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Watercress has several adaptations that make it an aggressive invader. (Jeff Mitton / For the Camera)
Watercress has several adaptations that make it an aggressive invader. (Jeff Mitton / For the Camera)

Trout Creek, which runs from Trout Creek Pass at the edge of South Park to the Arkansas River at Buena Vista, is home to beavers. I enjoy visiting their ponds, and although they always seem to elude me, I enjoy seeing the trout in their ponds.

Jeff Mitton. Natural Selections
Jeff Mitton. Natural Selections

This year I noticed that Trout Creek was packed with the small, bright white, green and yellow flowers of watercress, Nasturtium officinale.

Watercress was rooted at the banks of the stream, but its hollow stems allow the soft branches to float and colonize the open water. Although the stream was resplendent with floating flowers, they were virtually absent from the beaver ponds. Beavers eat watercress.

Leaves and stems of watercress are edible and tasty, but I was not tempted in this particular case, because unrinsed watercress can carry giardia. One of the earliest common names of giardia is beaver fever.

Watercress is native to Europe and Asia, but it has been introduced to all temperate and tropical regions around the world. But following introduction, it has demonstrated that it is an aggressive invader. For example, watercress was first reported in North America as a menu item at the first Thanksgiving in 1621. But watercress had already found its way across North America by 1804, when Lewis and Clark were exploring the lands acquired by the Louisiana Purchase. By 1920 it was widespread and common in large parts of both the U.S. and Canada.

Attributes contributing to its invasive character are that it is a perennial, it reproduces either by selfing or outcrossing, it grows adventitious roots from its stems at the leaf axils, and it can occupy space without competition by floating on the surface of the water. If a branch is severed from a plant, it can float downstream and then anchor itself with its adventitious roots when it touches a streambank or mudflat. A solitary branch in a new locality can produce seeds via selfing, a real advantage for invaders.

Watercress is in the family Cruciferae, called crucifers. The family name has more recently been Brassicaceae, and commonly called cabbages or mustards. Other familiar members of this group include mustard, radish, wasabi, arugula, broccoli, Brussels sprouts, collard greens, kale and cabbage.

The former family name, Cruciferae, comes from the shape of the flowers, which have four white petals, evenly spaced, that suggest a cross. Cruciferae means “cross bearing.”

In addition to humans and beavers, watercress is consumed by muskrats, racoons, deer, horses, cows, moose, ducks, geese and swans. This list might suggest that watercress has no chemical defenses to deter herbivores, but that is not the case. One of the characteristics that is common to crucifers is the glucosinolate-myrosinase system that has been described as a bomb triggered by chewing. Laboratory studies have demonstrated that this system deters a variety of aquatic herbivores, including amphipods, caddisflies and snails.

Crucifers or mustards synthesize a variety of metabolic compounds called glucosinolates, which release glucose when hydrolyzed. They also have the enzyme myrosinase, which catalyzes hydrolysis of glucosinolates to release an isothiocyanate. Each crucifer species metabolizes a different glucosinolate, which releases a different isothiocyanate, a unique defense.

In an intact leaf, myrosinase is compartmentalized in myrosin cells, but when watercress leaves are chewed by a caddisfly or diced by a chef, myrosinase mixes with glucosinolates, triggering the myrosinase–glucosinolate bomb,  releasing isothiocyanate. Watercress produces phenylethyl isothiocyanate, but cabbage and broccoli each have different isothiocyanates, determined by their specific glucosinolates, and effective at deterring their specific herbivores.

In addition to fending off herbivores, isothiocyanates provide defenses against bacteria, fungi, viruses and even some other plants. The myrosinase-glucosinolate system is the principal defense system for all of the crucifers.

Mustard, broccoli, arugula, kale, radish, wasabi and watercress are known for their unique, piquant flavors. I find it intriguing to appreciate that the chemical defenses evolved to be toxic for some herbivores are chosen from menus because they appeal to our palates.

The peppery taste of watercress is the outfall of the myrosinase-glucosinolate bomb. Bon appetit!