Nutritive Quality of Big Bluestem (Andropogon gerardii) and Eastern Gamagrass (Tripsacum dactyloides) Exposed to Tropospheric Ozone

Abstract

Tropospheric ozone (O3) is a phytotoxic air pollutant widespread in industrialized nations of the world. Ozone is produced by the photo-oxidation of hydrocarbons released into the atmosphere by combustion of fossil fuels. Studies demonstrate O3 can be transported from metropolitan areas to rural areas important to agricultural and forestry practices. Reports regarding O3 effects have focused on vegetation important to food production or agronomic crops of economic importance. However, relatively little is known about O3 effects on native plant species. The effects of tropospheric O3 on two warm-season grasses, eastern gamagrass (Tripsacum dactyloides L.) and big bluestem (Andropogon gerardii Vitman), were examined during June-September of 2003. Plants were fumigated with three levels of O3 in a randomized-block experiment with three replicates of each treatment. Grasses were grown in open-top chambers with introduced carbon-filtered (CF) air, characteristic of clean air quality; non-filtered (NF) air, representative of quality in Auburn, AL; and air with double (2X) the ambient concentration of O3. Because forage quality can be as important as quantity, we determined various effects on nutritive quality characteristics in addition to biomass yield. Big bluestem exhibited little response to O3 exposure. For eastern gamagrass, we generally found decreased nutritive quality with increasing O3 exposure as evidenced by increased concentrations of cell wall constituents and decreased concentrations of N. Regrowth of both species exhibited little treatment effect which emphasizes the importance of timing and duration of O3 exposures in relation to physiological stage of plant development. Decreased nutritive quality parameters observed for eastern gamagrass may have implications to diet selection and nutrient intake by ruminant herbivores. In addition, range managers can use species-specific information regarding O3 sensitivity to make decisions about mechanical harvesting and grazing regimes of these forages growing in areas exposed to elevated O3 concentrations.