Exogenous Melatonin Modulates Drought Response and Recovery in Wheat with Contrasting Grain Colour

Melatonin is recognised as a multifunctional regulatory molecule that enhances plant tolerance to abiotic stresses, but its effectiveness is often strongly genotype-dependent. This study aimed to elucidate how exogenous melatonin (200 µM) modulates the physiological and biochemical responses of wheat during drought and subsequent recovery in two genotypes with contrasting grain pigmentation: the standard cv. Bohemia (red grain) and an experimental purple-pericarp (PP) line. Plants were exposed to drought at the early vegetative stage (BBCH 15), and gas exchange, leaf water potential, and biochemical markers (proline, malondialdehyde, phenolics, and flavonoids) were assessed during drought and after rehydration. In cv. Bohemia, water deficit led to a pronounced decrease in CO2 assimilation, stomatal conductance, and leaf water potential, accompanied by strong increases in proline (Pro) and malondialdehyde (MDA). Melatonin application in this genotype markedly reduced the accumulation of Pro and MDA and accelerated the recovery of gas exchange, indicating a significant protective effect. The lower Pro levels in melatonin-treated Bohemia plants suggest that melatonin mitigated the perceived stress intensity, thereby reducing the physiological demand for osmotic adjustment. In contrast, the PP line exhibited higher inherent stability of the photosynthetic apparatus and more moderate biochemical shifts; its recovery was almost complete and independent of melatonin. Overall, these results indicate that the functional benefit of exogenous melatonin is greater in genotypes with a lower intrinsic stress-buffering capacity. This study highlights the importance of considering constitutive genotype traits and the recovery phase when using physiological regulators to improve wheat drought resilience.