Cowpea [Vigna unguiculata (L.) Walp.] seed coat pigmentation in relation to its chemical composition and function

Chemical compounds of seed coats of five cowpea [Vigna unguiculata (L.) Walp.] cultivars differing in testa colour, viz., a white, black eye, a white, brown eye, a brown and cream, a brown, and a dark brown, were determined and related to their colours and functions. Pigments were extracted in 1% HCl in methanol, loaded onto a Sephadex LH-20 column, and eluted with 95% ethanol followed by 50% aqueous acetone. Absorbances of 50 ethanol fractions containing non-tannin polyphenolic compounds and 30 acetone fractions containing tannins were measured and an elution profile constructed. The elution profile of the ethanol fractions showed three major peaks in the pigmented and the cream and brown seed coats compared to one in the unpigmented cultivars. The acetone elution profile showed one high peak in the two pigmented and the cream and brown cultivars compared to one low peak in the unpigmented cultivars. The absorbance per gram of the pigmented seed coats eluted in ethanol and acetone were 2-3 and more than 10 times higher, respectively, than of those of the white seeds. The dark brown colour of the acetone eluants (tannins) and copper pink colour pigments significantly influenced the seed coat colours of the pigmented cultivars. Spectral maxima and R-f values of the peak fractions revealed the nature and identity of the pigments that influenced cowpea seed coat pigmentation and function.