Melatonin: A novel and beneficial substance in sweet potatoes through selenium application

The effects of foliar Se (selenium) fertilizer on melatonin and its biosynthesis in four different colored sweet potatoes were studied. Solutions containing 1.25 mg Se/plant of inorganic selenium (ISe) and organic selenium (OSe) and a control check (CK) were applied three times during the swelling stage. Except for ISe in purple variety, both types of Se applications significantly increased melatonin in four colored varieties. The effect of OSe was greater than that of ISe, mainly because of higher concentration of tryptophan and activities of tryptophan decarboxylase, tryptamine-5 hydroxylase, 5-hydroxytryptamine N-acetyltransferase and N-acetyl-5-hydroxytryptamine methyltransferase. The orange variety had highest melatonin with the application of ISe and OSe, and highest melatonin among all applications was achieved by OSe in orange variety, followed by OSe in purple variety. These findings revealed that melatonin with extremely strong health benefits could be found and significantly increased in sweet potatoes through Se applications.