Annals of Botany

Reality of Rhinanthus parasitic potential is punishing for Phleum photosynthesis

Rhinanthus minor (yellow rattle) is an interesting representative of a group of hemiparasitic or facultatively parasitic species in the Scrophulariaceae. Although this phenomenon has been known for many years, we have little information about the effect of hemi-parasites on host photosynthesis compared with the impact of obligate parasites. This deficiency has been addressed by Cameron et al. (Aberdeen, UK and Montpellier, France, pp. 573–578). Seedlings of R. minor were grown in the same pots as either seedlings of the grass Phleum bertolonii (a good host) or of the forb, Plantago lanceolata (a poor host). In Phleum, host biomass was much reduced by the parasite, whereas there was no effect on Plantago biomass. This difference was reflected in the effects of parasitism on aspects of photosynthesis. In Phleum, there were significant reductions in steady-state quantum yield and chlorophyll content, and a non-significant reduction in Rubisco content. These reductions were taken to reflect, at least partly, the ability of the parasite to withdraw major nutrients from its host. In Plantago, the parasite had little or no effect on these parameters. Nevertheless, in both host species, maximum photosynthetic quantum yield was slightly reduced by parasitism (but not significantly so in the good host, Phleum). Thus, even in the absence of a vascular connection with Plantago, the parasite was able to exert some effect on this poor host. Perhaps the most telling differences were in the parasite itself: photosynthetic quantum yields (maximum and steady state) and Rubisco content were lower (much lower for steady-state yield) when Plantago was the host than when Phleum was the host, leading to very large differences in parasite biomass. Intriguingly, however, the increased parasite biomass on Phleum was insufficient to account for the reduction in biomass inflicted on its host.