Nebraskaland

NEBRASKAland May 2018

NEBRASKAland Magazine is dedicated to outstanding photography and informative writing with an engaging mix of articles and photos highlighting Nebraska’s outdoor activities, parklands, wildlife, history and people.

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44 NEBRASKAland • MAY 2018 O ne July evening two years ago, I picked a paper bag full of ripe alkali milkvetch seedpods from a prairie on our southeastern South Dakota farm and then placed them on a table in our enclosed front porch. The seeds would be sown into a prairie restoration that winter. The next morning when I came downstairs the house reeked with a pungent, garlicky, sulphurous odor. Not good. When my wife came downstairs her face took on "the look." "What is that smell?" she demanded. "I don't know," I responded with presumed innocence. Minutes later, a thought hit me. I quietly opened the porch door. "Dang!" Shutting the door behind me, I went to the table and stuck my nose into the bag of pods. My senses swooned from the scent of selenium. An Obligate Selenophile Belonging to the pea family, alkali milkvetch (Astragalus racemosus) ranges over the western Great Plains from Saskatchewan to Texas, typically growing on clay, shale, gypsum, chalk or sand soils rich in selenium. In Nebraska, the plant has a patchy distribution, occurring along the lower Niobrara and Missouri rivers in the northeast, the Republican River in the southwest and in the northwestern Panhandle. Here, it grows in dry prairies, primarily on outcroppings of Pierre Shale. The grayish-black shale, formed from muds deposited in marine waters 70 to 80 million ago, weathers to a sticky clay rich in selenium. Alkali milkvetch is among a small group of plants known as "selenium hyperaccumulators" or more poetically "obligate selenophiles" that grow almost exclusively on seleniferous soils. These plants contain compounds that confer selenium resistance, allowing them to accumulate the element into their tissues. Alkali milkvetch, for example, contains up to 15,000 parts per million (ppm) of selenium. Nebraska's other obligate selenophiles are princesplume (Stanleya pinnata) and two-grooved milkvetch (A. bisulcatus). Not surprisingly, they also grow mainly on Pierre Shale. Some sunflowers, asters and mustards growing on seleniferous soils can accumulate up to 50 to 100 ppm of the element, while some forage grasses and crops can accumulate up to 20 ppm. Most plant species cannot grow on these soils, while some exhibit stunted growth. Selenium is physically and chemically similar to sulphur, thus the sulphurous odor. It is an essential micronutrient for animals, necessary for DNA production, the immune system and other functions, but is toxic at higher levels. Plants are the main source of selenium in animal diets. Though not essential for plants, in some species selenium, at low doses, protects against stresses such as cold, drought and exposure to toxic metals in the soil. Selenophiles concentrate selenium in their sensitive young leaves and flowers as protection against herbivores, such as grasshoppers, prairie dogs and livestock; though generally avoided by livestock, starving animals forced to feed on the plants suffer from selenium poisoning. After flowering, the selenophiles transfer the element from their withering blooms and leaves to their seeds – protecting them and young seedlings from herbivory. The plants, including the seeds, vaporize selenium as dimethyldiselenide (the stench in our house) perhaps as a warning to potential grazers. Amazingly, a few insects have evolved a selenium resistance and feed on selenophiles. The larvae of a seed Alkali milkvetch in fruit in a prairie on the author's South Dakota farm. Uncommon among Great Plains milkvetches, the pods are triangular in cross-section and hang in pendulous clusters. Alkali Milkvetch and its strange, mysterious scent By Gerry Steinauer, Botanist Nebraska Game and Parks Commission PHOTO BY JUSTIN HAAG PHOTO BY GERRY STEINAUER PHOTO BY GERRY STEINAUER

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