Biochemica/SystemaEcs and Ecology, Vol. 14, No. 6, pp. 597-602, 1986. 0305-1978/86 $3.00+0.00 Printed in GreatBritain. © 1986Pergamon JournalsLtd. Patterns in the Phytochemistry of Three Prairie Plants RICHARD L. LINDROTH*~, GEORGE O. BA3-ZLI* and DAVID S. SEIGLERt *Ecology Program and Department of Ecology, Ethology and Evolution, Universi~ of Illinois, Urbana, IL 61801, U.S.A.; 1"Department of Plant Biology, Universiw of Illinois, Urbana, IL 61801, U.S.A. Key Word Index--Andropogon gerardii; Pensternon digitalis; Lespedeza cuneata; Microtus pennsylvanicus; protein; phenolics; alkaloids; boschniakine; seasonal patterns. Abstract--From 1980 to 1983 we monitored seasonal variation in leaf qualib/of three species of prairie plants; characteristics selected are known to influence feeding by herbivores. Leaf water and protein content generally declined with age in each species. Total and protein-binding phenolics declined with age in Andropogon gerardil; peaked in summer in Lespedeza cuneata, changed little or increased with age in Penstemon digitalis and were negatively correlated with protein content in Lespedeza and Penstemon. The alkaloid (boschniakine) content of Penstemon leaves showed consistent seasonal declines and was positively correlated with leaf protein but negatively correlated with leaf phenolics. Patterns in the phytochemistry of Penstemon fit the predictions of the plant apparency model, but those of Andropogon and Lespedeza do not. Seasonal patterns in the phyto- chemistry of these plants are most likely the product of still largely unknown physiological constraints on primary and secondary metabolism and the risks and costs of herbivory through time. Introduction The chemical defence system of a plant is influenced by the plant's requirement for effec- tive defence (including, for example, the risk and cost of herbivory) and its physiological pro- cesses and constraints (e.g. resource availability and competitive interaction between primary and secondary metabolic pathways). These factors and subsequent allocation of plant photosynthates to various contingencies change over time [1, 2]. Thus herbivores, particularly generalists, are confronted with a constantly changing mosaic of food items of different quality. Such seasonal changes in forage quality in turn affect consumption patterns [3-6] and population densities [7-9] of herbivores. Developmental changes in the levels of nutrients and allelochemicals have been reported from many plants, particularly ferns [10, 11] and woody angiosperms [2, 5, 12-16]; much less is known about seasonal variation in the phytochemistry of herbaceous angiosperms. Here we document the annual and seasonal changes in several leaf quality characteristics of three tallgrass prairie plants, big bluestem ~:Present address: Department of Entomology, 237 Russell Laboratories, 1630 Linden Drive, Madison, Wl 53706, U.S.A. (Received for publication 10 June 1986) (Andropogon gerardil), bush-clover (Lespedeza cuneata) and foxglove penstemon (Penstemon digitalis), and relate our findings to current theories of plant defensive chemistry. Andropogon is a perennial grass, Lespedeza is a perennial leguminous forb and Penstemon is a biennial forb. In east-central Illinois these plants commence growth in late April to early May, senesce by mid- to late-September and are the major foodplants of the meadow vole (Microtus pennsylvanicus) during the growing season [17]. A preliminary screening procedure showed that all of the plants contain phenolic compounds (Lespedeza contains condensed tannins) and Penstemon produces alkaloids [18]. The data presented here were collected as part of a study investigating the effects of grazing by voles on the levels of secondary compounds in their food plants. We found no such influence of meadow voles on their food [18, 19]. Results Levels of water and protein in Andropogon showed consistent seasonal patterns from year to year, decreasing rapidly from spring to summer (9 and 8% drop on average, for water and protein, respectively) and slightly, if at all, from summer to autumn (Fig. 1). Levels of total and protein-complexing phenolics also tended 597