Metabolism based on photosynthesis is the hallmark feature of plants, yet some flowering plant lineages have adopted a heterotrophic, parasitic lifestyle. Root hemiparasitic plants are distinctive because they photosynthesize yet can also parasitize host plants via specialized root structures called haustoria . To siphon host sap towards the haustoria, parasitic plants transpire much more than their hosts. This flow is one-directional, suggesting that hemiparasites should accumulate large amounts of xylem-mobile solutes. We experimentally evaluated this concept.
In several greenhouse experiments, ten hemiparasitic species from the subtribe Castillejinae (Orobanchaceae) were grown with various hosts. When the hemiparasites began to bud, plants were harvested, dried, and weighed. The multivariate suite of nutrients were subject to Principal Component Analysis. We related nutrient levels to hemiparasite identity, host identity, hemiparasite life-history status (annual or perennial), and degree of heterotrophy (parasitism allowed or prevented).
Results/Conclusions
As predicted, hemiparasites had higher nutrient concentrations than hosts, both in overall nutrient load and for most individual nutrients. However, hemiparasite nutrient status was affected in surprising ways by whether or not parasitism was allowed. P followed the expected pattern, being more abundant when parasitism was allowed than when it was prevented, but nutrients such as Ca, Mg and S were more abundant when parasitism was prevented. Furthermore, levels of other nutrients did not differ with degree of heterotrophy, suggesting that their levels are intrinsic to these species and are not a result of parasitism per se. Host nutrient status was largely unaffected by parasitism, suggesting that host plants compensate for the loss of nutrients. Host identity influenced the mineral nutrition of the hemiparasite. Preliminary data suggest that life-history status may predict mineral nutrition loads.
There are several ecological implications of this research. First, mineral nutrition analysis may help us understand why hemiparasites perform better on certain hosts. Given the known role hemiparasites play in structuring their ecosystems, such knowledge would inform community ecology theory. Differences in mineral nutrition based on whether or not parasitism was allowed suggest that a parasite’s foraging and nutrient cycling is plastic, based on host availability. Finally, it also suggests that parasites are particularly reliant on hosts for P, a largely immobile mineral nutrient.