Modulation of low-molecular-weight antioxidants in Amaranthus tricolor leaves exposed to cold stress during the ripening STA

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.