Seed coat tannins and bruchid resistance in stored cowpea seeds

Abstract: Seeds of wild species and varieties of Vigna were screened for their tannins and a-amylase inhibitor contents as defensive compounds against cowpea weevil. Seed coats contained condensed tannins that were positively correlated to their colour but not to their resistance against the insect. The a-amylase inhibitors were present in different amount in cotyledons of all species analysed. Amongst the cultivated lines assayed, Vigna unguiculata TVu 2027, an accession identified asmoderately resistant, was found to contain the higher amount of a-amylase inhibitor. When wild species were considered, V luteola and V vexillata (two resistant species) showed the highest content of a-amylase inhibitors. In addition, two cultivated accessions (Vita 7 and IT 84E-1-108) of cowpea seeds, both classified as susceptible accessions, showing a different degree of bruchid damage in storage, were also analysed. No a-amylase inhibitory activity was found in cotyledons of undamaged Vita 7 seeds, while the seed coat tannin content was found to be 13 times higher in undamaged Vita 7 seeds than in IT 84E-1-108 infested seeds. These latter results support the hypothesis that seed coat tannins must also be considered in biochemical defence mechanisms, which can deter, poison or starve bruchid larvae that feed on cowpea seeds.