Foliar spraying melatonin reduces the threat of chromium-contaminated water to wheat production by improving photosynthesis, limiting Cr translocation and reducing oxidative stress

Chromium (Cr)-contaminated in irrigation water poses a significant threat to the safety of wheat (Triticum aestivum L.) production safety. Recent studies suggest that melatonin (MT) could enhance crop tolerance to Cr pollution. This study aimed to investigate the effects of foliar spraying MT on alleviating Cr toxicity and accumulation in wheat irrigated with K₂Cr₂O₇ solution at concentrations of 5, 10, and 20 mg/kg Cr in the soil. Our results showed that Cr-contaminated water irrigation significantly reduced dry weight, grain numbers, grain weight, yield, harvest index, net photosynthetic rate (P_n), maximum and actual photochemical efficiency of photosystem II (F_v/F_m and Φ_PSII), chlorophyll contents, and the a/b ratio. It also increased PSII photodamage and oxidative stress in wheat leaves, resulting in high Cr accumulation in roots, leaves, and grains. Foliar spraying of MT alleviated Cr toxicity by improving P_n, F_v/F_m, and Φ_PSII, enhancing chlorophyll content, promoting dry matter accumulation and yield, and reducing oxidative stress and Cr translocation. Furthermore, MT application enhanced transcriptional regulation, alleviated oxidative stress by boosting antioxidant enzyme activities, and restricted Cr translocation from roots to leaves and grains by increasing the accumulation of secondary metabolites, such as lignin and metallothionein. These findings suggest that MT application could serve as a viable strategy for reducing Cr contamination in cereals and supporting phytoremediation efforts.