Scientific review points to the effects of melatonin on vegetables

The exogenous application of melatonin, known for its regulatory function in animal metabolism, has gained ground as a strategy for disease management in horticultural crops. A study by Ningxia University in China compiled robust evidence of its effectiveness against several pathogens that affect fruits and vegetables.

Research on tomatoes, apples, mangos, cucumbers, oranges and other species shows that melatonin activates defense mechanisms related to antioxidant enzymes, plant hormones and gene expression. The compound improves tolerance to biotic stresses, including infections by bacteria, fungi and viruses.

In bacterial control, melatonin has been shown to reduce the effects of cassava bacterial wilt and huanglongbing in citrus fruits. The substance increased the levels of salicylic acid and jasmonate, in addition to reducing the population of the bacteria Candidatus Liberibacter asiaticus.

In fungal diseases, the data are even more comprehensive. In apples, it mitigated leaf spot caused by Diplocarpon mali. In mango, it improved resistance to anthracnose by increasing the synthesis of lignin and flavonoids. In cucumber, it helped control downy mildew by increasing antioxidant activity and reducing damage to cell membranes.

The results are also consistent against viruses. In apples, the application reduced the viral load of stem notch virus. In cucumbers, it activated genes linked to antiviral immunity. In eggplant, it improved the response to alfalfa mosaic virus, increasing levels of chlorophyll and antioxidant enzymes.

In addition to its direct action against pathogens, melatonin promoted physiological gains in plants: increased leaf area, root growth, higher photosynthetic rate and hormonal balance. These effects were observed in crops such as apples, pears, tomatoes, cucumbers and bananas.

The molecular mechanisms identified include regulation of genes related to the salicylic acid, jasmonic acid, ethylene and auxin pathways. Melatonin also activates defense pathways such as the phenylpropanoid and pentose phosphate pathways.

However, the use of the substance requires caution. In citrus infected by Penicillium digitatum, the application of melatonin reduced resistance, worsening symptoms of the disease. The substance decreased hydrogen peroxide (H₂O₂) and compromised the action of antioxidant enzymes.

Another relevant limitation is the scarcity of field studies. Most experiments were carried out under controlled conditions. There is insufficient data on interactions with insects, nematodes or weeds. There is also a lack of information on the behavior of melatonin in different soil types and climates.

More information at doi.org/10.1093/hr/uhaf150