Nasturtium officinale and Gammarus pseudolimnaeus: role of secondary metabolites and nitrogen content in food preference.

Abstract

Nasturtium officinale (watercress) 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 glucosinolates to a toxic end product, 2-phenylethyl isothiocyanate. We investigated the potential role of 2-phenylethyl isothiocyanate in the avoidance of watercress by the co-occurring amphipod, Gammarus pseudolimnaeus. This was accomplished by performing a multiple choice food test. The Gammarus were allowed to feed for 24-hours on three types of watercress leaf tissues: 1) Fresh green watercress leaf tissue that had a high nitrogen content, as well as a high level of secondary chemicals, 2) Senesced yellow leaf tissue, which contained both low nitrogen and secondary chemical levels, and 3) green leaf tissue that was frozen and leached in stream water to deactivate the myrosinase enzyme, thus resulting in no secondary chemical content, yet maintaining a high nitrogen content. Observations were then recorded on the mean area eaten of each type of leaf tissue. The results of the multiple choice test show that Gammarus prefer food with low glucosinolate, but high nitrogen content. These results provide evidence that the glucosinolate-myrosinase system, recognized as the principal deterrent system of terrestrial crucifers, is the feeding deterrent in watercress, and also suggests that in the absence of a functioning deterrent system, nitrogen content may influence consumption.