The influence of the hemiparasitic angiosperm Cassytha pubescens on photosynthesis of its host Cytisus scoparius

Abstract

Infection with Cassytha pubescens R.Br, an Australian native hemiparasitic plant, can lead to death of the invasive shrub, Cytisus scoparius L. Link (Scotch broom). We examined the influence of C. pubescens on photosynthetic physiology of C. scoparius to determine whether this might contribute to death of infected plants. Infected C. scoparius had significantly lower photosynthetic rates, stomatal conductance and transpiration, and higher Ci (internal [CO2]), than uninfected plants. Rapid light response curves, determined using chlorophyll fluorescence, indicated significantly lower light-saturated electron transport rates and lower quantum yields for infected plants relative to uninfected plants. However, Rubisco content did not differ between infected and uninfected plants, suggesting the lower photosynthetic rates were most likely due to stomatal closure, rather than lower photosynthetic capacity. As a consequence of lower assimilation rates, PSII efficiency was lower in infected plants than uninfected plants across the diurnal cycle. Infected plants also had significantly lower pre-dawn Fv/Fm values and slower recovery from exposure to high light than uninfected plants. Our results suggest that infected C. scoparius are more susceptible to photodamage than uninfected plants. Combined with lower carbon fixation rates, this could contribute to the poor performance and even death of infected plants.

Native hemiparasite and light effects on photoprotection and photodamage in a native host

“…Thus, our pigment data can help explain why L. myrsinoides did not become photodamaged and shows tolerance to C. pubescens in terms of its overall growth in both the glasshouse and the field (Prider, Watling et al 2009;Cirocco, Facelli et al submitted). Similar investigations of pigment dynamics and PSII efficiency of introduced hosts may help explain why they are more severely affected by C. pubescens than native hosts such as L. myrsinoides (Prider, Watling et al 2009;Shen, Prider et al 2010). The effects of light treatment on both L. myrsinoides and C. pubescens were similar to those reported by others for a range of plants.…”

“…In a preliminary study, foliar nitrogen concentration of this host was found to be significantly Interactively, Chl a/b ratio of HL plants was unaffected by C. pubescens whereas that of LL plants decreased in response to infection (Table 1). In contrast, Shen, Prider et al (2010) found that Chl a/b ratio of C. scoparius stems increased in response to infection with C. pubescens under ambient light. Most other studies have reported no effect of parasitism on host Chl a/b ratio (Cechin and Press 1994;Hibberd, Quick et al 1996;Jeschke, Baig et al 1997;Logan, Huhn et al 2002;Reblin, Logan et al 2006;Cameron, Geniez et al 2008;Shen, Hong et al 2011;Shen, Xu et al 2013).…”

“…By contrast, Shen, Prider et al (2010) found that total chlorophyll of Cytisus scoparius stems was unaffected by C. pubescens. In a study by Logan, Huhn et al (2002) there was also no effect of infection by Arceuthobium pusillum on pigment content of Picea glauca needles.…”

“…A previous field study also found no infection effect on F v /F m for L. myrsinoides and the introduced host C. scoparius (Prider, Watling et al 2009). However, Shen, Prider et al (2010) myrsinoides (Fig. 1a, b).…”

Interactions between hemiparasitic plants and their hosts

“…In particular, annual hemiparasites tend to have highly negative effect on their host performance. 32,39,40 The decrease of the host biomass production is usually not compensated by the biomass production of the hemiparasite resulting in a general decrease of productivity of the whole community. 32,38 The hemiparasitic systems hence do not display source-sink relationship as has been shown for holoparasitic relationships.…”