Suppression of host photosynthesis by the parasitic plant Rhinanthus minor

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 (Φ_PSII) was significantly depressed in parasitized Phleum, but only at low irradiances in Plantago. Φ_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 Φ_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.