Phenolic compounds in peanut : studies on pod maturity, ellagic acid, elicitors, growth and aflatoxin production by A̲s̲p̲e̲r̲g̲i̲l̲l̲u̲s̲ f̲l̲a̲v̲u̲s̲, seed protein profiles, and isozymes

Abstract

Phenolic compounds were studied in relation to pod-rot and Aspergillus flavus resistance during pod development of peanuts. Amounts of free phenolic compounds (FPC) were abundant in immature (stage 2) while bound phenolic compounds (BPC) were greatest in mature (stage 7) pods by Folin-Ciocalteu assay. Resistant genotypes to pod-rotting fungi and to A. flavus invasion significantly contained high FPC and BPC levels at certain stages of pod maturity. Preformed caffeic, umbelliferone, p-coumaric, and unknown phenolic acids were present in high amounts. Ellagic acid (known to inhibit mutagenesis and carcinogenesis caused by aflatoxin-producing fungi) contained 80, 68, and 42 mg g^-1 in mature tissues of testae, cotyledons, and pods, respectively by a three-step gradient reverse-phase high performance liquid chromatography (HPLC). Chitosan enhanced elicitation of FPC but BPC decreased over time at water activity (Aw) levels of .85 and .95. A. flavus increased FPC after 9 h of incubation but chitosan + A. flavus caused a more pronounced increase in FPC. Major free and bound phenolic acids detected by reverse-phase HPLC were p-coumaric, ferulic and an unknown phenolic acid eluting at 22 min. Chitosan could limit A. flavus growth and subsequent aflatoxin production by inducing susceptible tissues to produce more preformed phenolic compounds. Analysis of liquid cultures of A. flavus revealed that p-coumaric, ferulic, and vanillic acids and their mixture slightly inhibited mycelial growth. Production of aflatoxin B1 was greatly affected by phenolic acids. By 48 h after inoculation of mature seeds with A. flavus, an early change in protein composition of moderately susceptible Starr was detected by non-denaturing gel electrophoresis. Resistant J-11 exhibited a delay in alteration of protein composition until 72 h. Chitosan and chitosan + flavus-treated seeds did not have any protein molecular weight changes over 72 h. This delay is likely an inherent defensive mechanism of resistance. Changes in isozymes revealed that shikimate dehydrogenase was the best marker for comparing the effect of elicitors on enhanced phenolic compound induction. Increased activities of some enzymes might be attributed to an activation of either pre-existing enzymes or de novo synthesis.