Responses of Flavonoids, Phenolics, and Antioxidant Activity in Rice Seedlings between Japonica and Indica Subtypes to Chilling Stress

Chilling is one of the major abiotic stress which limites yield and quality of many crops. The seedlings of rice varieties namely Koshihikari (Japonica subtype) chilling tolerant, and the susceptible NMR2 (Indica subtype) were treated at 25/15 °C and 5/4 °C day/night to determine the growth parameters, phenolic contents, and antioxidant activity. It was found that in all treatments, the growth of MNR2 including root and shoot lengths, and leaf and root weights were inhibited at greater levels than Koshihikari. There were seven phenolic acids identified in leaves of Koshihikari including caffeic acid, vanillin, ferulic acid, sinapic acid, benzoic acid, ellagic acid, and cinamic acid, but only benzoic acid and ellagic acid were found in leaves of MNR2. In contrast, only vanillic acid and ellagic acid were observed in roots of Koshihikari, whilst ellagic acid and cinnamic acid were found in roots of MNR2. It was found that rice reduced amount of phenolic acids but promoted quantity of total phenolic content (TPC) and total flavonoid content (TFC) and level of antioxidant activity in chilling stress, although the level of responses varied between Japonica and Indica subtypes. Tolerant rice possessed greater bound flavonoids, phenolics and phenolic acids, but susceptible rice accumulated greater free TPC and TFC in reduced temperature. Findings of this study highlighted that phenolic constituents in bound forms of phenols, polyphenols, and flavonoids may play an active role in rice plants than phenolic acids under chilling stress but need further elaboration.