Little Cross-Feeding of the Mycorrhizal Networks Shared Between C3-Panicum bisulcatum and C4-Panicum maximum Under Different Temperature Regimes

Common mycorrhizal networks (CMNs) formed by arbuscular mycorrhizal fungi (AMF) interconnect plants of the same and/or different species, redistributing nutrients and draining carbon (C) from the different plant partners at different rates. Here, we conducted a plant co-existence (intercropping) experiment testing the role of AMF in resource sharing and exploitation by simplified plant communities composed of two congeneric grass species (<i>Panicum</i> spp.) with different photosynthetic metabolism types (C₃ or C₄). The grasses had spatially separated rooting zones, conjoined through a root-free (but AMF-accessible) zone added with ¹⁵N-labeled plant (clover) residues. The plants were grown under two different temperature regimes: high temperature (36/32°C day/night) or ambient temperature (25/21°C day/night) applied over 49 days after an initial period of 26 days at ambient temperature. We made use of the distinct C-isotopic composition of the two plant species sharing the same CMN (composed of a synthetic AMF community of five fungal genera) to estimate if the CMN was or was not fed preferentially under the specific environmental conditions by one or the other plant species. Using the C-isotopic composition of AMF-specific fatty acid (C16:1ω5) in roots and in the potting substrate harboring the extraradical AMF hyphae, we found that the C₃-<i>Panicum</i> continued feeding the CMN at both temperatures with a significant and invariable share of C resources. This was surprising because the growth of the C₃ plants was more susceptible to high temperature than that of the C₄ plants and the C₃-<i>Panicum</i> alone suppressed abundance of the AMF (particularly <i>Funneliformis</i> sp.) in its roots due to the elevated temperature. Moreover, elevated temperature induced a shift in competition for nitrogen between the two plant species in favor of the C₄-<i>Panicum</i>, as demonstrated by significantly lower ¹⁵N yields of the C₃-<i>Panicum</i> but higher ¹⁵N yields of the C₄-<i>Panicum</i> at elevated as compared to ambient temperature. Although the development of CMN (particularly of the dominant <i>Rhizophagus</i> and <i>Funneliformis</i> spp.) was somewhat reduced under high temperature, plant P uptake benefits due to AMF inoculation remained well visible under both temperature regimes, though without imminent impact on plant biomass production that actually decreased due to inoculation with AMF.