Stem tissue mass density is linked to growth and resistance to a stem-boring insect in Alternanthera philoxeroides XIAOYUN PAN,* XIN JIA,* JING ZENG,* ALEJANDRO SOSA,† BO LI* and JIAKUAN CHEN* *Coastal Ecosystems Research Station of the Yangtze River Estuary, Ministry of Education Key Laboratory for Biodiversity Science and Ecological Engineering, Institute of Biodiversity Science, Fudan University, Shanghai, China; and †South American Biological Control Laboratory, USDA-ARS, Hurlingham-Buenos Aires, Argentina Abstract To investigate how stem anatomical structure is linked to growth and resistance to stem-boring insects in a herbaceous species, six populations of alligatorweed (Alternan- thera philoxeroides) were grown in a common garden. Stem growth rate (GR) of A. philoxeroides and pupation rate as an estimate of resistance to a stem-boring insect (Agasicles hygrophila) were quantified. Stem tissue mass density (TMD) was measured and stem anatomical traits were analysed on cross-sectional areas (CSA). Stem TMD was positively correlated with resistance (i.e. negatively correlated with pupation rate) and negatively correlated with GR. Stem cortex CSA (%) and vascular bundle (VB) density (no./mm 2 ) were positively related to stem TMD and negatively related to pupation rate. The GR was positively related to VB CSA (%) and negatively related to VB density. These results suggest that stem TMD, which results from a high fraction in cortex CSA and high VB density, is a key determinant of resistance to a stem-boring specialist in A. philox- eroides. The high resistance of plants with higher stem TMD may partially impose a cost to plant growth. Keywords: growth rate, herbivore resistance, stem anatomy, stem tissue mass density, stem- boring insect. Received 1 January 2010; accepted 29 August 2010 Introduction Recently, plant tissue mass density (TMD; the ratio of dry mass to fresh volume) has attracted considerable attention in comparative ecology (Westoby & Wright 2006; Chave et al. 2009; Poorter et al. 2010). It can be seen as a syndrome of traits involving many potentially competing functions, such as carbon gain and growth potential, mechanical strength, efficiency and safety of hydraulic transport, and resistance to herbivory (Sperry et al. 2008; Chave et al. 2009; Poorter et al. 2010). Wood density partially under- lies, for example, the growth–survival trade-off that is observed among woody plants (Poorter et al. 2010). Fast- growing species with low-density tissues are character- ized by leaves in which a high fraction of volume is occupied by mesophyll, the assimilatory tissue (van Arendonk & Poorter 1994). In contrast, slow-growing species produce high-density tissues, which favour both organ persistence and conservation of resources (Gleeson & Tilman 1994), and resistance to herbivorous insects (Coley et al. 1985). Although the relationship between TMD, anatomy and whole-plant functioning, such as growth, height and safety of hydraulic transport and mechanical stability, is widely recognized for stems, there is little information about such a relationship among stem TMD, growth and resistance to stem-boring insects. The aim of the present study was to investigate whether two key traits in whole-plant functioning, that is, growth rate and resistance to a stem-boring specialist, are related to stem TMD and stem anatomy in a herbaceous species, Alternanthera philoxeroides (Mart.) Griseb. (alligatorweed; Amaranthaceae). Previous studies have shown that under controlled conditions this species differs substantially in growth rate and resistance (Jia 2008). Six populations of A. philoxeroides were grown in a common garden. Anatomical Correspondence: Xiaoyun Pan Email: xypan@fudan.edu.cn Plant Species Biology (2011) 26, 58–65 doi: 10.1111/j.1442-1984.2010.00307.x © 2010 The Authors Journal compilation © 2010 The Society for the Study of Species Biology