The effect of salinity and nitrogen on a parasitic plant-host plant interaction

Background/Question/Methods
Nitrogen and salinity are two important factors affecting salt marsh plants. Complex interactions between these two factors have been reported to affect a plant’s physiology, growth, and interaction with herbivores. In this study, I investigated the effect of salinity and nitrogen on a parasitic plant (Cuscuta salina, salt marsh dodder) and its host plant (Sarcocornia pacifica, previously Salicornia virginica) to test whether the interaction between salinity and nitrogen affects the interaction between parasite and host. This avenue of research is intriguing because there are many unexplored parallels between parasitic plants and herbivores. To test this hypothesis, S. pacifica was grown in the greenhouse with salinity (4 levels), nitrogen (3 levels), and C. salina (present, absent) treatments in a fully factorial design.

Results/Conclusions
There was a strong interaction between nitrogen and salinity treatments on S. pacifica biomass, tissue salinity, and tissue nitrogen content. Plants grew the largest at intermediate salinities with intermediate nitrogen additions. Although C. salina did not affect S. pacifica biomass, the effects of the parasite on host tissue salinity and nitrogen interacted with nitrogen and salinity treatments, implying complex effects of the abiotic environment on the relationship between parasite and host. C. salina coiled significantly fewer host stems under the highest nitrogen treatment. Because there was no effect on the total number of coils per host, C. salina may forage less on hosts with high nitrogen content. There was no effect of salinity (host or treatment) on C. salina, suggesting that this halophytic parasite may be less sensitive than insect herbivores to salinity. Similarly, while C. salina responds to nitrogen additions like insect herbivores, there may be a threshold beyond which foraging ceases.