Causes of differences in seed quality among cowpea (Vigna unguiculata (L.) Walp) cultivars

Three cowpea cultivars differing in seed coat colour, IT83S-818 (white, black-eye), TVX 2724-01F (brown) and IT82D-32 (dark brown), were aged using the controlled deterioration method (20% seed moisture content and incubated at 40 oC) for 0, 1, 2 and 4 days. Time to 50 per cent germination (t50) and final germination percentage were determined to assess the vigour differences that exist among these cultivars. In unaged seeds, the white cultivar germinated quicker than the pigmented cultivars as a result of the characteristically higher rate of water uptake, which enhanced rapid hydration of the embryonic cells for quicker germination. When ageing progressed, however, the white cultivar germinated more slowly with a lower final germination percentage since the embryonic cells possibly became weaker and leaky leading to reduced vigour and eventually, loss of germinability. In contrast, the pigmented cultivars germinated more slowly before seeds were aged due to their slower rates of water uptake; germination became quicker as ageing progressed due to increasing softening of the seed coats. Thus, the pigmented cultivars showed decreases in the time to 50 per cent germination during ageing. They, however, retained high germination percentages throughout the ageing period, which is an indication of high vigour. These three cultivars, together with two other unpigmented cultivars, IT81D-1137 (white) and TVX 3236 (cream and brown) were subsequently examined for the incidence of disease pathogens. It was observed that the unpigmented cultivars had higher levels of the importasnt seed pathogens such as Fusarium species and Aspergillus flavus which cause reduction in germination. These seeds also showed susceptibility to mechanical damage caused by hand-beating of dry pods in sacs. These differences in behaviour between the two groups of cowpea cultivars, pigmented and unpigmented, were attributed to differences in tannin and lignin contents in their seed coats