Modulatory Effects of Pre- and Post-Drought Root Application of Melatonin on the Antioxidant Defense in Young Wheat Plants

Melatonin is a naturally occurring compound that regulates many aspects of plant growth and development. Recently, its stress-protective potential has been extensively studied. This study investigates the effect of exogenous melatonin on non-enzymatic antioxidants, gene expression, and activity of key antioxidant enzymes in young winter wheat plants subjected to 5 days of drought. Melatonin was root-supplemented 24 h before or after the stress. The parameters were analyzed in the leaves of two Bulgarian cultivars at the end of drought, and after recovery. Drought activated both enzymatic and non-enzymatic antioxidant defense in both cultivars, with distinct responses reflecting their tolerance. The drought-tolerant cv. Gines showed marked increase in total phenolics, thiol containing compounds, catalase (CAT) and glutathione reductase (GR) activities, and catalase (CATA, CAT3) and class III peroxidase (POX2) transcript levels. The less tolerant cv. Fermer exhibited more limited induction, primarily involving CATA, CAT3 and GR transcripts and the glutathione pool. Melatonin pre-treatment generally attenuated drought-induced antioxidant responses. This effect was more pronounced in cv. Fermer, where the most drought-responsive parameters were also the most alleviated by melatonin, whereas in cv. Gines only CATA expression, and CAT and GR activities were significantly influenced, suggesting cultivar-dependent modulation of antioxidant systems by melatonin. During recovery, both pre- and post-drought melatonin applications produced comparable effects on the antioxidant defense. The post-treatment selectively enhanced the studied transcripts in a cultivar-specific manner.