Over-compensation of the native herb Qiai (Artemisia argyi var. argyi cv. Qiai) to infection with alien field dodder (Cuscuta campestris).

Compensatory growth of crops is argued to alleviate detrimental parasitic effects. Cuscuta campestris causes severe crop losses worldwide, and is the most important parasitic weed. In a two-year consecutive field investigation, C. campestris was found on Artemisia argyi var. argyi cv. Qiai, one traditional medicinal herb with a long cultivation history in Qizhou, China. In 2020, anatomical validation was conducted on haustorial connection, and competition between carbon (C) and nitrogen (N) in this parasitic system was quantified with isotope signature (δ¹³C, δ¹⁵N) at five sites in 2021. Across sites, leaf senescence of low canopy of infected Qiai was greatly accelerated. Enhancement of resource remobilization from aging leaves (emerging source) mainly supported growth of the parasite (additional sink) also host stem, Qiai successfully abstained intense resource competition with C. campetris, and both species had similar δ¹³C and δ¹⁵N. A new source-sink balance re-established between the parasite and host. Infected Qiai held relatively steady canopy C fixation (constant photosynthetic N use efficiency, biomass and N concentration of green leaves), and branched in top canopy markedly, increasing light access. Together, Qiai appeared to enhance canopy C gain in response to infection. Increase in δ¹³C in this parasite-host association was mild. Finally, this parasite evidently over-stimulated the aboveground biomass of its host, irrespective of infection density and load, and was left behind. This study first reported C. campestris on Qiai, and the latter species execute over-compensation in the field.