Melanin Found in Wheat Spike Husks

Melanin is the dark polymer pigment found in all kingdoms of life. Plant allomelanin, formed through the oxidation and polymerization of phenolic compounds, does not contain nitrogen; however, it possesses similar properties to melanin of animal, fungal, or bacterial origin. The black coloration of awns, spike husk edges, and even complete spikes is well-known in wheat and occurs frequently in wild, but rarely in cultivated, wheat species. Previously, anthocyanins were considered the only pigments responsible for the black coloration of wheat ears. Recently, it has been shown that the black coloration of the husks in other cereals can be attributed to melanin or anthocyanins, or both of these pigments. In this study, using standard procedures for chemical extraction of anthocyanins and melanin, ultraviolet–visible–near-infrared spectroscopy, and hyperspectral imaging, we examined the pigment in Persian wheat (Triticum carthlicum Line 5999) black-colored spikes and found that it exhibits properties characteristic of melanin rather than anthocyanins. Also, using microscopy, we show that the dark pigment in the husks and awns of mature spikes is located mainly in the dead protoplasts of epiderma and sub-epidermal sclerenchyma cells. The localization of the pigment suggests that melanin may perform some protective or sunlight-to-heat transforming function.