Enriched CO2 and Root-Associated Fungi (Mycorrhizae) Yield Inverse Effects on Plant Mass and Root Morphology in Six Asclepias Species.

Rondy J Malik,James D Bever

doi:10.3390/plants10112474

Abstract

While milkweeds (Asclepias spp.) are important for sustaining biodiversity in marginal ecosystems, CO2 flux may afflict Asclepias species and cause detriment to native communities. Negative CO2-induced effects may be mitigated through mycorrhizal associations. In this study, we sought to determine how mycorrhizae interacts with CO2 to influence Asclepias biomass and root morphology. A broad range of Asclepias species (n = 6) were chosen for this study, including four tap-root species (A. sullivantii, A. syriaca, A. tuberosa, and A. viridis) and two fibrous root species (A. incarnata and A. verticillata). Collectively, the six Asclepias species were manipulated under a 2 × 2 full-factorial design that featured two mycorrhizal levels (−/+ mycorrhizae) and two CO2 levels (ambient and enriched (i.e., 3.5× ambient)). After a duration of 10 months, Asclepias 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 Asclepias biomass by ~12-fold, while enriched CO2 decreased biomass by about 25%. CO2 did not impact relative transportive roots, but mycorrhizae increased root organ’s response by more than 20%. Interactions with CO2 and mycorrhizae were observed for both biomass and root morphology (i.e., relative transportive root). A gene associated with CO2 fixation (rbcL) 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 CO2, while providing a potential pathway for mycorrhizae to ameliorate CO2 induced effects.

Highlights

Plants have a broad range of traits that vary in response to environmental cues [1,2], including atmospheric carbon
Interactions with CO2 and mycorrhizae were observed for both biomass and root morphology
A gene associated with CO2 fixation revealed that the two fibrous root species formed a phylogenetic clade that was distant from the four tap-root species

Summary

Introduction

Plants have a broad range of traits that vary in response to environmental cues [1,2], including atmospheric carbon. CO2 enrichment may increase root hierarchal branching, specific length, stele, cortex, and diameter [19,20], but it is unclear how root morphology varies in response to CO2 , especially across root order [17]. Roots that correspond to the hyper-fibrous end of the tap- versus fibrous root trait continuum, are less mycorrhizal dependent [21], which can lead to negative interactions (i.e., competition) with mycorrhizal extramatrical hyphae [22]. Unraveling how mycorrhizae and atmospheric carbon may interact with roots can potentially provide insight into CO2 -induced changes

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