THE WATERCRESS GLUCOSINOLATE-MYROSINASE SYSTEM - A FEEDING DETERRENT TO CADDISFLIES, SNAILS AND AMPHIPODS

Watercress (Nasturtium officinale) possesses the glucosinolate-myrosinase system. This system is regarded as a classic example of chemical defense for terrestrial crucifers. Damage of watercress initiates myrosinase-mediated hydrolysis of phenylethyl glucosinolate to a toxic endproduct, phenylethyl isothiocyanate. In multiple choice tests, the amphipod Gammarus pseudolimnaeus, the limnephilid caddisflies Hesperophylax designatus and Limnephilus sp., and the physid snail Physella sp. all strongly preferred (10X) yellowed senescent watercress (FY) over fresh green watercress (FG), despite the 2X higher nitrogen content of green watercress (6.9% for FG vs 3.8% for FY). Green watercress contained 10-40 X more glucosinolate than FY watercress (6.4 8.5 mg/g wet for FG vs 0.2 0.7 mg/g wet for FY). However, when the watercress was heated (ca 70-degrees-C), to deactivate the myrosinase enzyme, multiple choice tests showed that these species shift their preferences to heated green watercress (HG). Heating deactivated the deterrent effect and overall preference (consumption) was HG greater-than-or-equal-to HY > FY much greater than FG for Gammarus. HG > HY greater-than-or-equal-to FY much greater than FG for Hesperophylax, HG > FY greater-than-or-equal-to HY greater-than-or-equal-to FG for Limnephilus, and HG greater-than-or-equal-to FY > HY greater-than-or-equal-to FG for Physella. Thus heating resulted in a shift in preference from the low glucosinolate, but low nitrogen. unheated yellowed tissue to the high nitrogen green tissue. These results suggest that deactivation of the myrosinase enzyme, and hence isothiocyanate production, results in a shift in preference. Preliminary results with Hesperophylax indicate that addition of myrosinase to the test water, which resulted in the formation of isothiocyanate, results in a significant decrease in HG consumption from control levels (p < 0.001) and no change in preference for HY watercress. With Gammarus, myrosinase resulted in reduced consumption of both green and yellowed watercress, but no significant differential effect. These results provide evidence that the glucosinolate-myrosinase system, recognized as the principle deterrent system of terrestrial crucifers, is the feeding deterrent in watercress and also suggest that in the absence of a functioning deterrent system, nitrogen content may influence consumption.