Nutrient Density in Lettuce Cultivars Grown with Organic or Conventional Fertilization with Elevated Calcium Concentrations

Abstract

Calcium-rich vegetables in the diet could ameliorate the potential for calcium (Ca) deficiency in human nutrition. This study investigated the prospect of increasing Ca density of lettuce (Lactuca sativa L.) through cultivar selection and nutrient management in a greenhouse. Eighteen lettuce cultivars including butterhead, romaine, and loose-leaf phenotypes of heritage and modern genetics were tested. Organic fertilizer (3N–0.7P–3.3K) and commercial conventional fertilizer (20N–4.4P–16.6K) factored with three Ca levels (50, 100, 200 mg·L−1 as CaCl2) were the fertilizer regimes. Calcium in whole shoots was analyzed by atomic absorption spectrometry of oven-ashed samples. Heritage cultivars had a significantly higher Ca concentration (1.93% dry weight) than modern cultivars (1.54%). Loose-leaf phenotypes had the highest Ca concentration (2.06%) followed by butterhead (1.66%) and romaine (1.49%). Accumulation of Ca was higher with the conventional fertilizer (1.90%) than with the organic fertilizer (1.58%). Elevated Ca level in the fertility regimes raised the Ca concentration in lettuce from 1.56% at 50 mg·L–1 to a mean of 1.82% at 100 mg·L−1 and 200 mg·L−1. Large differences in Ca concentration occurred among individual cultivars with ranges from 1.27% to 3.05%. ‘Salad Bowl’, ‘Red Deer Tongue’, ‘Buttercrunch’, and ‘Bronze Mignonette’ were the top in cultivar ranking with mean Ca concentration of 2.50%, whereas ‘Adriana’, ‘Australe’, ‘Coastal Star’, and ‘Forellenschluss’ were low accumulators with a mean of 1.33%. Head size of cultivars had no correlation with Ca concentration. This experiment indicates that selection of nutrient regimes and cultivars can be used to increase Ca accumulation in lettuce.