2018 ASHS Annual Conference

Phosphorus (P) nutrition in plants can influence foliar coloration, typically due to increased concentrations of pigments such as anthocyanins. The foliar coloration of edible crops can affect their aesthetic appeal, perceived health benefits, and overall marketability to consumers. We conducted this study to record the effects of P fertilization on the leaf color, chlorophyll content, and anthocyanin concentration in ‘Salanova® Red Butter’ lettuce (Lactuca sativa L.). At the beginning of the first experiment, we fertilized the lettuce with P concentrations of 0, 2.5, 5, 10, and 20 mg·L^-1 P. After three weeks, half of the plants from each non-zero concentration were restricted to 0 mg·L^-1 P. We took biweekly measurements of height, diameter, and chlorophyll content, and a final determination of anthocyanin concentrations. Plants grown with low or no P for the duration of the study were dark purple in coloration with high concentrations of anthocyanins. However, these plants were extremely stunted and had developed severe necrosis on the lower leaves. Plants grown initially with 2.5–10 mg·L^-1 P but were later restricted to 0 mg·L^-1 P also developed necrosis on the lower foliage. Lettuce grown with 20 mg·L^-1 P for the duration of the study were greener in coloration, but were also the largest. Based on these results, we conducted a second experiment in which we used nine total treatments of 2.5, 20, and 40 mg·L^-1 P for the duration of the study, and 20 or 40 mg·L^-1 P restricted to 2.5 mg·L^-1 P after 3, 4, and 5 weeks. Regression demonstrated trends in plant size, weight, chlorophyll content, and anthocyanin concentration in response to P nutrition. Commercial lettuce growers can manipulate P fertilization in order to enhance the red pigmentation of ‘Salanova® Red Butter’ lettuce, but this may come at the cost of decreased fresh weight and deficiency symptoms if nutrition is not monitored sufficiently. Growers implementing this fertilization strategy should use ≥20 mg·L^-1 P during the first half of production, and then lower the P fertility 3 weeks before harvest. Further refinement and cultivar trialing is necessary to improve fertility recommendations for red lettuce.