Cost efficiency of nutrient acquisition and the advantage of mycorrhizal symbiosis for the host plant

Mycorrhizal symbiosis involves reciprocal transfer of carbon and nutrients between shoots on the one hand and roots colonized by symbolic fungi on the other. Mycorrhizas may improve the mineral nutrient acquisition rates, but simultaneously increase the belowground demand for carbon. Mycorrhizal pla nts will have a selective advantage over non-mycorrhizal ones if they are m ore cost-efficient in terms of carbon cost per unit of acquired mineral nut rient. However, we demonstrate here in a simple model system that this is n ot a necessary condition. Mycorrhizas may evolve even when they are less co st-efficient, provided that photosynthesis and/or growth are strongly nutri ent-limited. This result implies a unique hypothesis for the evolution of m ycorrhizal associations which may be inherently cost-inefficient as compare d to plant roots. Such symbioses may have evolved when the superior nutrien t acquisition rate of fungi combines with the relatively high photosyntheti c nutrient use efficiency of the hose plant. Consequently, provided that my corrhizas are really cost-inefficient, the selective advantage of mycorrhiz al plants will disappear when an increase in the nutrient acquisition rate is not associated with a sufficiently high nutrient use efficiency of photo synthesis, as at high soil nutrient levels or due to a loss of leaf area, s hading or low temperatures.