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

Commercial lettuce production requires adequate levels of nitrogen (N), phosphorus (P), and potassium (K) to provide high-quality postharvest attributes needed for longer shelf life. Factorial experiments were conducted in Salinas, CA, to evaluate yield and postharvest quality of both romaine and iceberg lettuce using fertilizers containing various levels of N, P, and K. Lettuce was evaluated for yield and postharvest quality parameters, including color, wilt, turgidity, glossiness, decay, brittleness, fringe burn, and salt burn. Uptake of N, P, K, calcium, and silicon by plants was also determined. Regardless of fertilizer treatment, shelf life and visual quality were better in the iceberg lettuce than romaine lettuce when cold-stored at 1 °C for 14 d. Yield increased with increased N application rate, but post-harvest quality fell at high levels of N (337 kg·ha−1) and P (225 kg·ha−1). The most economical treatment providing the highest yield and best post-harvest quality was the combination of 225 kg·ha−1 N and 112 kg·ha−1 P.

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Jorge M. Fonseca, Hyun-Jin Kim, Wesley L. Kline, Christian A. Wyenandt, Murshidul Hoque, Husein Ajwa, and Ned French

The effect of preharvest application of a newly developed second-generation harpin product (2G-Harpin) on shelf life of fresh-cut lettuce (Lactuca sativa) was investigated. The lettuce plants were grown in three locations in the United States: Watsonville, CA, Cedarville, NJ, and Yuma, AZ, and treated 5 days before harvest at 140, 280, and 420 g·ha−1 (30, 60, and 90 mg·L−1). Lettuce processed and bagged were stored at 1 to 3 °C and evaluated for quality for 20 days. Lettuce from California treated with 2G-Harpin at 280 to 420 g·ha−1 consistently showed better visual quality and lower microbial population than the control. Overall results in New Jersey showed no major differences among treatments. In Arizona, microbial population was lower and visual quality was higher in lettuce treated at 280 and 420 g·ha−1 during part of the storage period. In further experimentation, we examined the phenolic content of lettuce harvested 1 and 7 days after treatment with 2G-Harpin. The results showed that phenolic content was higher in all treated lettuce than in the control lettuce after 24 h. Six days later, the levels fell back to the initial stage. Antioxidants capacity increased by 40% in head leaves when plants were treated with 280 and 420 g·ha−1 2G-Harpin, but no change was observed in outer leaves. Overall, it was revealed that a field application of 2G-Harpin can improve quality of fresh-cut lettuce under environmental conditions that need to be determined. Our results with phenolic content and antioxidant activity suggested that improvement in quality is probably the result of alteration of metabolites' composition and demonstrated that increased phenolics do not correlate with lower quality of fresh-cut products.