Leaf Tissue Carotenoids and Chlorophyll Fluorescence Parameters Differ Among Sweet Corn Genotypes with Differential Sensitivity to Mesotrione

Leaf tissue carotenoid pigments play essential roles in the photoprotection of photosynthetic membranes.  Carotenoids contribute to non-photochemical quenching (NPQ) of excitation energy in antenna systems.  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 p-hydroxyphenylpyruvate dioxygenase (HPPD), essential to plastoquinone production.  Plastoquinone is a cofactor for phytoene desaturase, a critical enzyme for carotenoid biosynthesis.  Sweet corn is tolerant to mesotrione applications; however, differing sensitivity exist among genotypes.  What remains unclear is the impact of mesotrione on leaf tissue carotenoids and chlorophyll fluorescence parameters following post-emergent applications.  Our research objectives were to characterize leaf tissue: 1) visual injury; 2) carotenoid concentrations; and 3) chlorophyll fluorescence parameters follwoing post-emergence mesotrione applications 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, adjacent 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.  Leaf tissues were sampled 14 days after treatment (DAT) of the late post applications.  Leaf tissues were dark acclimated for 1 hour, and measured for chlorophyll fluorescence parameters.  The same leaf tissues were freeze-dried prior to HPLC analysis of carotenoid and chlorophyll pigments.  Leaf tissues had outgrown visual bleaching symptomology at time of tissue collection.  Data revealed that genotype contributed significantly to leaf tissues pigments of beta-carotene, chlorophyll b, and the xanthophylls energy dissipation cycle pigments (zeaxanthin, antheraxanthin, violaxanthin).  Xanthophyll cycle pigments contribute to NPQ in leaf tissues, and NPQ values also differed among genotypes.  Leaf tissue SPAD measurements were influenced by both genotype and herbicide treatment.  Post-emergence herbicide treatments did not impact concentrations of leaf tissue pigments or photochemical quenching and NPQ parameters when tissues were sampled 14 DAT.  Results demonstrate genotypic variation among leaf pigments and chlorophyll fluorescence parameters may contribute to sweet corn sensitivity to mesotrione applications.