Assessment of inhibitory substances in the seed coat of some cowpea cultivars for resistance against Callosobruchus maculates

Abstract

Laboratory experiment was conducted at the Faculty of Agriculture and Veterinary Medicine, Imo State University, Nigeria to assess the inhibitory substances in the seed coat of 15 cowpea cultivars for resistance against Callosobruchus maculatus. Fifty (50) seeds of the cowpea cultivars were collected from the International Institute of Tropical Agriculture, (IITA) Ibadan and placed in 15 different Petri-dishes with perforated tops and infested with 10 adult C. maculatus pest (4 males : 6 females). The Petri-dishes containing the seeds and the insects were allowed to stay undisturbed for 4 weeks. Another set of 50 seeds of the cowpea cultivars were soaked in water to allow for easy removal of the seed coats. The seed coats were dried in the oven at 65oC for about 25 min, and then ground into a fine powder. It was observed that the adult C. maculatus did not attack the cowpea. However, after 4 weeks, new C. maculatus adults were observed on the cowpea seeds. The number of emerged adult C. maculatus and the number of insect exit holes observed in the IT97K-568-18 and IT98K-131-2 cultivars were lower and significantly different from the other cultivars. The quantity of tannin and saponin in the seed coat of the cultivars IT97K-568-18 and IT98K-131-2 were significantly higher than the other cultivars. The ability of these two cowpea cultivars to resist the attack of C. maculatus was attributed to the presence of tannin and saponin in the seed coats. It is recommended that IT97K-568-18 and IT98K-131-2 cultivars be incorporated in the cowpea breeding programmes involving insect resistance.

Key words: Cowpea, Callosobruchus maculatus, Cultivars, Tannin, Saponin,Seed coat