Cassytha pubescens: germination biology and interactions with native and introduced hosts.

Abstract

The native hemiparasitic vine Cassytha pubescens infects and often kills the invasive weeds Cytisus scoparius and Ulex europaeus in the Mount Lofty Ranges, South Australia. This leads to the consideration of whether this parasite is a suitable biological control agent for these weeds. The aims of this study were to investigate germination characteristics of the parasite, the direct effects of the parasite on both invasive and native hosts, and the indirect effects of the parasite on interactions between native and invasive hosts. Seed dormancy and germination of C. pubescens were examined. Imbibition tests revealed that the seeds are enclosed in a water impermeable seed coat, which produces physical dormancy. Germination experiments showed that heat and scarification broke the physical dormancy, but the germination rate of heated seeds was over three times higher than that of scarified seeds. Thus this parasite may have evolved to share similar fire-related germination cues as some of its native hosts. The direct impact of C. pubescens on growth of Acacia myrtifolia (a native legume) and Cytisus scoparius (an invasive legume) was investigated in a pot experiment. None of the parasites on infected A. myrtifolia survived, so none of the A. myrtifolia was successfully infected with the parasite. In contrast, C. pubescens successfully infected C. scoparius. Host biomass accumulation was reduced by 21%, relative to uninfected plants. Photosystem II efficiencies were reduced but only on the infected branches. The total nitrogen content of infected plants plus parasite was the same as that of uninfected plants, and there was no impact of infection on nodulation by Rhizobium. Thus, it is likely that the removal of nitrogen by the parasite, reduces the supply to the host, and this limits the biomass accumulation of C. scoparius. The differences in resistance to the parasite by native and invasive hosts were studied. A pot experiment was conducted using [superscript]32P to examine the uptake of nutrients by the parasite from either C. scoparius or A. myrtifolia. In this experiment, C. pubescens was able to successfully attach to A. myrtifolia hosts. The parasite absorbed no [superscript]32P when attached to the native host, but did take up [superscript]32P from the invasive host. This suggests A. myrtifolia resists the formation of functional haustoria by the parasite, while the invasive host does not. It is likely that this resistance of the native host to the native parasite may have evolved through long-term coexistence, whereas the invasive host has had only a short-term association with the parasite. To investigate if the differences in host resistance to C. pubescens lead to changes in competitive outcomes between hosts, plants of the invasive weed Ulex europaeus were grown together with either A. myrtifolia, or a native non-legume, Leptospermum myrsinoides, and either with or without C. pubescens. There was no effect on either biomass accumulation of hosts or on the intensity of competition between hosts. However, as these are perennial species, it is possible that the experiment was too short to detect any effects. Long-term experiments and field monitoring may be required to resolve these competitive interactions. These results provide an important insight into the germination ecology of C. pubescens, and the nature of its impact on both native and invasive hosts. Unlike the morphologically similar holoparasites of the genus Cuscuta, C. pubescens does not seems to act as a carbon sink, thus had little effect on symbiotic nitrogen fixation. This suggests a different carbon-nitrogen economy model form the one proposed for the morphologically similar holoparasites, Cuscuta spp. The study also detected differences in resistance of hosts to the parasite; however, this appeared to have no effect on host competition in a short-term pot experiment.