Enriched CO and Root-Associated Fungi (Mycorrhizae) Yield Inverse Effects on Plant Mass and Root Morphology in Six Species.

While milkweeds ( spp.) are important for sustaining biodiversity in marginal ecosystems, CO flux may afflict species and cause detriment to native communities. Negative CO-induced effects may be mitigated through mycorrhizal associations. In this study, we sought to determine how mycorrhizae interacts with CO to influence biomass and root morphology. A broad range of species (n = 6) were chosen for this study, including four tap-root species (, , and ) and two fibrous root species ( and ). Collectively, the six species were manipulated under a 2 × 2 full-factorial design that featured two mycorrhizal levels (-/+ mycorrhizae) and two CO levels (ambient and enriched (i.e., 3.5× ambient)). After a duration of 10 months, responses were assessed as whole dry weight (i.e., biomass) and relative transportive root. Relative transportive root is the percent difference in the diameter of highest order root (transportive root) versus that of first-order absorptive roots. Results revealed an asymmetrical response, as mycorrhizae increased biomass by ~12-fold, while enriched CO decreased biomass by about 25%. CO did not impact relative transportive roots, but mycorrhizae increased root organ's response by more than 20%. Interactions with CO and mycorrhizae were observed for both biomass and root morphology (i.e., relative transportive root). A gene associated with CO fixation () revealed that the two fibrous root species formed a phylogenetic clade that was distant from the four tap-root species. The effect of mycorrhizae was most profound in tap-root systems, as mycorrhizae modified the highest order root into tuber-like structures. A strong positive correlation was observed with biomass and relative transportive root. This study elucidates the interplay with roots, mycorrhizae, and CO, while providing a potential pathway for mycorrhizae to ameliorate CO induced effects.