Accumulation of anthocyanins and phenolic acids and antioxidant activity of lettuce cultivars grown hydroponically and in the open ground

Abstract

We assess the level of anthocyanins and phenolic acids accumulated in the leaves of lettuce cultivars grown hydroponically and in the open ground. In addition, the antioxidant activity of the cultivars under study was determined. The plant material was macerated overnight using a selected extractant. The anthocyanin concentration was determined by spectrophotometry, while the composition of anthocyanins and phenolic compounds was determined by reverse-phase high performance liquid chromatography. In general, anthocyanins in the extracts of all red-leaved lettuce cultivars were represented by cyanidin-3-(6″- malonylglucoside) with a small amount of its isomer – cyanidin-3-(3″-malonylglucoside). The highest level of anthocyanins was detected in the leaves of c.v. Thurinus, amounting up to 0.200 g per 100 g of fresh product for the intensively painted leaf areas. The cultivation of lettuce under the green-house conditions (hydroponically) leads to a significant reduction in the anthocyanin concentration in its leaves. The loss of anthocyanins during leaf drying was established to exceed 50%. Along with anthocyanins, the derivatives of caffeic acid, including chlorogenic (5-caffeoylquinic) acid, 5CQA, chicoric (3,4-caffeoyltartaric) acid, and 3,5- dicaffeoylquinic acid are important water-soluble compounds with antioxidant effects. The 5CQA accumulation was the highest, comprising 140 mg per 100 g of fresh weight (Red Thurinus). Antioxidant properties correlating with the level of anthocyanin accumulation were determined using the Folin-Ciocalteu method. In comparison with green-leaved cultivars, red-leaved lettuce was found to exhibit a greater antioxidant activity, thus representing a more valuable and functional food product. According to the obtained results, a system of additional illumination is required for intensification of the anthocyan biosynthesis aimed at obtaining highquality products cultivated hydroponically in green-house facilities.