Mesotrione Applications Impact Kernel Carotenoid Concentrations Among Different Sweet Corn Genotypic Sensitivities

Mesotrione (Callisto^®) is currently labeled for selective pre- and post-emergence weed control in sweet corn (Zea mays var. rugosa).  Mesotrione competitively inhibits the enzyme 4-hydroxyphenylpyruvate dioxygenase (HPPD), an essential component for the conversion of tyrosine to plastoquinone.  Plastoquinone is a cofactor for phytoene desaturase, a critical enzyme for carotenoid biosynthesis.  Sweet corn is one of only a few vegetable sources high in zeaxanthin carotenoids.  Carotenoids serve antioxidant functions in photosynthetic processes, as well as in actions of disease reduction in mammalian systems.  Sweet corn is tolerant to mesotrione applications; however, differing sensitivity exist within genotypes.  What remains unclear is the impact of mesotrione on carotenoid concentrations in developing sweet corn kernels.  Our research objectives were to: 1) characterize crop visual injury; and 2) measure kernel carotenoid concentrations in response to post-emergence applications of mesotrione to cultivars of different sensitivities [‘Merit’(sensitive), ‘Temptation’(tolerant), ‘Incredible’(moderately sensitive)].  Cultivars were seeded in Knoxville, TN on 01 May 2008 in a randomized complete block design with four replications at two separate sites.  Post-emergence treatments included: 1) untreated control; 2) mesotrione (105 g ai/ha) as early post; 3) mesotrione (105 g ai/ha) as late post; 4) mesotrione (105 g ai/ha) + atrazine (560 g ai/ha) as early post; 5) mesotrione (105 g ai/ha) + atrazine (560 g ai/ha) as late post; 6) atrazine (560 g ai/ha) as early post; and 7) atrazine (560 g ai/ha) as late post.  Corn plants were 5-10 and 15-20 cm in height for early-post and late-post applications, respectively.  At harvest, eight uniform ears were collected from the center of the sprayed area, and stored 24-48 h at 4 °C.  Kernels were cut from a 5-cm section of each ear for carotenoid analysis.  Kernels were freeze-dried and ground in liquid nitrogen prior to carotenoid extraction.  HPLC separation was used for kernel carotenoid quantification.  Data revealed that no post-emergence treatment with mesotrione alone negatively impacted the concentration of kernel carotenoids, as compared to the untreated control.  Visual injury among genotypic sensitivities followed previous research findings; however, greater visual injury following post-emergence applications correlated with higher kernel carotenoid concentrations, especially for early-post applications.  Early-post applications of mesotrione + atrazine to the moderately sensitive genotype resulted in significantly higher kernel lutein and zeaxanthin concentrations.  No such results were found for the tolerant genotypes.  Data revealed a strong genotypic effect on responses of kernel carotenoids to mesotrione alone, or when tank mixed with atrazine.