Carbon allocation and partitioning in Populus tremuloides are modulated by ectomycorrhizal fungi under phosphorus limitation

The fate of carbon (C) captured by forest trees during photosynthesis is influenced by the supply of other resources. Fixed C may be partitioned among biomolecules within the leaf and/or allocated throughout the tree to growth, storage and maintenance activities. Phosphorus (P) availability often limits tree productivity due to its high biological demand and strong interactions with soil minerals. As ectomycorrhizal (ECM) fungi play critical roles in enhancing phosphate (Pi) acquisition by their hosts, these symbioses will influence the fate of C within trees and forested ecosystems. Using Populus tremuloides Michx. (trembling aspen) in symbiosis with Laccaria bicolor (Marie) P.D. Orton or Paxillus involutus (Batsch) Fr., we assessed C acquisition, allocation and partitioning under Pi limitation, specifically focusing on primary and secondary C compounds. Both ECM fungi moderated the effects of low P on photosynthesis and C partitioning among carbohydrates and secondary metabolites by sustaining Pi uptake and translocation in P. tremuloides under Pi limitation. As leaf P declined, reductions in photosynthesis were accompanied by significant shifts in C partitioning from nonstructural carbohydrates (NSCs) to phenolic glycosides and tannins. Carbon partitioning in roots exhibited more complex patterns, with distinct increases in NSCs in nonmycorrhizal (NM) plants under Pi limitation that were not evident in plants colonized by either ECM symbiont. In general, aspen colonized by L. bicolor exhibited C partitioning patterns intermediate between those of NM and P. involutus aspen. The C cost of symbiosis was pronounced for plants supporting P. involutus, where ECM plants exhibited maintenance of photosynthesis yet reduced biomass in comparison with NM and L. bicolor aspen under Pi replete conditions. Our results indicate that the ECM symbiosis affects the disposition of C in forest trees in part by altering the acquisition of other limiting resources from soils, but also through ECM species-specific influences on host physiology. This modulation of C partitioning will have broad implications for forest ecosystem C capture, storage and cycling where nutrient resources may be limited.