1080 American Journal of Botany 88(6): 1080–1087. 2001. FAMILY- AND POPULATION-LEVEL RESPONSES TO ATMOSPHERIC CO 2 CONCENTRATION: GAS EXCHANGE AND THE ALLOCATION OF C, N, AND BIOMASS IN PLANTAGO LANCEOLATA (PLANTAGINACEAE) 1 DAWN JENKINS KLUS, 2 SUSAN KALISZ, 2,5 PETER S. CURTIS, 3 JAMES A. TEERI, 4 AND STEPHEN J. TONSOR 2,5,6 2 Michigan State University, W.K. Kellogg Biological Station, 3700 E. Gull Lake Drive, Hickory Corners, Michigan 49060 USA; 3 The Ohio State University, Department of Plant Biology, Columbus Ohio 43210 USA; and 4 The University of Michigan Biological Station, Pellston, Michigan 49769 USA To ascertain the inheritance of responses to changing atmospheric CO 2 content, we partitioned response to elevated CO 2 in Plantago lanceolata between families and populations in 18 families in two populations. Plants were grown in 35 Pa and 71 Pa partial pressure of CO 2 (pCO 2 ) in open-top chambers. We measured above- and belowground mass, carbon (C), nitrogen (N), hexose sugar, and gas exchange properties in both CO 2 treatments. Families within populations differed in mass, mass allocation, root : shoot ratios, above- ground percentage N, C : N ratio, and gas exchange properties. The CO 2  family interaction is the main indicator of potential evolutionary responses to changing CO 2 . Significant CO 2  family interactions were observed for N content, C : N ratio, and photo- synthetic rate (A: instantaneous light-saturated carbon assimilation capacity), intercellular CO 2 concentration, transpiration rate (E), and water use efficiency (WUE = A/E), but not for stomatal conductance. Families differed significantly in acclimation across time. The ratio of A in elevated vs. ambient growth CO 2 , when measured at a common internal CO 2 partial pressure was 0.79, indicating down- regulation of A under CO 2 enrichment. Mass, C : N ratio, percentage, C (%C), and soluble sugar all increased significantly but overall %N did not change. Increases in %C and sugar were significant and were coincident with redistribution of N aboveground. The observed variation among populations and families in response to CO 2 is evidence of genetic variation in response and therefore of the potential for novel evolutionary trajectories with rising atmospheric CO 2 . Key words: biomass allocation; elevated CO 2 ; gas exchange; genetic variation; nitrogen assimilation; photosynthesis; Plantago lanceolata; Plantaginaceae. Many studies have shown that whole-plant mass in C 3 spe- cies typically increases in response to elevated CO 2 (reviewed by Poorter, 1993; Curtis and Wang, 1998). However, as the variety of species studied and the length of experiments have increased, considerable interspecific variation in the magnitude and duration of this ‘‘typical’’ response to elevated atmospher- ic CO 2 has been documented (Bazzaz, Coleman, and Morse, 1990; Garbutt, Williams, and Bazzaz, 1990; Hunt et al., 1995). Substantial interspecific variation has also been observed in the physiological responses to elevated CO 2 , in changes in nitrogen acquisition, and in the partitioning of overall mass, carbon, and nitrogen among plant parts and functions (Sage, 1994; Curtis and Wang, 1998). While interspecific differences in responses to elevated CO 2 can have important implications for ecological interactions, 1 Manuscript received 4 May 2000; revision accepted 15 August 2000. The authors thank Jon Ervin, Pam Woodruff, Nina Consolatti, Chris Vogel, Sandy Halstead, Hal Collins, R. Kelman Wieder, Brenda Casper, Phillip Brau- tigam, and Kim Hollingshead for technical help and the graduate students at W.K. Kellogg Biological Station for discussions on this topic. DJK gives most special thanks to John and Nicholas Klus. Mark Vandermeulen and the Tonsor lab group at The University of Pittsburgh provided useful comments on the manuscript. This project was supported by a Grant-in-Aid of Research through Sigma Xi and an NSF Predoctoral Fellowship to DJK, an NSF Research Training Group at Kellogg Biological Station (CIR-9113598), NSF grant BSR 8906283 to SJT, a Michigan State All University Research Initiation Grant to SK, the University of Michigan Biological Station, and the University of Michigan Global Change Project. 5 Current address: A234 Langley Hall, The University of Pittsburgh, De- partment of Biological Sciences, Pittsburgh Pennsylvania 15260 USA. 6 Author for reprint requests (e-mail: tonsor@pitt.edu). they also indicate that genetic variation in responses exists, at least at a macroevolutionary level. Intraspecific differences arise and are translated into interspecific variation through the actions of drift and divergent selection. The extent of intra- specific variation indicates, at least in a qualitative way, the potential for rapid adaptation to elevated CO 2 . Thus while ex- tant interspecific differences in response to elevated CO 2 dem- onstrate that evolution in CO 2 responses can occur on a geo- logic time scale, the extent of intraspecific variation indicates the likelihood of near-term genetic adaptation to shifting at- mospheric CO 2 composition on something closer to ecological time scales. Yet the presence and nature of that intraspecific variation is largely unexplored. To understand the potential evolutionary consequences of increasing atmospheric CO 2 , we need to examine the extent of genetic variation in relevant traits both at the level of popu- lations and at that of families. In some species, genetic vari- ation is distributed mainly among rather than within popula- tions, largely as a function of dispersal and mating system effects (Loveless and Hamrick, 1984). When genetic variation is distributed mainly among populations, and the within-pop- ulation heritabilities are low, the movement of genes between populations can be as important a source of genetic variance as mutation and recombination, especially in small populations (Weber, 1990, 1992). Variation at the family level withinpop- ulations in contrast is the classic quantitative genetic measure of genetic variation, among-sibling variance being the basis of commonly used heritability estimates (Hallauer and Miranda, 1981). It is therefore important to assess genetic variation both within and among populations.