Factors Affecting Pre-Harvest Aflatoxin Contamination of Peanut (Arachis Hypogaea L.)

Abstract

Aflatoxin contamination of peanuts is one of the greatest concerns in peanut production throughout the world. Aflatoxin in peanut can be severe when plants are subjected to late season drought stress. Available carbon source in the media is proved to be another factor affecting aflatoxin production under in vitro studies. Immature and drought-stressed peanuts are reported to have greater carbohydrate levels than mature, non-stressed seed. Most of the research to date to understand the influence of sugars in aflatoxin production has been done to evaluate carbon sources that optimize or support aflatoxin production. However, it is important to understand how sugars in peanut seed influence the synthesis of aflatoxin. Supplemental soil calcium has also been reported to reduce aflatoxin accumulation in peanuts. This project was initiated with the following objectives: 1) to evaluate peanut sugar concentrations for the growth of and aflatoxin production by Aspergillus flavus and 2) to determine the effects of drought and soil calcium on aflatoxin contamination and soluble sugar (i.e., sucrose, glucose, and fructose) concentrations of peanut seed in the greenhouse and field. For objective 1, A. flavus was grown in media containing different concentrations of sugars in the range that naturally occurs in peanut. Aflatoxins and mycelial dry weights were determined after defined periods of incubation. Mycelial weight and total aflatoxin produced consistently increased as sucrose concentration within the range in peanut seed rose. No significant differences in aflatoxin content and mycelial weight were noted due to total reducing sugars. For objective 2, peanuts were grown in soils at various levels of soil calcium and drought periods in a greenhouse experiment and two field experiments located at Wiregrass Research and Extension Center, Headland, AL and E.V. Smith Research Center, Plant Breeding Unit, Shorter, AL. The greenhouse experiment consisted of 5 soil calcium levels and 3 drought treatments, the Wiregrass experiment consisted of 2 soil calcium levels and 2 drought treatments, and the E.V. Smith experiment consisted of 2 drought treatments. Peanut seeds were analyzed for total aflatoxins (i.e., B1, B2, G1, and G2) and soluble sugars (i.e., sucrose, glucose, and fructose). In all experiments, total aflatoxin levels were significantly higher in drought-stressed compared to non-drought-stressed peanuts. Total aflatoxins significantly declined with increases in soil calcium in the greenhouse but not in the Wiregrass experiment. In the greenhouse experiment, negative correlations were also observed between aflatoxins and calcium contents of leaves, shells and kernels. In the greenhouse and E.V. Smith experiments, total soluble sugars and sucrose levels were significantly higher in drought-stressed-peanuts compared to non-drought-stressed peanuts. Further, positive correlations were observed with peanut total soluble sugars, sucrose and aflatoxins. However, no differences in the concentrations of total reducing sugars were found due to drought. In the Wiregrass experiment, no differences in peanut sugars were noted due to drought. In all experiments, soil calcium content did not influence sugar levels in peanut.