PRIMARY AND SECONDARY HOST PLANTS DIFFER IN LEAF-LEVEL PHOTOSYNTHETICRESPONSE TO HERBIVORY - EVIDENCE FROM ALNUS AND BETULA GRAZED BY THE ALDER BEETLE, AGELASTICA-ALNI

Field-grown trees of Alnus incana (L.) Moench, Alnus glutinosa (L.) Ge artner and Betula pendula Roth displayed pronounced differences in res ponses of light-saturated net photosynthesis (A(sat)) to herbivory by the alder beetle (Agelastica alni. L., Galerucinae), a specialized ins ect which primarily defoliates alders. We found that photosynthetic ra tes of grazed leaves increased following herbivory in Alnus but not in Betula. Area- and mass-based A(sat) of grazed leaves declined linearl y with increasing amount of leaf perforation in B. pendula, by as much as 57 %. By contrast Alnus glutinosa and Alnus incana increased area- based rates of A(sat) by 10-50 % at all levels of leaf grazing. Given increased A(sat) in the remaining portion of grazed leaves, a net redu ction in photosynthesis per leaf occurred only when the proportion of leaf area grazed exceeded 40 % for Alnus incana and 23 % for Alnus glu tinosa. Since vein perforation by Agelastica alni was observed much mo re frequently in leaves of Betula than in Alnus, we hypothesized that declining A(sat) in herbivorized Betula was related to this disruption of water transport. A field experiment with artificial leaf perforati on demonstrated a greater decline in A(sat) in vein-perforated Betula leaves than perforated leaves with midrib veins intact. However, regar dless of leaf perforation regime, birch never showed post-perforation increases in A(sat). In all species, rates of transpiration of grazed leaves linearly increased and water-use efficiency decreased with incr eased amount of leaf perforation. In grazed Alnus incana leaves, incre asing leaf area consumption by Agelastica alni resulted in an increase of total phenols, a reduction in starch content and no changes in nit rogen concentration in the remaining portion. The increase in photosyn thesis in Alnus incana might be related to declining leaf starch conce ntration or increasing stomatal conductance, but was unrelated to leaf nitrogen concentration. These gas exchange and leaf chemistry measure ments suggest that in contrast to B. pendula, Alnus incana and Alnus g lutinosa, which are the major host species for Agelastica alni, posses s leaf-level physiological adaptations and defence mechanisms which ca n attenuate negative effects of herbivory by the alder leaf-beetle.