Abstract
Glycine, the simplest amino acid in nature and one of the most abundant free amino acids in soil, is regarded as a model nutrient in organic nitrogen studies. To date, many studies have focused on the uptake, metabolism and distribution of organic nitrogen in plants, but few have investigated the nutritional performance of plants supplied with organic nitrogen. Lettuce (Lactuca sativa L.), one of the most widely consumed leafy vegetables worldwide, is a significant source of antioxidants and bioactive compounds such as polyphenols, ascorbic acid and tocopherols. In this study, two lettuce cultivars, Shenxuan 1 and Lollo Rossa, were hydroponically cultured in media containing 4.5, 9, or 18 mM glycine or 9 mM nitrate (control) for 4 weeks, and the levels of health-promoting compounds and antioxidant activity of the lettuce leaf extracts were evaluated. Glycine significantly reduced fresh weight compared to control lettuce, while 9 mM glycine significantly increased fresh weight compared to 4.5 or 18 mM glycine. Compared to controls, glycine (18 mM for Shenxuan 1; 9 mM for Lollo Rossa) significantly increased the levels of most antioxidants (including total polyphenols, α-tocopherol) and antioxidant activity, suggesting appropriate glycine supply promotes antioxidant accumulation and activity. Glycine induced most glycosylated quercetin derivatives and luteolin derivatives detected and decreased some phenolic acids compared to nitrate treatment. This study indicates exogenous glycine supplementation could be used strategically to promote the accumulation of health-promoting compounds and antioxidant activity of hydroponically grown lettuce, which could potentially improve human nutrition.
Highlights
A balanced diet is essential to ensure physical development and health
After recovering the seedlings in water for 3 days, the seedlings were cultivated for 30 days in nitrogenous nutrient solution (1.25 mM Mg, 3.5 mM K, 1.25 mM S, 2.05 mM Ca, 1 mM P, and 6.6 mM Cl, pH 5.8) containing different forms and concentrations of nitrogen: 9 mM nitrate, 4.5 mM glycine (4.5Gly), 9 mM glycine (9Gly) or 18 mM glycine (18Gly)
The anthocyanidin level peaked at 34.72 ng per cm2 in Lollo Rossa leaves exposed to 18 mM glycine, which was significantly higher than the plants treated by 4.5 or 9 mM glycine
Summary
A balanced diet is essential to ensure physical development and health. Numerous epidemiological studies have suggested high daily consumption of fruits and vegetables lowers the risk of several chronic diseases, such as cancer, cardiovascular disease and diabetes; the protective effects of fruit and vegetable consumption are mainly attributed to the presence of bioactive phytochemicals such as polyphenols, vitamin C and vitamin E (Arts and Hollman, 2005; Hooper and Cassidy, 2006; Russo et al, 2012; Chen and Chen, 2013; Wang et al, 2017). The economically valuable vegetable crop lettuce (Lactuca sativa) is a minimally processed food product available throughout the entire year, and is a significant source of natural healthpromoting compounds. Multiple factors, such as environmental conditions, agronomical manipulation, harvest time, watering and fertilization can strongly influence the levels of healthpromoting compounds in horticultural plants (Liu et al, 2007; Li and Kubota, 2009; Becker et al, 2014; Tavarini et al, 2015). Nitrogen fertilization plays an essential role in balancing the yield and quality of edible plants, especially the levels of secondary metabolites. Glycine is regarded as a model amino acid in plant organic nitrogen research
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