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
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
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
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
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
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
Mark Ritenour and Mikal E. Saltveit Jr.
Activity of phenylalanine ammonia-lyase (PAL) is critical in the induction of russet spotting (RS) in leaves of Iceberg lettuce (Lactuca sativa L.). RS is a major postharvest disorder of lettuce caused by exposure to ppm levels of ethylene at = 5C. Both PAL and RS are decreased when lettuce tissue previously exposed to ethylene is stored at = 15C or is transferred from = 5C to = 15C. To study the induction and inactivation of PAL, we exposed lettuce leaves to air ± 10 ppm ethylene at 5C for four days to initially induce high PAL levels. After four days, leaves were treated with water ± 2 mg/L cycloheximide, and transferred to air at 5 or 15 C. In leaves previously exposed to ethylene, PAL activity decreased rapidly to baseline levels within two days in non-cycloheximide treated leaves transferred to 15C. PAL activity remain elevated in the same treatment held at 5C. In leaves treated with cycloheximide and transferred to 15C, PAL did not begin to decrease until after four days. Cycloheximide treated leaves held at 5C showed increased PAL activity both two and four days after treatment.
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.
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.
K.K. Tanino, D.R. Waterer, S.R. Abrams, and L.V. Gusta
Seeds of celery, spinach, onion, cress, water cress, iceberg lettuce, Great Lakes lettuce, cabbage, tomato, sweet corn and celery were pre-treated with 0.1 μM/g seed of both ABA and analogs of ABA. The chemicals were dissolved in a mixture of methanol:hexane (9:1/v:v) and applied to the seeds for approximately 3 minutes. The solvent was removed from the seeds within 5 minutes by rotary evaporation under reduced pressure. Effects on petri plate germination and soil emergence were monitored daily at 5, 10 and 15°C. The methanol/hexane solvent alone improved spinach seed emergence at 10°C from 10% to 100% and from 50% to 90% at 15°C in celery. Certain ABA analogs reduced time to 50% emergence in celery by at least 7 days at 15°C. Two ABA analogs synchronized emergence in celery and effect was temperature-dependent. One analog improved seed germination in tomato from 15% to 90% at 10°C. In most cases treatment effects on radicle germination on petri plates was not a good indicator of treatment effects on emergence from a soil based system.