Mineral Nutrient Density of Lettuce In Response to Cultivar Selection and Nutritional Regimes

Mineral nutrient deficiency in vegetable-based foods is a substantial concern in human diets. Depleted soil fertility and high yielding cultivars have been associated with low nutrient contents in vegetables. Research is needed to develop systems that introduce nutrient-dense crops to the markets. This study explored if mineral nutrient densities of lettuce (Lactuca sativa L.) can be increased though selection of cultivars and nutritional regimes. Eighteen cultivars including butterhead, romaine, and loose-leaf phenotypes of heritage and modern origins were studied in greenhouse and field experiments at the University of Massachusetts. In the greenhouse, Hoagland solution #1, commercial inorganic fertilizer (20-10-20 peat-lite special plus additional Ca), and a commercial organic fertilizer (3-1.5-4 containing calcium) were the nutrient regimes.  In the field, conventional (10-10-10), compost, and an organic fertility regime of soybean meal, bone meal, and potassium sulfate were assessed.  Compost was applied at 40 Mg/ha, and chemical or organic fertilizers provided 75 kg N-75 kg P₂O₅-75 kg K₂O/ha. Elements in whole shoots were determined by plasma spectrophotometry. In the greenhouse, heritage cultivars had about 10% higher P, K, Ca, Mg, and Zn concentrations than modern cultivars with no differences occurring for the other elements. Differences for each element occurred among phenotypes and nutritional regimes but with no consistent trends for phenotypes or regimes. In the field, few differences occurred with nutrient concentrations among the fertility regimes, phenotypes, or origin of the cultivars. In the greenhouse or field, differences among individual cultivars for each element were large with some cultivars having nearly twice the concentrations of nutrients of others and with considerable uniformity in cultivar rankings among the elements. Some interactions occurred between nutritional regimes and cultivars. This work suggests that cultivars can be selected for production of nutrient-dense lettuce.