Mycorrhizal functioning in Taraxacum hosts shifts along the mutualism-parasitism continuum in response to glacial through future changes in atmospheric [CO2]

Background/Question/Methods

Over recent geologic history, atmospheric [CO₂] has risen from a minimum value of 180 ppm during the last glacial period to the current value of 390 ppm. In addition, atmospheric [CO₂] is expected to reach 700-1000 ppm by the end of this century. There is strong evidence that low [CO₂] of the past produced major carbon limitations within plants and that rising [CO₂] has alleviated those constraints on plant carbon dynamics. However, little to nothing is known about the effects of low [CO₂] on mycorrhizal associations, and therefore it is unclear how these interactions functioned across this temporal [CO₂] gradient. We examined mycorrhizal responses to glacial through predicted future [CO₂] in a controlled growth chamber experiment. Taraxacum ceratophorum and T. officinale (Asteraceae) were grown under constant nutrient conditions at 180, 270, 390, 700, and 1000 ppm [CO₂]. Half of the plants were inoculated with arbuscular mycorrhizal fungi by adding living soil collected from the host populations. Plant responses to fungal colonization and [CO₂] were assessed after 30 and 60 d. Given the potential impact of [CO₂] on plant carbohydrate production and nutrient demand, we hypothesized that mycorrhizal associations would shift from a more parasitic to a more mutualistic state with increasing [CO₂].

Results/Conclusions

Our results indicate that plant size increased with [CO₂]; however, this effect was greater for the faster growing T. officinale relative to T. ceratophorum. Additionally, the size of T. officinale peaked at 700 ppm [CO₂], whereas T. ceratophorum was largest at 1000 ppm [CO₂]. The responsiveness of T. ceratophorum and T. officinale to mycorrhizal fungi also varied between host species and with [CO₂]. Based on measurements of total plant biomass, T. ceratophorum was most responsive to fungal colonization at 270 ppm [CO₂], while T. officinale was most responsive at 1000 ppm [CO₂]. These results suggest that [CO₂] affects the functioning of mycorrhizal associations; however, in contrast to our original hypothesis, mycorrhizal fungi may actually be most beneficial to some host plants at [CO₂] below the modern value. Furthermore, our results suggest that plant growth strategies may influence mycorrhizal responses to rising [CO₂]. Ultimately, this work provides novel insights into how mycorrhizal associations may have functioned prior to human influence as well as potential responses to rapid climate change in the future.