Shading decreases plant carbon preferential allocation towards the most beneficial mycorrhizal mutualist

Preferential allocation towards the most beneficial mutualist could maintain mycorrhizal mutualism. Context dependence of preferential allocation could then determine environmental patterns in abundance of mycorrhizal mutualists. We assessed the preferential allocation of carbon (C) and differential phosphorus (P) uptake across four light treatments between the host plant Allium vineale and two arbuscular mycorrhizal (AM) fungi within a split-root system. The ratios of C allocation and P uptake between the beneficial and nonbeneficial AM fungi were measured using isotopic labelling.Allium vineale preferentially allocated more C towards roots infected with the most beneficial AM fungus in high light and, in return, received more P from the beneficial fungus. Preferential allocation declined with shading, as A.vineale allocated 25% of labelled C to roots infected with beneficial AM fungi in high light, but only 15% with shading, a similar percentage to that allocated to roots infected with nonbeneficial fungi regardless of shading. Our findings demonstrate that plant preferential allocation towards the most beneficial mycorrhizal mutualist depends upon above-ground resources, suggesting that the abundance of beneficial mycorrhizal fungi will increase with amount of above-ground resources, with implications for mycorrhizal mediation of plant productivity with anthropogenic change.