Effects of Cultivation Temperature on Phenolic Compound Content and Antioxidant Capacity in the Leaves and Stems of the Vegetable Gynura bicolor (Roxb. ex Willd.) DC

The leafy vegetable Gynura bicolor (Roxb. ex Willd.) DC. is cultivated in Taiwan and is popular because of its abundant antioxidant components. In this study, the effects of cultivation temperature on the antioxidant levels and antioxidant activity of methanolic extract from G. bicolor leaves and stems were studied. In cultivated plants, total phenolics were the most abundant components of the methanol extract. The levels of antioxidant compounds from the aboveground portions were highest when cultivated at 25 °C, followed by those when cultivation at 20, 30, and 35 °C. The antioxidant capacity of the leaf extract was highest when G. bicolor was grown at 25 °C, scavenging 69% of the 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical. The antioxidant capacity was lower in stems, scavenging 26% to 41% of the DPPH. A principal component analysis (PCA) was performed to explore the correlation between phenolic compound levels and antioxidant capacity under different cultivation temperatures. The PCA confirmed that growth at 25 °C resulted in the highest levels of phenolic compounds in the leaves G. bicolor .