Parasitic plants extract resources from host vascular tissues but their responses to environmental fluctuation experienced by the host are poorly studied. Three frequently-cited hypotheses for effects of environmental stress on plant resistance to herbivores predict decreased, increased, or fluctuation in herbivore performance in response to drought stress. We tested which hypothesis best accounts for how drought stress applied to a perennial herb affects growth of the holoparasite, Cuscuta gronovii Willd. ex Roem. & Schult. (common dodder), an obligate shoot parasite. Verbesina alternifolia (L.) Britton ex Kearney (wingstem) supporting single, young C. gronovii were exposed to continuous, pulsed, or no water stress for 32 days and then dry mass of each parasite was determined. Consistent with the plant vigor hypothesis, C. gronovii grew significantly better on well-watered hosts. Continuous and pulsed drought stress of the host resulted in similar growth reduction relative to no drought stress. In addition to reducing absolute growth of the holoparasite, continuous and pulsed drought stress reduced the growth of the holoparasite relative to host growth. Although functionally similar to insect phloem feeders, growth of holoparasites such as C. gronovii is constrained by source–sink relations. Our results suggest drought stress experienced by a host weakens source strength and reduces uptake by the holoparasite acting as a sink.
Oviposition site preference of the bamboo gall maker, Aiolomorphus rhopaloides (Hymenoptera: Eurytomidae), on bamboo in terms of plant-vigor hypothesis