Isotopic evidence of partial mycoheterotrophy in the gentianaceae: Bartonia virginica and obolaria virginica as case studies

Premise of the study:
An estimated 10% of plant species have evolved to steal C from their symbiotic fungal partners (mycoheterotrophy), and while physiological evidence for full and partial mycoheterotrophy is well developed in the Orchidaceae and Ericaceae, it is lacking for the majority of other mycoheterotrophic taxa. The family Gentianaceae not only contains several lineages of achlorophyllous mycoheterotrophs, but also contains species that are putative partially mycoheterotrophic. The North American genera Bartonia and Obolaria (Gentianaceae) are green but have leaves reduced to scales or foliose bracts and so have ambiguous mycoheterotrophic status.

Methods:
We investigated the natural abundance ¹³ C and ¹⁵ N profiles of both genera along with total N and chlorophyll content and investigated mycorrhizal infection using light microscopy. Key results: The shoots of B. virginica were significantly more enriched in ¹⁵ N than the surrounding vegetation but not in ¹³ C. In contrast, the shoots of O. virginica are not enriched in ¹⁵ N compared to the surrounding vegetation but were significantly enriched in ¹³ C. Total N concentrations were significantly higher than the surrounding vegetation in B. virginica, while the collaroid roots of both species were infected by arbuscular mycorrhizal fungi.

Conclusions:
This microscopic evidence coupled with the natural abundance stable isotope profiles strongly suggests that both species are partially mycoheterotrophic. However, differences in the root-shoot stable isotopic patterns relative to surrounding vegetation between B. virginica and O. virginica are suggestive of the utilization of different physiological pathways or extent of commitment to mycoheterotrophic C gain.