Sensitivity of Eleven Milkweed (Asclepias) Species to Ozone

We exposed 11 milkweed species to ozone within continuous stirred-tank reactor (CSTR) chambers in a greenhouse to determine species sensitivity and potential use as bioindicators to detect phytotoxic levels of ambient ozone. Asclepias syriaca (Common Milkweed), A. ovalifolia (Oval-leaf Milkweed), A. sullivantii (Prairie Milkweed), A. speciosa (Showy Milkweed), A. asperula (Spider Milkweed), A. incarnata (Swamp Milkweed), A exaltata (Tall Milkweed), and A. curassavica (Tropical Milkweed) developed typical ozone-induced dark stipple on the adaxial surface of older leaves. Tropical Milkweed also exhibited significant premature defoliation (accelerated leaf senescence). Asclepias tuberosa (Butterfly Milkweed), A. hirtella (Green Milkweed), and A. verticillata (Whorled Milkweed) were tolerant to ozone. Foliar stipple on Common Milkweed increased with ozone concentration and time. In addition to Common Milkweed, a bioindicator commonly used to detect phytotoxic levels of ozone, the other 7 ozone-sensitive milkweed species should be evaluated further as potential ozone bioindicators.