Abstract
Holoparasites are nonphotosynthetic plants that acquire all resources from hosts. The holoparasite Cuscuta gronovii is native to much of the US with a broad host range including Verbesina alternifolia, an understory perennial. Both species grow in moderate to moist soils and occur in habitats that may experience prolonged or episodic drought. We applied the Wise-Abrahamson Limiting Resource Model (LRM) developed for plant-herbivore relations to examine the effects of pattern of drought stress on tolerance of V. alternifolia to parasitism by C. gronovii. Individual plants were assigned one of six treatments that were combinations of parasite (none or addition of parasite) and drought stress (well-watered, continuously-stressed, or pulse-stressed). After pulse-stressed plants had experienced two wet-dry cycles all plants were harvested. Parasitism strongly reduced both shoot and root mass and well-watered hosts exhibited the greatest decline, indicating reduced tolerance to parasitism when water was readily available. This is consistent with the LRM if parasitism limits photosynthates available to the host. However, parasitism increased allocation to shoot and this effect did not differ between well-watered and drought-stressed plants, indicating equal tolerance. This outcome is in accord with an alternative prediction of the LRM if hosts are not carbon limited. Total pot productivity was reduced by parasitism and drought stress, and this effect was greater for pulse-stressed than for continuously-stressed hosts. We discuss the applicability of the LRM for understanding the effects of drought on tolerance to parasitism.
Highlights
Being non-photosynthetic, holoparasites obtain all needed resources from hosts via root-like haustoria [1]
In this paper we present the results of host growth within the framework of the Limiting Resource Model of plant tolerance
Drought stress and parasitism significantly reduced the growth of the host roots and shoots, but the effect of the parasite differed among drought treatments (Table 1A) and this effect was more strongly expressed in the shoots
Summary
Being non-photosynthetic, holoparasites obtain all needed resources from hosts via root-like haustoria [1]. Parasites can greatly reduce host growth but the impact is likely to be a product of resource supply for the host. Because water is a key resource affecting the ability of a plant to acquire and transport minerals [2], to fix carbon [3], and distribute photosynthates, water availability is likely to affect plant-parasite interactions. Stress reduces source strength by reducing photosynthesis, and sink strength by inhibiting growth which diminishes translocation [5]. Plants experiencing moderate to severe drought stress may show slowed nutrient uptake and transport and a reduction in photosynthate translocation [6] and a change in nutrient availability to a consumer
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