Uptake and Metabolism of Host Plant Alkaloids by Parasitizing Cuscuta Species

131

Herbivores and pollinators can simultaneously exert selective pressures on plant traits via direct and indirect effects. Net selection on plant traits, such as defensive chemistry, may be difficult to predict from studying either of these interactions in isolation. In this study, alkaloids were manipulated experimentally in the hemiparasitic annual plant Castilleja indivisa (Scrophulariaceae; Indian paintbrush) by growing these parasites with bitter (high‐alkaloid) or sweet (low‐alkaloid) near‐isogenic lines of the host Lupinus albus (Fabaceae) in the field. To evaluate the effect of herbivores, half of the Indian paintbrush plants were randomly assigned to a reduced‐herbivory treatment using insecticide, and the other half to a natural‐herbivory treatment. Floral traits, bud and fruit herbivory, pollination, alkaloids, and plant performance were measured. These variables were used in a path analysis to dissect the direct and indirect effects of herbivory and pollination on lifetime seed set, and the direct and indirect effects of alkaloids on seed set via herbivory and pollination. Bud herbivory and fruit herbivory directly decreased seed production, whereas pollination had a direct positive effect. In addition, bud herbivory had negative indirect effects on seed set by reducing the number of open flowers, which reduced pollinator visits. Alkaloids directly reduced bud herbivory but did not significantly affect pollination or fruit herbivory directly. However, because bud herbivory indirectly reduced seed set by reducing pollinator visits to flowers, alkaloids also had additional indirect benefits for plants by increasing pollination. Overall, the net benefit of alkaloid uptake was due to both reduction in herbivory and an increase in pollinator visits to flowers. This study demonstrates the importance of considering multiple interactions simultaneously when attempting to understand the mechanisms underlying correlations between plant traits and fitness.

Section: Introductionmentioning

confidence: 99%

Direct and Indirect Effects of Alkaloids on Plant Fitness via Herbivory and Pollination

Adler

Karban

Strauss

2001

131

“…However, in the field, generalist parasitic plants are presented with a range of host species that vary in many aspects including defensive chemistry. Lupine species are common hosts of several parasitic plant species (Stermitz and Harris 1987, Stermitz et al 1989, Arslanian et al 1990, Schneider and Stermitz 1990, Boros et al 1991, Baeumel et al 1992, Stermitz and Pomeroy 1992, , , ), and they differ from nonleguminous hosts in their ability to fix nitrogen (via association with symbionts) as well as in their alkaloid content. The impact of lupines or other alkaloid‐containing host species on herbivores of a parasitic plant has only been assessed in a handful of studies to date, each with different conclusions (Stermitz et al 1989, , , ).…”

Section: Introductionmentioning

confidence: 99%

Host Effects on Herbivory and Pollination in a Hemiparasitic Plant

Adler¹

2002

The indirect effects of hosts on interactions between parasites and other species are not well understood, and it may be difficult to predict the outcome of host species effects on parasite performance due to the complexity of potential direct and indirect effects. For example, parasitic plants obtain defensive compounds as well as nutrients from their hosts, and thus many attributes of parasitic plants are dependent on the quality of their host species. Here I measure the effect of a lupine host species (Lupinus argenteus) compared to other host species on herbivory, pollination, and female plant fitness in the hemiparasite Indian paintbrush (Castilleja miniata) using a series of field experiments. Association with lupine was determined in the field by assaying Indian paintbrush leaves for lupine alkaloids. I found that Indian paintbrush plants parasitizing lupines experienced reduced herbivory from plume moth larvae, agromyzid fly larvae, and deer, relative to Indian paintbrush plants parasitizing other host species. However, there was no correlation between alkaloid content of inflorescences and plume moth performance. Host species did not affect pollinator preference for Indian paintbrush in the field. Indian paintbrush para-sitizing lupines produced twice as many seeds overall as Indian paintbrush parasitizing other host species. Correlations suggest that this benefit arises both from reduced herbivory and increased nitrogenous resources. The reduction of herbivory in Indian paintbrush plants parasitizing lupines indicates that host species can affect performance of hemiparasites via indirect pathways, and that the larger community of herbivores could alter the impact of a host on its plant parasites.

“…However, in the field, generalist parasitic plants are presented with a range of host species that vary in many aspects including defensive chemistry. Lupine species are common hosts of several parasitic plant species (Stermitz and Harris 1987, Stermitz et al 1989, Arslanian et al 1990, Schneider and Stermitz 1990, Boros et al 1991, Baeumel et al 1992, Stermitz and Pomeroy 1992, Jeschke et al 1994, Marvier 1995, 1998a, Adler and Wink 2001, and they differ from nonleguminous hosts in their ability to fix nitrogen (via association with symbionts) as well as in their alkaloid content. The impact of lupines or other alkaloid-containing host species on herbivores of a parasitic plant has only been assessed in a handful of studies to date, each with different conclusions (Stermitz et al 1989, Marko et al 1995, Marvier 1995, 1998a.…”

Section: Introductionmentioning

confidence: 99%

Host Effects on Herbivory and Pollination in a Hemiparasitic Plant

Adler

2002

Abstract. The indirect effects of hosts on interactions between parasites and other species are not well understood, and it may be difficult to predict the outcome of host species effects on parasite performance due to the complexity of potential direct and indirect effects. For example, parasitic plants obtain defensive compounds as well as nutrients from their hosts, and thus many attributes of parasitic plants are dependent on the quality of their host species. Here I measure the effect of a lupine host species (Lupinus argenteus) compared to other host species on herbivory, pollination, and female plant fitness in the hemiparasite Indian paintbrush (Castilleja miniata) using a series of field experiments. Association with lupine was determined in the field by assaying Indian paintbrush leaves for lupine alkaloids. I found that Indian paintbrush plants parasitizing lupines experienced reduced herbivory from plume moth larvae, agromyzid fly larvae, and deer, relative to Indian paintbrush plants parasitizing other host species. However, there was no correlation between alkaloid content of inflorescences and plume moth performance. Host species did not affect pollinator preference for Indian paintbrush in the field. Indian paintbrush parasitizing lupines produced twice as many seeds overall as Indian paintbrush parasitizing other host species. Correlations suggest that this benefit arises both from reduced herbivory and increased nitrogenous resources. The reduction of herbivory in Indian paintbrush plants parasitizing lupines indicates that host species can affect performance of hemiparasites via indirect pathways, and that the larger community of herbivores could alter the impact of a host on its plant parasites.