Reducing Sugars and Minerals from Lint of Unopened Cotton Bolls as a Substrate for Aflatoxin and Kojic Acid Synthesis byAspergillus flavus

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Abstract
Invasion of developing cotton (G. hirsutum) bolls by A. flavus and subsequent formation of aflatoxins in cottonseed within such bolls is predicated on growth of the fungus on the lint. Unopened cotton bolls in the field and a water extract made from lint of unopened cotton bolls were evaluated as substrates for mycelial growth and synthesis of kojic acid and aflatoxin by A. flavus. Solids from the water extract were 47.3% carbohydrate and 6.96% protein that was partly peroxidase. Reducing sugars, which comprised 76% of the carbohydrate fraction, were a mixture of equal amounts of glucose and fructose. These reducing sugars on lint in unopened cotton bolls decreased from 15% 20 days after flowering to 0.1% 40 days after flowering, but the ratio of glucose to fructose remained constant. All minerals detected on the lint were also detected in the water extract, but the levels were lower in the extract than in the lint. The water extract was an excellent substrate for fungal growth and production of kojic acid and aflatoxin. In contrast, even though the fungus grew well in field-inoculated cotton bolls and kojic acid was produced, no toxin formed on the lint. Toxins were detected only in the seed. The possible involvement of Zn and enzymes present on lint in living cotton bolls in secondary metabolism of A. flavus is hypothesized.