The Influence of Water Relations on the Response of Cutleaf Coneflower (Rudbeckia laciniata) to Ozone

Roberts, Melinda

Ozone is the most important gaseous air pollutant causing foliar injury to plants. Cutleaf coneflower (Rudbeckia laciniata), a native perennial sunflower in Great Smoky Mountains National Park (GSMNP), is sensitive to ozone and has the ability to live in environments from full shade to full sun. Because of its susceptibility to ozone, and the availability of plant populations in GSMNP, cutleaf coneflower has become the subject of much investigation for how ozone affects native wildflowers. The objective of my study was to determine the interactions between ozone and the water relations of cutleaf coneflower. I hypothesized that droughted or shaded plants would have lower stomatal conductances, resulting in less ozone uptake and reduced physiological damage. As a corollary, plants that received extra water would have the most injury from ozone because of their greater stomatal conductance. Effects of light and either drought or additional water were tested in five blocks in both shade and sun plots. Sub-plots within each block were subjected to no manipulation (control), additional water (2x average annual amounts) or water withheld (droughted) treatments, for a total of 30 sub-plots. The growing season in 2004 had record high rainfall and record low ozone with a SUM00 (total of hourly ozone values) of 202 ppm*hrs and a SUM60 (total of hourly values >= 60 ppb) of 40.5 ppm*hrs. Even with the excessive rainfall, soil moisture content was progressively lowered throughout the season in the droughted sub-plots, while watered and control sub-plots had similar volumetric soil water contents at the end of the season. There were no effects of the soil moisture treatments on stomatal conductance, water potential or D13C values, indicating that the differences in soil moisture between treatments were not large enough to elicit measurable physiological responses. Nevertheless, shaded plants had significantly lower stomatal conductances than sun plants, but only in the watered treatment. Neither treatment nor shading affected the amount of foliar injury. However, it is interesting to note that even though the SUM60 was a record low value, this species still exhibited moderate to high levels of foliar injury, further confirming its extreme sensitivity to ozone, and making it one of the most ozone sensitive plants in Great Smoky Mountains National Park.