Effects of root hemiparasitic infection on host performance: Reduced flower size and increased flower asymmetry

Abstract

We conducted two pot experiments to examine the relationship between hemiparasitic plant (Rhinanthus serotinus, Scrophulariaceae) infection and host (Linum usitatissimum, Linaceae and Brassica rapa ssp. oleifera, Brassicaceae) performance. We were especially interested in the effects of hemiparasitism on the size and shape asymmetry of host flowers, since neither subject has been studied before. We also conducted a field experiment to examine the effects of shape asymmetry of B. rapa ssp. oleifera flowers on pollination success. The shape of flowers produced by both L. usitatissimum and B. rapa ssp. oleifera plants grown without parasites was less asymmetric, and for both host species, the flowers of plants grown without parasites had significantly longer petals than flowers produced by hosts with parasites. Significantly more bumblebees visited the inflorescences of B. rapa with untreated flowers compared to inflorescences with highly asymmetrical flowers (two of the four petals clipped shorter), whereas no difference was found in the number of visits made by syrphid flies. Parasitism significantly reduced host biomass and reproduction in both host species.