BACKGROUND AND AIMS:Parasitism is well understood to have wide-ranging deleterious effects on host performance in species thus far characterized. Photosynthetic performance reductions have been noted in the Striga-Zea mays association; however, no such information exists for facultative hemiparasitic plants and their hosts, nor are the effects of host species understood.METHODS:Chlorophyll fluorimetry was used to study the effects of parasitism by the hemiparasite Rhinanthus minor on the grass Phleum bertolinii and the forb Plantago lanceolata, and the effects of host species on the photosynthetic apparatus of R. minor.KEY RESULTS:Parasitism by Rhinanthus led to a significant decrease in the host, and total (host + parasite) biomass in Phleum; however, in Plantago, no significant repression of growth was noted. Maximum quantum yield (F(v)/F(m)) was reduced in parasitized Plantago, relative to control plants, but not in Phleum. F(v)/F(m) was significantly lower in R. minor parasitizing Phleum than Plantago, suggesting Phleum to be a superior host to Plantago for R. minor. Steady-state quantum yield (Phi(PSII)) was significantly depressed in parasitized Phleum, but only at low irradiances in Plantago. Phi(PSII) was very low for R. minor grown on Plantago, but not Phleum.CONCLUSIONS:Shown here is the first evidence of the suppression of host photosynthesis by a facultative hemiparasitic plant, which has significant effects on total biomass production. Host identity is a significant factor in parasite success, with the forb Plantago lanceolata exhibiting apparent chemical as well as previously identified physical defences to parasitism. It is proposed that the electron transport rate (as denoted by Phi(PSII)) represents the limiting factor for biomass accumulation in this system, and that Plantago is able to suppress the growth of Rhinanthus by suppressing the electron transport rate.

The root parasite Rhinanthus minor feeds on the xylem of a diverse range of species. Grasses and legumes are the best hosts, while on forbs R. minor typically shows poorer growth. It has been hypothesized that host quality is linked to the expression of defences against the parasite seen in forb roots, but never in grasses. The efficacy of these defence mechanisms in preventing resource loss has not, however, been measured directly. Here we combine histological characterization of haustoria formed on Cynosurus cristatus (a grass), Leucanthemum vulgare and Plantago lanceolata (forbs) with (15)N tracers supplied to the host to quantify the efficacy of these defence responses. Rhinanthus minor penetrated only the xylem of C. cristatus, abstracting an average of 17% of the (15)N tracer taken up, but only 2.5 and 0.2%, respectively, when attached to L. vulgare and P. lanceolata. For the first time, this study has established that the resistance mechanisms of the forbs are effective in preventing the parasite from directly accessing their xylem solutes.

Background and Aims Rhinanthus minor is a root hemiparasitic plant that attacks a wide range of host species which are severely damaged by the parasite. Rhinanthus minor also attempts unsuccessfully to form connections to a range of non-hosts which in contrast are not damaged by the parasite; however, the underlying physiological basis of these differences is not fully understood.Methods Biomass of host-parasite combinations was studied, and histology, electron microscopy and FT-IR microspectroscopy were used to determine the cellular-level interactions between Rhinanthus haustoria (the parasite's connective structure) and the roots of a range of potential host species.Results Two distinct defence responses were observed in the non-host forbs Plantago lanceolata and Leucanthemum vulgare. Firstly, L. vulgare was able to encapsulate the parasite's invading structures preventing it from gaining access to the stele. This was supported by FT-IR microspectroscopy, used to monitor lignification in response to Rhinanthus haustoria. Secondly, host cell fragmentation was observed at the interface between the parasite and P. lanceolata. Growth data confirmed the non-host status of the two forbs whilst, in contrast, grasses and a legume which were good hosts showed no evidence of defence at the host/parasite interface.Conclusions Variable resistance to Rhinanthus is shown for the first time to be controlled by cellular-level resistance to haustoria by either cell fragmentation or lignification at the host/parasite interface.

Parasitic plants can significantly influence the species to which they attach. The host response is variable however, and ranges from death of the host to no detectable effects in terms of both growth and physiology. The parasite can directly influence its hosts through resource abstraction, and indirectly by influencing inter- and intra-specific interactions. Abiotic factors interact with these direct and indirect effects to moderate the potential outcome of the host parasite interaction. This paper sets out to review a series of experiments that have been undertaken in our laboratory over a number of years that examine these effects and help us to understand mechanisms underpinning the variability in host response.

Agricultural soil samples containing survival structures of the fungal crop pathogens Verticillium dahliae, Sclerotinia sclerotiorum, Sclerotium cepivorum, Pythium ultimum, potato cyst nematodes Globodera rostochiensis and G. pallida and weeds Chenopodium album and Agropyron repens [Elymus repens] were treated in the laboratory with aerated steam at temperatures ranging from 40 to 80°C in a specially constructed apparatus. Steaming at 50 or 60°C for 3min, followed by an 8-min resting period in the steamed soil and immediate removal from the soil thereafter, resulted in 100% kill of all weeds, diseases and nematodes. When steamed at 45°C, there was a small but significant reduction in the survival of V. dahliae microsclerotia but no reduction in survival of S. cepivorum.

We studied the effect of light availability on the growth of an angiospermatic root hemiparasite,Rhinanthus minor. When attached to its host, height growth increased in response to shading, demonstrating thatR. minor was able to detect alterations in light quality and/or quantity. However, this reduced illumination did not affect its biomass, number of haustoria, or the amount of15N transferred from the hosts, compared with its performance under non-shaded conditions. Therefore,R. minor is unlikely to have difficulty in extracting host resources under shading. This result may have been mediated by a loweredR. minor transpiration rate in response to fluctuations in external conditions, including shading and water stress, compared with non-parasitic plants. Therefore, we suggest that, as long as the extent of resources diverted from host to parasite is not significantly altered by shading, growth of the attachedR. minor will be unaffected by reduced light availabilityper se.

Soil samples containing resting structures of fungal crop pathogens (Verticillium dahliae, Sclerotinia sclerotiorum, Sclerotium cepivorum, Pythium ultimum), potato cyst nematodes (Globodera rostochiensis and Globodera pallida) and weeds (Chenopodium album and Agropyron repens) were treated with aerated steam in the laboratory at temperatures ranging from 50-80oC in a specially constructed apparatus. Steaming at 50 or 60oC for three minutes, followed by an eight-minute resting period in the steamed soil and immediate removal from the soil thereafter, resulted in 100% kill of all weeds, fungi and nematodes. Low temperature/ short duration soil steaming could become a sustainable alternative to chemical or high-temperature steam soil disinfestation.

This report was presented at the UK Organic Research 2002 Conference. Soil samples containing resting structures of fungal crop pathogens (Verticillium dahliae, Sclerotinia sclerotiorum, Sclerotium cepivorum, Pythium ultimum), potato cyst nematodes (Globodera rostochiensis and Globodera pallida) and weeds (Chenopodium album and Agropyron repens) were treated with aerated steam in the laboratory at temperatures ranging from 50–80oC in a specially constructed apparatus. Steaming at 50 or 60oC for three minutes, followed by an eight-minute resting period in the steamed soil and immediate removal from the soil thereafter, resulted in 100% kill of all weeds, fungi and nematodes. Low temperature/ short duration soil steaming could become a sustainable alternative to chemical or high-temperature steam soil disinfestation.

A method was used for applying a uniform mechanical impedance to plant roots using sand packed at different bulk densities with depth within each growth cylinder. In a growth experiment, replicate cylinders were packed with sand to give the following mechanical impedances: 0.25 MPa penetration resistance (negligible impedance), 1.40 MPa (moderate impedance) and 2.30 MPa (severe impedance). Seedlings of Lolium perenne L., Trifolium repens L. and Agrostis capillaris were grown for 23 d in each impedance treatment and effects on both roots and shoots were studied. Severe mechanical impedance affected both root and shoot growth rates for all three species resulting in smaller leaves and shorter roots. For the grasses, the root-to-shoot ratio at harvest was the same for all the treatments, but a delay in the initiation of both shoots and roots was observed in the severe mechanical impedance treatment. The results are discussed in relation to the possibility that roots penetrating the impeding treatments may have caused signalling that kept shoot and root growth in synchrony. Differences observed in the response to mechanical impedance of T. repens compared to the other two species may be a function of differences in the physiology between mono- and dicotyledonous species.

Plant height, light-saturated rates of photosynthesis (Amax) and foliar nitrogen concentration (N1) were measured forBartsia trixago under field conditions in Mallorca. All three variables were postively correlated, and were also positively related to the abundance of nitrogen-fixing legumes in the associated vegetation (putative host species).Amax forB. trixago ranged from 7.7 to 18.8 μmol m-2 s-1; similar rates were measured for a second hemiparasiteParentucellia viscosa, and both species were within the range of rates measured for six putative hosts (10.6–19.2 μmol m-2 s-1). Fertilization of unattachedB. trixago plants with inorganic nitrogen (ammonium nitrate) elicited neither the growth nor the photosynthetic responses observed in plants considered to be parasitic on legumes and in receipt of an enriched organic nitrogen supply. Both hemiparasites had high diurnal leaf conductances (gs) (469–2291 mmol m-2 s-1) and were at the upper end of the range of those measured in putative hosts (409–879 mmol m-2 s-1). In contrast with the latter, high nocturnal rates ofgs were also recorded for the two hemiparasites (517–1862 mmol m-2 s-1). There was no clear relationship between eitherAmax orN1 and eithergs, transpiration (E) or water use efficiency (Amax/E) inB. trixago plants. The economics of water loss appear to be independent of both the supply of nitrogen from the host and autotrophic carbon fixation.