Field pennycress (Thlaspi arvense L.) was introduced to North America from Eurasia at a very early date. By 1818 English botanist Thomas Nutall classified it as a common weed around Detroit (13), but more likely the introduction dates back to the founding of the colony on the Detroit River by Antonine de la Mothe Cadillac in 1701 (2). Field pennycress is conceded to be a significant agricultural weed and competes keenly with crops for moisture and space, causing profound reductions in yield. Adding to the problem is
seed distribution by spring floods. When once established, the soil soon becomes contaminated with its seeds (5,31). The plant bears an unpleasant odor, making it easy to identify. Because of its abominable smell it became widely known as stinkweed (34). And stink weed is easily understood by any one who has ever handled the weed, or tasted milk or butter from a cow which has eaten it (26). The most characteristic attribute of this species is the round or oval wafer-like pod which justifies the name pennycress (11), and in England during the last century the pods were likened to silver pennies (26). Additionally, other common English names for field pennycress include fanweed, frenchweed, pennycress, mithridate mustard, bastard cress, and wild garlic.
ETYMOLOGY AND HISTORY
Thlaspi, a name used by Dioscorides (17), is from the Greek thlao, to flatten, and aspis, shield, alluding to the shape of the fruits. The specific name arvense is from the Latin arvens, meaning "of the field." The genus contains 60 species of annual to perennial, low herbs with white to purple-red flowers; 26 of the species are European. Thlaspi is in the family Cruciferae (21, 27) and Heywood (20) places it in the tribe Lepidieae, along with such genera as Lepidium, Capsella, and Camelina.
Field pennycress was collected at Fort Garry, Manitoba, in 1860 and at Anticosti, Quebec, in 1865 (9). It was introduced to the prairies with the first settlers and traders as early as 1882 in bales of trading and food stuffs (5, 36). Muenscher (29) observed that the high incidence of field pennycress on the Canadian prairies appeared to correspond to its similar abundance in the Northwestern States. By 1937, field pennycress was distributed throughout the U.S. from Maine to Florida and westward to California and Washington (10). In 1935 the species was recorded in New South Wales for the first time (3).
Field pennycress is a smooth annual or winter annual, reproducing by seeds. The seedling develops as a compact vegetative rosette. Stems are erect to 80 cm in height, simple or branched above. The leaves are glabrous, alternate, undivided, with margins entire or sparingly toothed. Basal leaves are stalked and ephemeral. The stem leaves clasping, with an arrow-shaped base (2).Flowers are produced in racemes, petals white, 3 to 4 mm long, equal, and are twice as long as the sepals. Silicles (seed capsules) are borne on slender upward curving stalks. They are round and flat like a palm-leaf fan, about 1 cm in diameter, broadly winged and notched at the tip, flattened at right angles to the partition, and dehiscent by 2-winged valves, each with 2 to 8 seeds (2, 35).
The seed capsules are bright green but become yellowish or greenish-orange at maturity, making the plant easily noticed when growing among crops (5). Seeds are compressed, ovoid and dark reddish brown to black, and are 1.5 to 2 mm long and obovate to ovate in shape (12,16, 28, 31, 34). The flattened surface of the seeds has 5 or 6 loop-like lines, which start at the basal scar or notch and run concentrically around a central groove. No mucilage develops on these seeds when soaked in water (5). When growing in unfavorable conditions, the stem remains unbranched and may reach only 1.0 cm in height; in fertile soil and with little competition, many flowering lateral branches are produced from buds at basal nodes, and stems may grow to a height of 80 cm. During the summer plants are distinguishable by differences in leaf shape. The leaves of the late-flowering plants have longer petioles and deeper serrations than those of the early-flowering strain (1 ).
Field pennycress may produce from 1600 to 15 000 seeds that shatter readily. The older the seedling at the onset of winter, the greater the number of seed pods produced the following year (25). A field heavily infested with field pennycress may yield as much as 1345 kg/ha of pure seeds, exactly the weight of 20 bu of wheat (2). Seeds mature and many scatter before and during harvest. Dispersal is chiefly by wind, but Helgelson (19) found that 90% of field pennycress seeds would float for 24 h on still water. Dry-stored seeds lose their viability completely within 2 yr, whereas seeds from the same sample were alive after 10 yr burial in the soil (24); indeed, even after 20 yr burial some seeds are still capable of germination (8). However, the dormancy of most field pennycress seeds in cultivated land does not survive for more than 6 yr (4). This persistent viability of field pennycress seeds in the soil, their capacity to germinate when brought to the surface by cultivation, and the very large reservoir of dormant seeds present in the soil of a heavily infested area are all factors that contribute significantly to the persistence of this troublesome weed (2).
Field pennycress occurs throughout the U.S. (35) and in all provinces of Canada, being particularly abundant on the heavier soils in the Prairie Provinces (12). It is often plentiful in areas where winter wheat (Triticum aestivum L.) is grown (2). It grows in mostareas of the British Isles, except in the outer Hebrides and the Shetland Islands, and is also found in Europe to 79° N, northern Africa, western Asia, Siberia, and Japan (6). It occurs on wasteland and in other disturbed areas in west Greenland where it appeared to have become established in a few locations as far north as Spitsbergen. The species also occurs at several locations in Alaska (32). Since the distribution of field pennycress is remarkably widespread, it is apparent that it is adapted to a profusion :)f environmental conditions, especially on the North American continent where climatic factors have not significantly curtailed its spread. It is adapted to both dry and wet habitats-from exposed knolls to moist valleys, from ;ea-level (2) to an altitude of 3300 m in Colorado (18).
Field pennycress occupies grain fields, clover (Trifolium spp.), and other hay fields, grasslands, gardens, roadsides, and waste places. It was frequently introduced into he eastern U.S. with western feed (13, 28). It occurs as isolated plants, in small patches, or in pure stands. While i pioneer of disturbed soil, it still readily invades cultivated land (2).
Thlaspi arvense is classified as a therophyte, i.e., the species overwintering either as seeds or as vegetative rosettes (15). During its vegetative phase, the rosettes of field pennycress protect the overwintering plants from freezing temperatures and drying winds, enabling them to survive under he insulating effect of the snow cover (2). In fact in Alaska winter snow cover plays a significant role in the survival overwintering rosettes. Field pennycress is one of the four primary colonizers in the annual stage of succession on abandoned land in mixed prairie, and stabilization of such areas is achieved within 15 to 40 yr (7). It is an aggressive competitor and causes critical reductions in crop yields. In a Canadian study, a 16% infestation of field pennycress (percentage of total vegetation on an air-dried basis) reduced wheat yields by an average of 36% over a 5-yr period. Where a 61 % infestation occurred, the yields were reduced by 51 % over hand-weeded checks (2). Other crops cannot crowd it out, for, as Georgia (14) remarked, "It is the better crowder."
Plants in bloom when winter sets in become frozen, but they thaw out in spring and continue to grow and mature their seeds without the slightest injury. Fruits continue ripening until frost. The seeds of these early plants are ripe early in July. Plants that grow from seeds in the spring are not ripe until some weeks later. After the middle of June they are too far advanced to be plowed down (5). At the turn of the century, T. N. Willing, Chief Weed Inspector of the Northwest Territory, said: "It will pay well to drop all other work and fight this weed when it is first noticed" (14).
EMERGENCY FOOD SOURCE FOR HUMANS
Field pennycress is a popular food plant in various parts of the world, often being cultivated in Europe. It is used when the shoots are young and tender, utilized raw as a salad, or cooked as a potherb like spinach. It is high in the vitamins C and G and contains a relatively large amount of sulfur; it may have the health effect of sulfur and molasses (18). Since the plant belongs to the Cruciferae, its leaves have the characteristic mustard "bite." To prepare, boil the shoots for 15 to 25 min and change the water once or twice. Even then a slight bitterness is present, so mix the greens with those from some blander tasting plants like pigweeds (Amaranthus species) (18). The young tender leaves, when used in a salad, are rather bitter tasting, so either mix them with those from other plants or use a strong flavored salad dressing. Seeds and fruits have been used to flavor other food. But be cautious as field pennycress has caused illness when fed to cattle in hay (18).
POISONOUS PRINCIPLES AND OFF FLAVORS
Field pennycress seeds contain abundant oil glucocides. When they are acted upon by enzymes, mustard oil or similar substances are liberated. Allyl isothiocyanate is undoubtedly responsible for the gastric distress in livestock (23), and results when grain containingfield pennycress seeds is ingested (33); mustard oils are very strong irritants. Livestock (horses, cattle, and pigs) consuming consequential quantities of groundseeds may develop chronic enteritis, hemorrhagic diarrhea, colic, abortion, nephritis and hematuria, apathy, and paralysis of heart and respiration. Pouring boiling water over the feed containing ground field pennycress seeds tends to inactivate the enzymes that release mustard oils (30). Feeds containing excessive amounts of its seeds may produce off-flavors in meat products (13). Onion (Allium cepa L.) flavors appear in milk 4 or 5 min after feeding, and will require 5 to 6 h of grazing for the savor to disappear. However, the flavor and odor of field pennycress does not disappear until 7 to 8 h after it is eaten (22). If eaten by milk cows, the disgusting garlicky odor spoils dairy products. A very few of these seeds ground by accident with wheat ruin the flour, and grain that contains them is very sharply reduced in price (14).
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