The influence of abiotic factors on the impact of a native stem hemiparasite on introduced versus native hosts

Abstract

Over 100 years, the impact of parasitic plants on their hosts has been a major and fascinating field of research. Recently, there is evidence for native parasites having a greater effect on growth of introduced compared with native hosts. However, there is little known about the mechanisms behind these differential impacts. Further, there have been surprisingly very few studies in the field in general, that have incorporated the influence of abiotic factors on parasite effects on their hosts. A series of glasshouse studies were conducted to explore these gaps in the literature. Light experiment (Ch. 2): The influence of high (HL) or low light (LL) on the effects of the Australian native stem hemiparasite Cassytha pubescens on the native and introduced perennial evergreen shrubs Leptospermum myrsinoides and Ulex europaeus, respectively. It was hypothesised that as a result of decreased parasite photosynthesis in LL, C. pubescens would become more dependent on host carbon and have a greater effect on host performance (particularly, U. europaeus) in these conditions. Parasite photosynthesis was significantly lower in LL relative to HL when infecting either host. However, contrary to my prediction, light was not found to influence the effect of C. pubescens on overall growth of these two hosts. Independent of light, the parasite did have a significant negative impact on overall growth of U. europaeus but not L. myrsinoides and also grew much more vigorously on the introduced host. Pigments (Ch. 3): The influence of high (HL) or low light (LL) and C. pubescens on pigment dynamics and photo-damage of L. myrsinoides. It was hypothesised that excess light would occur as a result of infection effects on host photosynthesis in HL and in response; the native host would increase its photo-protective capacity (VAZ/Chl) and engagement (de-epoxidation state) in these conditions. As total xanthophyll (VAZ) and chlorophyll content (Chl) significantly decreased in parallel in response to infection, regardless of light, VAZ/Chl of L. myrsinoides was unaffected by C. pubescens in either HL or LL. The de-epoxidation state of the host was also unaffected by infection in both HL and LL. Consequently, infected L. myrsinoides had the same photo-protective capacity/engagement as uninfected plants and thus, showed no signs of photo-damage. These findings may explain why this native host shows tolerance to C. pubescens both in the light experiment (Ch. 2), and in the field. Nitrogen experiment (Ch. 4): The influence of nitrogen (N) when supplied (HN) or not (LN) on the effect of C. pubescens on two leguminous hosts, (native: Acacia paradoxa; introduced: U. europaeus). It was hypothesised that the combination of infection along with the added carbon burden of rhizobia at LN would result in C. pubescens having a greater effect on hosts in these conditions. Contrary to this prediction, N was not found to influence the effect of the parasite on overall growth of hosts. Similarly, as with the light experiment (Ch. 2), C. pubescens had a significant negative effect on total biomass of U. europaeus but not that of A. paradoxa, regardless of N and also grew significantly greater on the introduced host, irrespective of N. Maximum electron transport rates (ETR𝚖𝚊𝚡) of U. europaeus, but not A. paradoxa were also found to be affected by C. pubescens which may explain the parasite’s negative effect on growth of U. europaeus. Water experiment (Ch. 5): The influence of water on the effect of C. pubescens on U. europaeus. It was hypothesised that the parasite would grow better and have a greater effect on the host in well-watered (HW) compared with low water (LW) conditions. Again, as with the experiments above, the parasite negatively affected growth of this introduced host, but in contrast, water did influence the effect of the parasite. Supporting this hypothesis, total biomass of U. europaeus was affected by C. pubescens in both treatments, but more severely in the HW treatment. This greater effect may be explained by the significantly higher photosynthetic performance (Fv/Fm) [v subscript and m subscript] and increased growth of the parasite in the HW compared with LW. Thus, it seems a more hydrated healthy C. pubescens in HW was capable of removing more resources and therefore had a greater effect on growth of U. europaeus in these conditions. These studies have revealed that light and N (specifically when hosts are legumes) may not be important in modulating the effects of stem hemiparasites on their hosts. By contrast, water was an important factor, with the parasite having a more severe effect when the host was well hydrated. It seems that from these experiments, parasite performance is controlled by host supply rather than parasite demand. Such ‘fine tuning’ between parasite and host has also been reported for the stem holoparasitic vine Cuscuta. Nevertheless, studies looking at the effects of the parasite when these abiotic factors are combined will further clarify potential outcomes of these associations. Results from these experiments also consolidate the idea that native parasites more negatively affect introduced compared with native hosts. Consequently, my data continues to support the potential-use of C. pubescens as a native bio-control agent against major introduced weeds in Australia. At the same time, my information adds to the discussion on pre-existing ecological theory; are introduced species successful invaders because their newly encountered enemies lack the effective arsenal. Or are they naïve invaders in the sense that new enemies do have an effective arsenal, my findings support the latter hypothesis.