Changes in enzyme activities and aflatoxin elaboration in sorghum genotypes followingAspergillus parasiticus infestation

Abstract

Sorghum is a relatively poor substrate for aflatoxin production compared with high-risk agricultural commodities like maize and groundnut, even though it is susceptible to fungal attack. Fungal infestation of sorghum results in a varied biochemical composition of the deteriorated grain. In this study, six sorghum genotypes (red—AON 486, IS 620; yellow—LPJ, IS 17 779; white—SPV 86, SPV 462) were inoculated with a toxigenic strain of Aspergillus parasiticus (NRRL 2999) in order to evaluate the changes in the activities of various hydrolytic enzymes (α- and β-amylases, protease and lipase) in comparison with those in uninfected grains. Enzyme activities were measured at different times after fungal infestation, and the enzymatic activities were correlated with the aflatoxin production. Alpha-amylase activity was observed to be greater than β-amylase activity in all six genotypes under both healthy and infected conditions. The increase in α-amylase activity during the period of infection was higher in white genotypes than in red sorghum genotypes. Alpha-amylase activity in all the genotypes increased up to day 6 after fungal infection, but was significantly lower in infected grains than in healthy grains. The variability in the basal enzyme activities among the six sorghum genotypes was quite high compared with the amount of induction of each specific enzyme due to infection and germination. Higher protease activity was observed in the infected grains than in healthy grains. The enzyme activities in high tannin red genotypes were less than those in yellow and white genotypes. The α- and β-amylase activities were positively correlated (r = 0.406 and 0.436; P < 0.05) to aflatoxin production. Inherent lipase activity was highest (on day 0) in AON 486, SPV 462 and SPV 86, as compared with the activity in infected grains. The total aflatoxins produced (quantified by TLC-fluorodensitometry) were lower in red genotypes than in yellow and white genotypes, suggesting that red genotypes were least susceptible to aflatoxin elaboration among the various genotypes tested. All four aflatoxins, (B1, B2, G1 and G2) were present in five genotypes (IS 620, LPJ, IS 17 779, SPV 86 and SPV 462) at all the stages of infection, but, aflatoxin could not be detected in the red genotype AON 486 on day 3 after infection. White genotypes SPV 86 and SPV 462) showed maximal aflatoxin (total) production on day 6 after infection. © 2000 Society of Chemical Industry