WATERCRESS ALLELOCHEMICAL DEFENDS HIGH-NITROGEN FOLIAGE AGAINST CONSUMPTION - EFFECTS ON FRESH-WATER INVERTEBRATE HERBIVORES

Watercress (Nasturtium officinale) is a spring-stream macrophyte that possesses glucosinolates, which are hydrolyzed to feeding deterrent is othiocyanates when the enzyme myrosinase is released by tissue damage. Previous studies indicated that frequently associated aquatic shredde rs strongly prefer yellowed-senescent leaves over fresh-green foliage, because the latter releases much more isothiocyanate than senescent w atercress. When the action of myrosinase was blocked by heating the ti ssue, the shredders' preference shifted to heated-green tissue, which contains much more nitrogen than that found in senescent tissue.Here w e report a series of no-choice experiments oil various tissue types de signed to determine intermediate and long-term consumption rates and a ssociated impacts on growth and survival of shredders associated with watercress. The amphipod Gammarus pseudolimnaeus, the caddisflies Pycn opsyche sp., Hesperophylax designatus, and Limnephilus sp., and the sn ail PHysella gyrina, all consumed much more senescent than fresh-green tissue. In long-term tests with G. pseudolimnaeus, H. designatus, and Limnephilus sp., growth was negative or zero on the defended fresh-gr een tissue and often highest on heated-green tissue. Reaction to gluco sinolates differed among shredders. For the caddisflies, growth and su rvival were positively correlated with bulk tissue and nitrogen consum ption, which were highest for heated-green tissue. However, amphipod s urvival was significantly lowered on heated-green watercress, although it was readily consumed. Therefore, consumption of high-glucosinolate tissue was detrimental to the amphipods but not to caddisflies. Our r esults indicate that the glucosinolate-myrosinase system defends live green watercress against herbivory by an array of aquatic invertebrate s. These generalist herbivores forgo eating high-quality fresh-green l eaves because of plant defenses and instead settle fur lower quality s enescent leaves. When the defense system was experimentally shut down, these herbivores readily consumed young green tissue and frequently s howed higher growth rates than those achieved on the senescent, but un defended, leaves they typically consume. Evidently, these shredders fa ce a trade-off of high nitrogen and high defense vs. low nitrogen and low defense.