Plant of the Week
Watercress (Nasturtium officinale)
By Dave Moore
Watercress is an aquatic or semi-aquatic perennial herb with bright white flowers that resemble the shape of a cross; hence, an old name (Cruciferae) for the mustard family, to which watercress belongs. Watercress is commonly found in cold, alkaline waters of springs, spring runs, and similar streams throughout the State of Missouri, as well as the majority of the North American continent. Watercress is also worldwide in distribution, and is usually considered an introduced species in North and South America, Australia, South Africa and New Zealand.
Watercress is a member of the mustard family, which includes many well-known leafy and tuberous vegetables such as collared greens, kale, turnips, and radishes, as well as many problematic weeds such as garlic mustard. It is widely cultivated and is the same watercress commonly used as a salad green. It is also used as a garnish for meats and other dishes where a peppery or pungent flavor is desired (Steyermark 1963).
The pungent, spicy, and/or peppery taste of members of the Mustard family is due to a defense system known as the glucosinolate-myrosinase system. When tissue damage, due for example to herbivory, mechanical damage, or pathogen attack, occurs, the mechanism isolating the two compounds (glucosinolate and myrosinase) within the plant tissues breaks down and results in the production of several mustard oils that have a variety bioactive properties (Newman, et al 1992). This arrangement is known as “the myrosinase-glucosinolate bomb.” This “bomb” is thought to be active against herbivores, fungi, viral and bacterial pathogens, nematodes, and even other plants. Thus, the distinctive flavors of many members of the Mustard family are due to the types and amounts of hydrolyzed glucosinolate products released.
With watercress, the glucosinolate-myrosinase system is employed as a defensive mechanism against herbivory by various aquatic herbivores such as caddis flies, amphipods, and snails. However, yellowed leaves of watercress are more readily consumed by these same aquatic herbivores due to the low levels of glucosinolate and myrosinase in the leaf tissues.
Plants collected from the wild should be washed carefully prior to consumption to avoid accidental ingestion of microscopic parasites, such as the protozoan Giardia, that may be present in untreated water (Yatskievych 2006). Ducks, muskrats, and deer eat the leaves of watercress, and the plants serve as shelter for small aquatic life.
For More Information
Newman, R.M., Z. Hanscom, and W. C. Kerfoot. 1992. The watercress glucosinolate-myrosinase system: a feeding deterrent to caddis flies, snails and amphipods. Oecologia Vol. 92, No. 1 (1992), pp. 1-7.
Steyermark, J.A. 1963. The Flora of Missouri. The Iowa State university Press, Ames, IA. 1728 pp.
Yatskievych, G. 2006. Steyermark’s Flora of Missouri, Vol II, revised ed. Missouri Botanical Garden Press. St. Louis, MO. 1181 pp.