Changes in the phenolic acid content and antioxidant activity in colored maize grain during kernel development and analysis of the correlation with mycotoxin contamination at harvest maturity.

Maize (Zea mays L.) is one of the most varied grain crops that is widely cultivated and consumed worldwide. Maize red and pink ear rots are two of the major fungal diseases affecting maize production worldwide. The predominant species responsible for maize red ear rot in Europe are Fusarium graminearum and Fusarium culmorum, whereas pink ear rot is caused by Fusarium verticillioides and Fusarium proliferatum. These pathogens could be responsible for the production of mycotoxin such as deoxynivalenol (DON) and fumonisins (FUM). The identification of naturally occurring mechanism in plants that lead to reduced mycotoxin accumulation has gained a lot of interest. Several constitutive or pathogen-induced plant endogenous compounds, such as phenylpropanoids, reduce in vitro fungal growth and mycotoxin accumulation1. In this investigation, maize ears were randomly handpicked from plants at four growth stages, namely: end of the silking stage (about 5 days after silking (DAS)), blister stage (about 7 DAS), dough stage (about 32 DAS) and harvest maturity (about 75 DAS). The aim of this study was to determine the evolution of phenolic acids and Total Antioxidant Activity (TAA) during kernel development of four maize open-pollinated varieties and two representative hybrids, cultivated at the same site and characterized by a wide range of colors (dark red, red, yellow and white) in order to evaluate if they could have a protective effect towards mycotoxin contamination. TAA, measured by the QUENCHER method2, showed significant differences among maize types at different stages of development. In general the highest TAA was observed at the beginning of kernel development. At the dough stage, the TAA decreased 2 to 5 times less than values detected at the blister stage. The lowest TAA was observed for all maize types at the harvest maturity. Total free and cell wall-bound phenolic acids were quantified by a spectrophotometric method, while phenolic acid profile was analyzed by LC-MS/MS analyses. The content of total cell wall-bound- was higher than the total free phenolic acid in agreement with previous results both for maize3 and other cereal samples4. Total cell wall-bound- and free phenolic acids ranged from 1.88 to 12.52 mg g-1, and from 0.23 to 6.54 mg g-1 of dry weight, respectively depending on the type of maize and kernel stage. On average, the dark red variety showed the highest total cell wall-bound and free phenolic acid content at all stages of kernel development, while the white hybrid exhibited the lowest one. Similar to results on TAA, the highest total cell wall-bound and free phenolic acid concentrations were observed at the end of the silking stage and at the blister stage. On average, a significant decrease in content of both total cell wall-bound and free phenolic acids was observed at the dough stage and at harvest maturity. Ferulic, p-coumaric and caffeic acid were the major cell wall-bound phenolic acids during kernel development, but their relative proportions changed depending on the stage of development. Chlorogenic acid was the main free phenolic acid detected during kernel development followed by ferulic acid and vanillic acid, the latter of which was detected mainly at the beginning of kernel development. DON and FUM contamination of samples collected at harvest maturity was analyzed by LC-MS/MS. Significant negative correlation was observed between free phenolic acids and TAA at the beginning of kernel development and DON contamination at harvest maturity, while no significant correlation was observed with FUM contamination. Therefore, the higher the free phenolic acid concentration at the beginning of kernel development, the lower the DON contamination at harvest maturity. Moreover results suggest that both free chlorogenic and ferulic acid, in particular, could be related to ear rot resistance. The findings on phenolic acids provide insight into their evolution during kernel development, evidence of their correlation with mycotoxin contamination and information about bioactive compound content of maize varieties and hybrids characterized by a wide range of color.