2017 ASHS Annual Conference

A major cause of decrease in yield and quality of lettuce, a cool-weather crop, is bolting due to high temperatures. The ability to grow lettuce during high temperatures could result in an overall increase in production of lettuce because it could be grown through the summer in regions where high temperatures limit production. The purpose of this research was to determine how lettuce yield, biomass accumulation, phenolic acid, flavonoid, and anthocyanin concentrations are affected by varying levels of nitrogen (N) fertilizer, location, and cultivar. This study was carried out at two sites, the Mississippi State University (MSU) and the Washington State University (WSU). The experiment was a randomized complete block design arranged in a 2 (cultivar) x 2 (location) x 4 (nitrogen rate) factorial with 4 replications. Treatments included two cos (romaine) lettuce cultivars, Salvius and Thurinus. Nitrogen treatments included 42 (1x), 63 (1.5x), 105 (2.5x) and 189 (3.5x) kg·ha^-1 N. Lettuce plants were transplanted on May 6, 2016 at MSU (average temperature is 21.4 °C in May and 25.6 °C in June) and June 14, 2016 at WSU (average temperature is 13.3 °C in June and 16.1 °C in July). Results indicated no significant differences in lettuce yield and biomass accumulation with increasing N treatments. Salvius had a significant increase in fresh weight (FW) and dry weight (DW), and the DW:FW ratio when compared to Thurinus. Additionally, the WSU location had higher lettuce FW and DW. There were no significant differences in phenolic acid, flavonoids, and anthocyanin concentrations with N treatments, except for quercetin-3-glucoside (Q-3-G). The flavonoid Q-3-G increased significantly with increasing N concentrations. There were interactions between location and lettuce cultivar for chlorogenic acid, Q-3-G, and quercetin/luteolin glucuronide (QL-G). In general, there was increased chlorogenic acid content in Salvius at the MSU location and increased concentrations of Q-3-G and QL-G in Thurinus at the WSU location. The lettuce cultivar Thurinus had significantly more chicoric acid and quercetin-malonyl-glucoside (QM-G) when compared to Salvius. Furthermore, the lettuce grown at the WSU location had significantly more chicoric acid and cyandidan-3-glucoside (C-3-G). Overall, lettuce grown at the WSU location and the cultivar Salvius had greater yields and biomass accumulation. Lettuce grown at the WSU location also had greater amounts of phenolic acids, flavonoids, and anthocyanin except for chlorogenic acid. The cultivar Salvius had greater amounts of phenolic acids and Thurinus had greater amounts of flavonoids and anthocyanins.