The performance of root hemiparasites depends strongly on host species identity, but it remains unknown whether there exist general patterns in the quality of species as hosts for hemiparasites and in their sensitivity to parasitism. In a comparative approach, the model root-hemiparasites Rhinanthus minor and R. alectorolophus were grown with 25 host species (grasses, forbs and legumes) at two nutrient levels. Hosts grown without parasites served as a control. Host species identity strongly influenced parasite biomass and other traits and both parasites grew better with legumes and grasses than with forbs. The biomass of R. alectorolophus was much higher than that of R. minor with all host plants and R. alectorolophus responded much more strongly to higher nutrient availability than R. minor. The performance of the two species of Rhinanthus with individual hosts was strongly correlated, and it was also correlated with that of R. alectorolophus and the related Odontites vulgaris in previous experiments with many of the same hosts, but only weakly with that of the less closely related Melampyrum arvense. The negative effect of R. minor on host biomass was less strong than that of R. alectorolophus, but stronger relative to its own biomass, suggesting that it is more parasitic. The impact of the two parasites on individual hosts did not depend on nutrient level and was correlated. Several legumes and grasses were tolerant of parasitism. While R. minor slightly reduced mean overall productivity, R. alectorolophus increased it with several species, indicating that the loss of host biomass was more than compensated by that of the parasite. The results show that closely related parasites have similar host requirements and correlated negative effects on individuals hosts, but that there are also specific interactions between pairs of parasitic plants and their hosts.

In a comparative approach, the model root-hemiparasites Rhinanthus minor and R. alectorolophus were grown with 25 host species (grasses, forbs and legumes) at two nutrient levels. Hosts grown without parasites served as a control. The performance of the two species of Rhinanthus with individual hosts was also related to that of R. alectorolophus and the related Odontites vulgaris and Melampyrum arvense in previous experiments with many of the same hosts.

Explanation of variables in "Data Experiment" (Experiment with 2 parasites x 25 Hosts x 2 nutrient levels)

Parasite: No parasite / Rhinanthus minor / Rhinanthus alectorolophus

Host: Host species

Nutrients: Low or high nutrient level

FunctGroup: Functional group of host (Grass, forb, legume)

HNutrientStatus: Nutrient status of host habitat

PMass: Mass of parasite (mg)

LogPMass: Log10 of parasite mass

LeafLength4Weeks: Length of longest leaf of parasite after 4 weeks of growth (mm)

Host mass: Mass of host (mg)

LogHostMass: Log10 of host mass

TotalMass: Total mass per pot (parasite + host) (mg)

LogTotalMass: Log10 of total mass per pot

DaysToFlowering: Days until parasite flowered

InfloLength: Sum of length of all inflorescences (mm)

LogInfloLength: Log10 of inflorescence length

LengthLongestLeaf: Length of longest leaf of parasite at harvest

Height: Height of parasite (mm)

LogHeight: Log10 of height

HostSeedMass: Mean seed mass of host (mg)

LogHostMassNoP: Mean log10 of biomass of host grown without a parasite

HostInitialMass: Host mass at time of parasite transplanting (mg)

LogHostInitialMass: Log10 initial host mass

LogHostSeedMass: Log10 of mean host seed mass

RGR: Relative growth rate of host

Survived: Whether the parasite survived or not

 

Explanation of variables in "Data Comparison" (Data on performance of parasites with the same hosts in various experiments)

Host: Host species

LogBiomMinor: Log10 biomass of Rhinanthus minor in current experiment

LogBiomAlec: Log10 biomass of Rhinanthus alectorolophus in current experiment

LogBiomMelam: Log10 biomass of Melampyrum arvense in study of Matthies (2017)

LogBiomS&M: Log10 biomass of Rhinanthus alectorolophus in study of Sandner & Matthies (2018)

LogBiomOdontites: Log10 biomass of Odontites vulgaris in study of Geppert (2012)