Plant parasites span a continuum from complete dependence upon their host for resources (i.e., holoparasitic) to partially dependent on the host and partially autotrophic (i.e., hemiparasitic). Many molecules transfer between hosts and parasites leading to competition for some nutrients, especially nitrogen, when multiple parasites are attached to a single host. Although hemiparasitic plants can acquire carbon through photosynthesis, whether or not hemiparasites compete for host-derived carbon or regulate autotrophy relative to availability of host-derived carbon is unknown. We assessed how the degree of heterotrophy (carbon acquired from host) varied among mistletoes (Phoradendron californicum) and mesquite hosts (Prosopis velutina), and how this changed with removal of mistletoe up or down the same xylem stream (i.e., branch). Photosynthetic gas exchange, elemental analyses, and vegetative traits of mesquites and mistletoes were measured prior to and after mistletoe removal and across seasons varying in canopy closure (i.e., light availability) and monsoonal moisture in the southwestern US.
Results/Conclusions
Removal of mistletoe altered photosynthesis in adjacent host tissue over time depending on whether the parasite was near the branch end or close to the trunk, linking location on host branch and parasite load to virulence. Mistletoe also altered its gas exchange in response to removal, with increased photosynthesis over time under no removal, suggesting host carbon may not be enough to support multiple parasites. Conductance changed even when photosynthesis did not, suggesting nutrients within the xylem stream may modulate mistletoe photosynthetic ability. After removal, heterotrophy decreased for mistletoe near branch tips but did not change for mistletoe near host trunks in spring. By monsoon season, heterotrophy decreased for mistletoe near both branch tips and host trunks. These changes in heterotrophy highlight the seasonal variability in mistletoe virulence, and the location-dependent crosstalk that occurs when multiple parasites depend upon a single host. These data also demonstrate context-dependency in competition among hemiparasites for host-derived carbon and maintenance of autotrophy relative to host health.