Combining leaf gas-exchange and stable carbon isotopes to assess mycoheterotrophy in three species of Pyroleae

Determining the extent of mycoheterotrophy (MH) in plants, primarily through the use of stable isotope methods, has gained considerable attention in the last decade. The aim of this study was to characterize the rates of photosynthesis (PS) and several gas-exchange parameters, as well as stable carbon isotope composition (δ13C) of partially mycoheterotrophic (PMH) Pyroleae compared with autotrophic reference species of Ericaceae. An end-member mixing model was applied to δ13C, deriving estimates of % C gained via fungi (CDF). The δ13C was significantly enriched for Orthilia secunda and Pyrola chlorantha (relative to autotrophs) resulting in estimates of CDF ranging from 13.8% to 20.8%. Despite significantly lower PS rates for O. secunda and P. chlorantha, as well as lower conductance and transpiration, there were no significant differences in the Ci:Ca ratios across all of the species, suggesting that the C isotope inferences for these two species were reflective of fungal C gains. By contrast, results for all of the variables indicated primarily autotrophic C nutrition for Chimaphila umbellata. Further studies, such as isotope labelling experiments or assessments of biochemical constraints to autotrophy, may resolve the uncertainties in these species, allowing more accurate understanding of the complex nutritional mode of these plants.