Flavonol content and composition of spring onions grown hydroponically or in potting soil

Two experiments were conducted as part of an effort to evaluate the suitability of onions as a candidate crop for testing in a closed, controlled environment, hydroponic-based plant facility designed for long-term manned space missions (NASA Engineering Development Unit). Composition and total flavonol content of the plants were determined as they matured in a hydroponic-versus a soil-based system. ‘Purplette’ onions (Allium cepa L.) were grown hydroponically in a greenhouse for as long as 77 days. Composition of the plant tissue was determined at weekly or biweekly intervals. Ca, Mg, K, and N (wet matter basis) all decreased as plants matured. Dry matter (DM) and S contents were constant regardless of age averaging 10.6% and 187 mg/100 g, respectively. Total flavonol (TF) content increased as plants matured (226–538 mg/100 g at 14 and 77 days, respectively). Onions grown in hydroponic units or in potting medium had similar composition for all constituents examined (10.38%, 0.550%, 4.15%, and 0.97% DM, N, C, and ash, respectively; and 126.0, 55.5, 270, 185 and 453 mg/100 g Ca, Mg, K, S and TF, respectively). Based on phenotypic characteristics and composition determined in this study, onions were well suited to hydroponic propagation.