Amaranth is an indispensable C₄ agricultural crop with considerably reduced productivity under hypothermia loading. During seed ripening, chilling stress (1 °C–2 °C) can harm the photosynthetic organs in the plants and reduce the efficiency of low-molecular-weight defense systems. The studies on the content of low-molecular-weight antioxidants commenced in the leaves of the Amaranth cultivar Valentina cultivated in an open field in the post-stress period (after a chilling stress). After chilling at 2 °C in the post-stress period, older leaves of the main shoot displayed partial damage, while young leaves of the lateral shoots visually maintained a native appearance. The ascorbic acid (AA) content showed significant variations in the leaves. The content of possessing antioxidant properties revealed red-colored amaranthine decreased during this period, i.e., 1.5–1.9 times in young leaves, 3.5 times in leaves damaged by cold (DC), and non-damaged by cold (NDC) leaves showed a 1.1-times decrease. The decline in photosynthetic pigment content varied from 14% for carotenoids (Cars) to 60% for chlorophylls a (Chl a) and b (Chl b) in NDC leaves. The water and ethanol-soluble antioxidant contents improved with repeated cold stress (2 °C) in young leaves. The same pattern was also evident for the ascorbic acid and amaranthine content enhancement (20%–25% and 30%, respectively). The formation of hypothermia-induced tolerance in the leaves of different ages in autumn, which are the prime producers of low-molecular-weight antioxidants, signified a close relationship to the functioning of hydrophilic and hydrophobic antioxidants.

Keywords: Amaranthus tricolor L., red-colored Valentina cultivar, cold stress, low-molecular-weight antioxidants, amaranthine, ascorbic acid

Key findings: New data have been notable from the amaranthine pigment contributing to increasing the resistance of amaranth plants to chilling stress. Variations in the low-molecular-weight antioxidants’ content and functional activity, which have become traditionally resistance actors, were also evident.