title: Does light influence the relationship between a native stem hemiparasite and a native or introduced host?
abstract: OBJECTIVE
  There have been very few studies investigating the influence of light on the effects of hemiparasitic plants on their hosts, despite the fact that hemiparasites are capable of photosynthesis but also access carbon (C) from their host. In this study we manipulated light availability to limit photosynthesis in an established hemiparasite and its hosts, and determined whether this affected the parasite's impact on growth and performance of two different hosts. We expected that limiting light and reducing autotrophic C gain in the parasite (and possibly increasing its heterotrophic C gain) would lead to an increased impact on host growth and/or host photosynthesis in plants grown in low (LL) relative to high light (HL).
  
  
  METHODS
  The Australian native host Leptospermum myrsinoides and the introduced host Ulex europaeus were either infected or not infected with the native stem hemiparasite Cassytha pubescens and grown in either HL or LL. Photosynthetic performance, nitrogen status and growth of hosts and parasite were quantified. Host water potentials were also measured.
  
  
  RESULTS
  In situ midday electron transport rates (ETRs) of C. pubescens on both hosts were significantly lower in LL compared with HL, enabling us to investigate the impact of the reduced level of parasite autotrophy on growth of hosts. Despite the lower levels of photosynthesis in the parasite, the relative impact of infection on host biomass was the same in both LL and HL. In fact, biomass of L. myrsinoides was unaffected by infection in either HL or LL, while biomass of U. europaeus was negatively affected by infection in both treatments. This suggests that although photosynthesis of the parasite was lower in LL, there was no additional impact on host biomass in LL. In addition, light did not affect the amount of parasite biomass supported per unit host biomass in either host, although this parameter was slightly lower in LL than HL for U. europaeus (P = 0·073). We also found no significant enhancement of host photosynthesis in response to infection in either host, regardless of light treatment.
  
  
  CONCLUSIONS
  Despite lower photosynthetic rates in LL, C. pubescens did not increase its dependency on host C to the point where it affected host growth or photosynthesis. The impact of C. pubescens on host growth would be similar in areas of high and low light availability in the field, but the introduced host is more negatively affected by infection.
authors.affiliation: School of Biological Sciences, The University of Adelaide, Adelaide 5005, Australia and robert.cirocco@adelaide.edu.au.
authors.forename: Robert Michael
authors.initials: RM
authors.lastname: Cirocco
authors.affiliation: School of Biological Sciences, The University of Adelaide, Adelaide 5005, Australia and.
authors.forename: José Maria
authors.initials: JM
authors.lastname: Facelli
authors.affiliation: Faculty of Health and Life Sciences, Northumbria University, Newcastle upon Tyne NE1 8ST, UK.
authors.forename: Jennifer Robyn
authors.initials: JR
authors.lastname: Watling
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D018533:Biomass; D004579:Electron Transport / Q000528:radiation effects; D006790:Host-Parasite Interactions / Q000528:radiation effects*; D058865:Introduced Species; D031649:Leptospermum / Q000469:parasitology / Q000528:radiation effects; D008027:Light; D009584:Nitrogen / Q000378:metabolism; D010271:Parasites / Q000254:growth & development / Q000502:physiology* / Q000528:radiation effects; D010788:Photosynthesis / Q000528:radiation effects; D018520:Plant Shoots / Q000502:physiology / Q000528:radiation effects; D018547:Plant Stems / Q000469:parasitology* / Q000528:radiation effects*; D054046:Plant Stomata / Q000502:physiology / Q000528:radiation effects; D039762:Ulex / Q000469:parasitology / Q000528:radiation effects
chemical_list: D009584:Nitrogen
keywords: Biomass; Cassytha pubescens; Leptospermum myrsinoides; Ulex europaeus; gas exchange; hemiparasite–host association; light; nitrogen; photosynthesis; water potential
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