Watercress is a semi-aquatic plant with alternate toothed leaves and small white flowers (figure 1). The watercress plant is native to Western and central Europe, and can be found in the United Kingdom growing in streams and ditches. The popularity of watercress as a food has caused its distribution to spread, and the plant is now cultivated all over the World for use as a salad dressing. Watercress is a member of the mustard (brassica) family of vegetables (also called cruciferous vegetables) that include cauliflower, radish, brussels sprouts, horse radish, cabbage, bok choy and rapeseed. Nutritionally watercress possesses many of the same nutritional quantities of other brassica family vegetables which include a high content of vitamin C, and this explains its historical use as a cure for scurvy. The bitter taste of the herb make it a useful addition to salads. However this bitter taste is caused by the presence of chemicals called glucosinolates, and these give watercress some interesting medicinal properties. Glucosinolates are also found in other brassica family vegetables.
Glucosinolates are a group of naturally occurring sulphur and nitrogen containing compounds that are thought to play a role in the defence of plants from predators. Upon tissue damage to the plant, glucosinolates are brought into contact with a group of enzymes called myrosinases, which convert the glucosinolates to isothiocyanates such as allyl isothiocyanate, benzyl isothiocyanate, sulforaphane and indole-3-carbinol. These isothiocyanates give watercress and other brassica family vegetables their pungent taste. Isothiocyanates are bioavailable and once absorbed are thought to provide a potent anti-cancer effect in mammals. In particular isothiocyanates may improve detoxification pathways in the liver and provide antioxidant defence. In this way isothiocyanates may increase the elimination of and also limit the genetic damage caused by carcinogenic compounds. As well as vitamin C and glucosinolates, watercress also contains high levels of carotenoids, various B vitamins, vitamin D, vitamin E and various minerals including iodine, manganese, calcium and iron.