Submitted to: Environmental Pollution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/9/2005
Publication Date: 10/1/2006
Citation: Burkey, K.O., Neufeld, H.S., Souza, L., Chappelka, A.H., Davison, A.W. 2006. Seasonal profiles of leaf ascorbic acid content and redox state in ozone-sensitive wildflowers. Environmental Pollution. 143 (3): 427-434. Interpretive Summary: Ground-level ozone is an air pollutant that has negative impacts on both crop plants and natural vegetation. Certain plants tolerate ozone better than others, possibly due to differences in the concentration of antioxidant compounds such as vitamin C that help protect against oxidative stress. In this study, three ozone-sensitive wildflower species native to the Great Smoky Mountains National Park (USA) were compared for differences in vitamin C content. Natural populations of coneflower (Rudbeckia laciniata L.), crownbeard (Verbesina occidentalis L.), and tall milkweed (Asclepias exaltata L.) located within the Park were compared in June, July, and August to examine vitamin C metabolism during the period when ozone symptoms appeared on the foliage. Ozone injury was greatest for coneflower and was associated with very low levels of vitamin C in the leaves. The results suggest that certain plant species within natural ecosystems do not have sufficient antioxidant capacity to protect against the effects of ambient ozone.
Technical Abstract: Coneflower (Rudbeckia laciniata L.), crownbeard (Verbesina occidentalis L.), and tall milkweed (Asclepias exaltata L.) are wildflower species native to the Great Smoky Mountains National Park (USA) that are known to be sensitive to elevated ambient ozone concentrations. Natural populations of each species were analyzed for ascorbic acid (AA) and dehydroascorbic acid (DHA) in leaves to assess the role of ascorbate in protecting the plants from ozone stress. Tall milkweed contained greater quantities of AA in leaf tissue (7-10 micromol g-1fresh weight) than crownbeard (2-4 micromol g-1fresh weight) or coneflower (0.5-2 micromol g-1fresh weight). DHA was elevated in crownbeard and coneflower relative to tall milkweed suggesting a diminished ascorbate-glutathione pathway for converting DHA into AA. Species differences in leaf extracellular metabolites were found. Tall milkweed plants accumulated AA in the leaf apoplast (30-100 nanomol g-1 fresh weight) with individuals expressing ozone injury symptoms having less apoplast AA than those without symptoms. In contrast, AA was not present in the leaf apoplast of either crownbeard or coneflower. Unidentified antioxidant compounds were present in the leaf apoplast of all three species, but coneflower was unique in that apoplast antioxidant capacity was induced several-fold as part of the ozone response mechanism. Overall, distinct differences in antioxidant metabolism were found in the wildflower species that could differentially affect response to ozone stress.