The plant vigor hypothesis applies to a holoparasitic plant on a drought-stressed host

Abstract

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.

High water availability increases the negative impact of a native hemiparasite on its non-native host

“…Following on, C. pubescens had higher biomass per unit of host biomass in HW compared with LW conditions, although this was only significant at α <0.07. Similarly, Cuscuta gronovii grew significantly larger in absolute and per unit host biomass terms in wet than in droughted treatments ( Evans and Borowicz, 2015 ). As mentioned above, parasite growth in HW may have been greater because of increased resource removal from the host, but also because of increased photosynthesis in the parasite.…”

The Physiological Effect of a Holoparasite Over a Cactus Along an Environmental Gradient

“…The seasonal differences could result from the distinct nature of the parasites (holoparasite vs. hemiparasite), together with the ability of cacti to store sugars in their stem which increases during the wet season. Other holoparasites such as Cuscuta gronovii , C. campestris , or C. australis have also recorded a significant reduction in the photosynthesis of their host under dry conditions ( Shen et al, 2007 ; Evans and Borowicz, 2013 , 2015 ; Le et al, 2015 ). Interestingly, the contrast between healthy and infected cacti increased in higher irrigated populations ( Figure 3 ), which are the ones under a weaker precipitation seasonality.…”

“…Solely regarding water stress, irrigation significantly reduces the harmful effect of the holoparasite Cuscuta australis over the photosynthesis of Mikania micrantha ( Le et al, 2015 ). However, it decreases the biomass of Verbesina alternifolia when infected by Cuscuta ( Evans and Borowicz, 2013 , 2015 ). Environmental gradients also provide for attractive natural studies, although scarcely explored due to the difficulty of finding a host-parasite interaction along a gradient.…”

Plant Parasites under Pressure: Effects of Abiotic Stress on the Interactions between Parasitic Plants and Their Hosts

“…In addition, they may alter the photosynthetic performance of their hosts. Some reports showed that the host's photosynthetic capacity is increased in order to compensate for the organic compounds sink from the host to the parasite [74]. By contrast, stomatal conductance, photosynthetic rates and carboxylation efficiency severely decreased in Mikania micrantha Kunth infected with Cuscuta campestris [110] and this effect was exacerbated by drought in the case of Cuscuta australis infection [111].…”

“…Nonetheless, stem parasitic plants may be indirectly affected by drought stress incurred by their hosts, which can decrease a host's growth rate, thereby limiting the resources available to the parasite, as reported for the stem parasite Cuscuta gronovii Willd. ex Schult.-Verbesina alternifolia (L.) Britton ex Kearney [74] and Amyema miquelii (Lehm. ex Miq.)…”