Traditionally, parasitic plants were studied for their direct negative impacts on host plants, particularly agricultural crops. More recent research, however, has highlighted the importance of parasitic plants’ range of interactions, both positive and negative, with neighboring plants and hosts. This growing body of research suggests that parasitic plants play an outsize role in structuring communities, particularly through indirect effects.
Here I used stable isotopes to investigate interactions between a root hemiparasite, Castilleja applegatei, and a N-fixing host, Ceanothus prostratus. Because many parasitic plants have high transpiration rates, and/or affect N cycling via deposition of N rich litter, I looked at the effects of parasitism and host-type on transpiration and the movement of N.
I established 120 1x1m plots within Sagehen Natural Reserve, CA. 50% included C. applegatei. I surveyed each plot noting in particular the presence/absence of C. applegatei and C. prostratus. In a subset of the plots I collected leaf samples, and took physiological measurements of representative species including C. applegatei, C. prostratus, and three non N-fixing possible hosts. Leaf samples were processed for δ13C, δ15N and C/N.
Results/Conclusions
In the presence of C. prostratus, δ15N of the parasite, C. applegatei, was depleted and the leaf percent N was higher. In the presence of C. applegatei, δ15N of C. prostratus was depleted, however, percent leaf N stayed relatively constant regardless of the C. applegatei’s presence. Lower δ15N is generally indicative of biologically-fixed N, suggesting that a) C. applegatei obtains N from C. prostratus and b) C. prostratusincreases N fixation rates when parasitized, thereby compensating for N loss to the parasite.
In the presence of C. prostratus, δ13C of C. applegatei was relatively enriched. In the presence of C. applegatei, δ13C of C. prostratus was relatively depleted. The δ13C of leaf tissue reflects the balance of carboxylation and stomatal conductance. If we interpret these patterns as driven by changes in stomatal conductance (because the transpiration stream is essential to parasitism) then that suggests a) the host transpires more rapidly when parasitized, engaging in a sort of arms race with the parasite and b) when N is more abundant C. applegateidecreases transpiration.
These results highlight the range of interactions that shape host-parasite relationships, and open the door for further work on indirect effects of host-parasite interactions.