chlorophyll-containing fruit [apple (Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.), pear (Pyrus communis L.), banana (Musa ×paradisiaca L.), kiwifruit (Actinidia deliciosa C.S. Liang & A.R. Ferguson), mango (Mangifera indica L.), and avocado (Persea americana Mill.)] and vegetables (cabbage (Brassica oleracea L. Capitata Group), green pepper (Capsicum annuum L. Grossum Group), iceberg and romaine lettuce (Lactuca sativa L.)). In all of these fruit and vegetables, Fα was able to indicate the presence of low-O2 stress. As the O2 concentration dropped below threshold values of 0 to 1.4 kPa, depending on the product, the Fα value immediately and dramatically increased. At the end of the short-term study, O2 was increased above the threshold level, whereupon Fα returned to approximately prestressed values. A 9-month study was undertaken with `Summerland McIntosh' apple fruit to determine if storing the fruit at 0.9 kPa O2, the estimated low O2 threshold value determined from Fα, would benefit or damage fruit quality, compared with threshold + 0.3 kPa (1.2 kPa O2) and the lowest recommended CA (1.5 kPa O2). After 9 months, the threshold treatment (0.9 kPa) had the highest firmness, lowest concentration of fermentation volatiles (ethanol, acetaldehyde, ethyl acetate) and lowest total disorders. Sensory rating for off-flavor, flavor and preference indicated no discernible differences among the three treatments.

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Francisco A. Tomás-Barberán, Julio Loaiza-Velarde, Antonio Bonfanti, and Mikal E. Saltveit

The phenolic composition of whole heads and excised midrib sections of iceberg, butter leaf, and romaine lettuce (Lactuca sativa L.) was followed at 5 and 10 °C during the first 3 days after wounding or during continuous exposure to 10 μL·L-1 ethylene in air. After 3 days of storage at 5 and 10 °C, only 5-caffeoylquinic acid (chlorogenic acid), 3,5-dicaffeoylquinic acid (isochlorogenic acid), caffeoyltartaric acid, and dicaffeoyltartaric acid were detected in wounded lettuce midribs. Of these four compounds, chlorogenic acid accumulated to the highest level in all three lettuce types. The content of caffeic acid derivatives increased 3- and 6-fold after 72 hours of storage at 5 and 10 °C, respectively. The synthesis of caffeoyltartaric acid was not induced by wounding in iceberg lettuce, while chlorogenic acid increased 5-fold at 5 °C and 10-fold at 10 °C. Similar relative phenolic compositions were detected in the three lettuce types studied, although at different concentrations. Changes observed in the content of individual phenolic compounds during the first 3 days of ethylene exposure seemed to follow the same pattern observed during wound induction of the synthesis of phenolic compounds. Chlorogenic acid increased 5-fold and isochlorogenic acid increased 10-fold, while the content of caffeoyltartaric derivatives were not significantly altered by ethylene treatment. Isochlorogenic acid, which was only present in low amounts in the control, was synthesized in the later steps of wound and ethylene induction. Similar kinetics for the induction of phenolic compounds were observed in the three lettuce types studied, suggesting that the mechanisms by which wounding induces phenylpropanoid synthesis are common for the different lettuce types.

Open access

William D. Afton, Kathryn K. Fontenot, Jeff S. Kuehny, and Carl E. Motsenbocker

in the form of salads or as an accompaniment for hamburgers, sandwiches, and tacos. When consumed fresh, it is an excellent source of bulk and fiber ( Swiader and Ware, 2002 ). In the United States, romaine lettuce and leaf lettuce production have

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Huangjun Lu, Alan L. Wright, and David Sui

has been identified, as Nuessly and Nagata (1994) reported that ‘Valmaine’, a romaine lettuce cultivar, had a high level of resistance to SL. This cultivar was later found to be resistant to BCB ( Huang et al., 2002 ; Sethi et al., 2008 ) and two

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Mark A. Mikel

%, romaine 25%, and leaf lettuce 10% of U.S. lettuce production area. A 5-year trend (2000 through 2005) shows romaine lettuce production hectarage increasing 66% and leaf lettuce increasing by 26%, whereas crisphead lettuce production has remained relatively

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Yunwen Wang, Huangjun Lu, Richard N. Raid, Gregg S. Nuessly, and Georgy Faroutine

). California and Arizona are the two largest growing states of lettuce, producing 96% of head and romaine lettuce, and 98% of leaf lettuce in the United States ( Glaser et al., 2001 ). Lettuce is an important winter vegetable crop in Florida. The EAA is the