© CSIRO 2003 10.1071/FP03117 1445-4408/03/121239 Functional Plant Biology , 2003, 30, 1239–1247 www.publish.csiro.au/journals/fpb CSIRO PUBLISHING Does water status of Eucalyptus largiflorens (Myrtaceae) affect infection by the mistletoe Amyema miquelii (Loranthaceae)? Anthony C. Miller A , Jennifer R. Watling A,C , Ian C. Overton B and Russell Sinclair A A Environmental Biology, School of Earth and Environmental Sciences, University of Adelaide, SA 5005, Australia. B CSIRO Land and Water, Waite Campus, PMB 2, Glen Osmond, SA 5064, Australia. C Corresponding author; email: Jennifer.watling@adelaide.edu.au Abstract. The impact of soil salinity and host water status on the host/parasite association between Eucalyptus largiflorens (F.Muell.) and Amyema miquelii (Lehm. ex Miq.) Tiegh. was investigated in a semi-arid floodplain environment in southern Australia. Water status of potential hosts (i.e. uninfected E. largiflorens) was assessed at a range of sites with different soil salinities and then compared with mistletoe infection at the same sites. Pre-dawn leaf water potentials (ψ) of uninfected E. largiflorens declined with increasing salinity, while leaf δ 13 C values increased. The proportion of infected E. largiflorens at each site decreased significantly with increasing soil salinity. A significant relationship was found between the proportion of infected trees at each site and leaf δ 13 C values, but not pre-dawn ψ, of potential hosts. The impact of mistletoes on water status of infected trees was also investigated. Among infected trees, we found no significant relationship between mistletoe volume and either pre-dawn ψ or leaf δ 13 C values of hosts. However, there was a significant relationship between host midday leaf ψ and mistletoe volume, with hosts exhibiting increased stress as mistletoe volume increased. The data suggest that increasing water and/or salinity stress make E. largiflorens a less suitable host for A. miquelii. Keywords: parasitic plants, soil salinity, stable carbon isotopes, water potential. Introduction Mistletoes (Loranthaceae and Viscaceae) are hemi-parasites that extract water and dissolved nutrients, including carbon, from the xylem stream of their hosts (Calder 1983; Marshall et al. 1994). They often have mutualistic relationships with other organisms such as birds and mammals (Norton and Reid 1997; Amico and Aizen 2000), and because of these associations they are considered a ‘keystone’ resource in the forests and woodlands where they occur (Norton and Reid 1997; Watson 2001). However, undoubtedly the closest associations formed by mistletoes are those with their host plants, without which the parasite would be unable to obtain water and nutrients. Like other parasitic angiosperms, mistle- toes extract these resources via the haustorium, a specialised structure that forms the interface between host and parasite (Stewart and Press 1990). A water potential (ψ) gradient between host and mistletoe provides the mechanism by which water and dissolved solutes flow towards the parasite (Fisher 1983; Schulze and Ehleringer 1984; Ehleringer et al. 1986; Davidson et al. 1989). The gradient in ψ is maintained through a combination of high parasite transpiration rates and high resistances in the hydraulic pathway between host and parasite, especially at the haustorial interface (Whittington and Sinclair 1988; Davidson and Pate 1992). This ψ gradient is maintained under most conditions, although there have been occasional reports of no gradient or even reversed gradients between host and parasite (Whittington and Sinclair 1988; Strong and Bannister 2002). Given that mistletoes need to maintain a ψ gradient in order to abstract water and nutrients from their hosts, it seems likely that host water status may be an important determinant of mistletoe growth and success, and there have been several reports that mistletoes are, in fact, more abundant in wetter areas (McLuckie 1923; May 1941; Hartigan 1960). This includes road verges where potential hosts receive runoff from the road and reduced competition for water from other trees (Lamont and Southall 1982; Norton and Stafford Smith 1999), and along salinity gradients in mangrove systems (Rozema et al. 1986; Goldstein et al. 1989) in which parasite infestation decreases with increasing salinity. Mistletoe growth and survival have also been linked to water availability. Canopy cover and fruit production of the north American mistletoe, Phoradendron californicum Nutt., which is parasitic on either Acacia gregii A.Gray (Lei 1999) or epiparasitic on Abbreviations used: WUE, water-use efficiency; δ 13 C, stable carbon isotope ratio; ψ, water potential.