Palatability of Thlaspi caerulescens for snails:: influence of zinc and glucosinolates

The hypothesis that zinc (Zn) hyperaccumulation defends Thlaspi caerulescens against herbivores is tested with the snail Helix aspersa. We investigated the effects of leaf zinc, cadmium, glucosinolate, nitrogen and dry matter concentrations on the feeding preferences of snails. Four T. caerulescens populations from southern France (two from metalliferous and two from normal soils) were grown on low- and high-Zn soils to obtain contrasting leaf Zn concentrations. Plants were also collected in the field, and binary feeding choices involving low- and high-Zn leaves were conducted. Foliar Zn, Cd, N and dry matter concentrations did not affect the feeding choices of snails, whereas glucosinolate had a significant negative effect on herbivore preferences. Compared with metallicolous plants, nonmetallicolous ones appeared to be better protected against snails, whatever their Zn concentration. These results do not support the defence hypothesis, as glucosinolates appear to decrease the degree of herbivory when Zn does not.