Wheat can tolerate a mild water deficit, but prolonged drought causes a number of detrimental physiological changes resulting in a substantial decrease in productivity. The present study evaluates the potential of the natural plant growth regulator melatonin to alleviate the negative effects of moderate drought in two Bulgarian winter wheat cultivars at the early vegetative stage. Melatonin doses of 75 µM were root-supplemented 24 h before or after the stress period. The levels of several biometric parameters, osmolyte content and stress indicators as well as the expression of genes coding for key enzymes of the proline biosynthesis pathway were analyzed in leaves at the end of the drought stress and after two and four days of recovery. Applied alone, melatonin did not exert significant effects on most of the monitored parameters. Water deprivation negatively affected seedlings’ fresh weight and water content and increased the stress markers and osmolyte levels. These were accompanied by a high accumulation of TaP5CS and TaP5CR transcripts coding for the enzymes Δ-pyrroline-5-carboxylate synthase and Δ-pyrroline-5-carboxylate reductase, respectively. The effect of melatonin in reducing drought stress was similar whether applied before or after exposure, though slightly more effective when used as a pre-treatment.

Drought stress affects many aspects of plant biochemistry, with photosynthesis being one the most significantly impaired physiological processes. Melatonin is a natural antioxidant with growth-regulating properties in plants. Its diverse physiological functions have been extensively studied in recent decades. Changes in leaf gas exchange and chlorophyll a fluorescence parameters were investigated in young wheat plants (Triticum aestivum L.) cv. Fermer and cv. Gines which were characterized to differ in their responses to drought, with cv. Gines being more tolerant than cv. Fermer. The plants were subjected to drought for five days by withholding their water supply. Melatonin was applied as a root supplement to the irrigation water before or after the drought period. Analyses were performed before and at the end of the stress period, as well as during the recovery phase. Changes in leaf pigment content, photosynthetic rate, stomatal conductance, and transpiration, as well as some chlorophyll a fluorescence parameters, were recorded. Melatonin alone did not cause considerable changes in the measured traits. We found a significant decrease in leaf gas exchange parameters, Fv/Fm and Fv/F0 values, and leaf pigments due to drought, especially in cv. Fermer. The data show that the application of melatonin favorably influenced the efficiency of the photosynthetic apparatus under water deprivation and during plant recovery. The pre-treatment with melatonin maintained the photosynthesis-related parameters closer to the control levels during the stress period. Both melatonin treatments supported the recovery of photosynthesis when the water supply was restored and the post-drought treatment showed a similar but weaker effect than pre-drought treatment.

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.