THE EFFECT OF NITROGEN-FERTILIZATION AND RUST FUNGUS INFECTION, SINGLY AND COMBINED, ON THE LEAF CHEMICAL-COMPOSITION OF RUMEX OBTUSIFOLIUS

1. The chrysomelid beetle Gastrophysa viridula occurs on Rumex obtusif olius growing in a range of nutrient conditions and also on plants inf ected with the foliar fungus Uromyces rumicis. In a controlled environ ment, we investigated the effect of fertilizing plants with eight nitr ate and four ammonium concentrations, with or without infection, on le af nutritional quality. 2. Increasing nitrate fertilization increased leaf oxalate, total nitrogen and nitrate concentrations and water cont ent, and decreased total non-structural carbohydrate (NSC) concentrati ons. Increasing ammonium fertilization increased leaf nitrogen concent ration and water content, decreased nitrate and NSC concentrations, an d had no effect on oxalate concentrations. 3. Infection produced a mai nly additive effect to fertilization, increasing NSC and oxalate, and decreasing nitrate and nitrogen concentration in whole plants fed nitr ate, and increasing nitrate and NSC in whole plants fed ammonium. 4. Y oung leaves on infected plants remained uninfected and had greater nit rogen and NSC concentrations, and lower oxalate and nitrate concentrat ions, than infected leaves on the same plant. 5. These results are dis cussed in relation to changes in C:N and NSC:organic nitrogen ratios, the effect of nitrate and oxalate, and the known feeding and ovipositi on preferences of the beetle. The results suggest that there is an opt imum nitrogen fertilization level for G. viridula development.