Effects of nitrogen fertilization on leaf chemistry and beetle feeding are mediated by leaf development

Leaf expansion is known to be affected by N fertilization. Arthropod herbivore performance is known to be affected by changes in leaf chemistry that occur as leaves expand. Nitrogen (N) fertilization is known to affect arthropod herbivore performance, and the effects are assumed to be directly associated with changes in leaf chemistry. However, there is little known about the link between leaf expansion, leaf chemistry and herbivore responses to N fertilization. We examined how the developmental stage of 'young' Populus deltoides sapling leaves influenced the effects of N fertilization on leaf chemistry and the feeding performance of two folivorous beetles, Chrysomela scripta and Plagiodera cersicolora. One-year-old saplings were grown at low, moderate, and high steady-state rates of N-addition in a glasshouse. Three leaf positions near the stem apex were used in feeding assays. Leaves were categorized into three leaf developmental stages based on continuous measurements of leaf expansion as follows: rapid expansion, completion of expansion, and cessation of expansion. Total leaf N, total soluble protein-N, total phenolic glycosides, and salicin concentrations were measured. Sapling growth rates and leaf development were both changed by rates of N-addition. Leaf N concentration was affected by rates of N-addition, but not changes in leaf development. Leaf protein-N concentration was not affected by either rates of N-addition or changes in leaf development. Total leaf phenolic glycoside and salicin concentration were only affected by changes in leaf development. C. scripta feeding was affected by both rates of N-addition and changes in leaf development. P. versicolora feeding was only affected by changes in leaf development. These data indicated that: (1) N fertilization affected leaf development, (2) changes in leaf chemistry caused by N fertilization were not directly or consistently associated with herbivore feeding behavior, and (3) changes in leaf development caused by N fertilization primarily accounted for herbivore feeding behavior.