短期缺水對芥藍菜(Brassica oleracea)硝酸鹽、草酸鹽含量與抗氧化能力的影響

Abstract

Chinese kale, Brassica oleracea, is a leafy vegetable from the Brassicacea family and is widely consumed in Asian cuisine. The edible portions of the plant include the leaves and some parts of the stem at maturity, 45 days after emergence (DAE). Concerns about nitrate and oxalate levels in plants that have experienced drought stress caused by water deficit and probable coupling of nitrate (NO3-) reduction and oxalate biosynthesis incited this study. Six varieties were surveyed in order to select two varieties for more detailed study. These varieties were Big Stem Yellow (V1), Green Leaf (V2), Tainan Black (V3), Round Leaf White (V4), Tall Black (V5) and Veg-Gin (V6). Two varieties of Chinese kale, Big Stem Yellow (V1) a locally grown variety and Round Leaf White (V4), a cultivated variety for commercial purposes were chosen to study in more detail to determine the stress response of different varieties of the same species. They were subjected to a carefully monitored period of water deficit that lasted for seven days; thereafter they were irrigated and allowed to recover. High Performance Liquid Chromatography (HPLC) analysis was conducted during vegetative stage, water deficit stage and recovery stages. It was found that there was an increase in nitrate concentration only in the leaves of drought stressed plants (DS) for both varieties and differed from the control (CK) more than 300µg/g, the bigger difference seen in the V1 variety. There was also increase nitrate concentration in the stems and whole plants higher than 500 µg/g when compared to the control in the V1 variety. There was a decrease in soluble oxalate concentration in the leaves by as much as 30 mg/g for the V1 variety when compared to the control during the stress stage. There was approximately 85 mg/g difference however for the V4 variety. Also, the concentration in the stem was comparable to the control for both varieties. Soluble and insoluble oxalate concentrations increased throughout the stages for both varieties. The total oxalate concentrations for the DS treatment for leaves and stems and whole plants were significantly lower than its CK counterpart for both varieties; whole plant concentrations were significantly lower than CK treatment. Concentrations were also lower in the CK treatment by as much as 99 mg/g for soluble oxalate in the V4 variety. Meanwhile, the DS for the V1 variety was much as 61mg/g for soluble oxalate concentration. These findings were compared to that of spinach bought from the market. It was determined that V1 had better recovery from short term drought stress. This variety was further examined for its antioxidant capacity against peroxyl radicals, hydroxyl radicals and transition metals. The assay was conducted and the data was obtained by using flourescence spectrophotometry for detecting oxygen radical absorbance capacity (ORAC). It was found that there was a general increase in trend of antioxidant capacity for the control but during the stress stage there was a marked decrease in antioxidant capacity which caused a significant reduction in the total score when compared to the control.