Caracterização bioquímica e avaliação nutricional das proteínas de genótipos de feijão-de-corda susceptíveis e resistentes ao CpSMV (Vírus do mosaico severo do feijão-de-corda)

Abstract

Cowpea [Vigna unguiculata (L.) Walp.] is a very important legume human dietary constituent supplying protein, fiber, vitamin and energy. In the Northeast of Brazil, breeding efforts involving cowpea have been directed towards the selection of high yielding varieties associated with traits of resistance to diseases, grain quality and plant architecture. However, studies on the nutritional quality of these improved genotypes are scanty. The research was conducted with four cowpea genotypes, two resistant to the Cowpea Severe Mosaic Virus - CpSMV (TE93-244-23F and BR17-Gurgúeia) and the other ones susceptible (Canapuzinho and Sempre Verde) to evaluate whether the probable factors related to the resistance would interfere with the nutritional value. Amongst the several parameters analyzed was not possible to establish a direct relationship with the resistance or susceptibility to CpSMV. Proximate composition reinforced that cowpea is a good source of protein (23.32 to 26.12 g/100 g dry matter), carbohydrate (62.9 to 67.4 gí100 g dry matter) and crude fiber (3.77 to 6.23 g/100 g dry matter), presenting low contents of oil (1.44 to 1.77 g/100 g dry matter). Regarding to the main types of seed protein the globulin fraction was the most representative, followed by basic glutelins and albumins. Polyacrylamide gel electrophoresis of the flours and protein fractions showed that no qualitative differences were found. Data on amino acid composition showed similar profiles for all genotypes. The hemagglutinating activity against enzyme-treated rabbit erythrocytes was detected only in the seed crude extracts and globulin fractions. The trypsin inhibitory activity of the crude extract varied from 12.20 to 20.42 Ul/g flour. The albumin presented a higher content of trypsin inhibitor (2.43 to 4.26 Ul/g flour) than the globulin fractions (1.44 to 2.88 UI/g flour). Urease activity varied amongst the crude extract cowpea genotypes (15.81 to 20.49 U/g flour), being higher than those detected in the protein fractions. Differences in the tannin contents were also observed in the cowpea flours (4.76 to 13.42 mg/100 g flour). Amongst the defense proteins, the endochitinase activity was much higher in the resistant genotypes, BR17-Gurguéia (17.58 nKat/g flour) and TE93-244-23F (20.53 nKat/g flour), than in the susceptible ones, Sempre Verde (14.52 nKat/g flour) and Canapuzinho (14.26 nKat/g flour). The seed flour and protein fraction amino acid compositions were deficient in methionine + cysteine, showing chemical scores of 17.1 to 22.5% and 32.6 to 34.9%, respectively. Seed flours of TE93-244-23F and Canapuzinho genotypes, even supplied with methionine, were not able to maintain the appropriate growth of the rats, presenting low values of NPU, digestibility, biological value and alimentary conversion when compared to the animals fed the high quality diet containing egg white. However, albumin and globulin fractions complemented with egg white, showed a nutritional value similar or even superior to positive control. Based on these results, it is possible to suggest that the breeding strategies used to improve the cowpea resistance to CpSMV did not interfere its nutritional quality.