Evol Ecol Res 8: 577-590 (2006)     Full PDF if your library subscribes.

Plant growth and the optimal sharing of photosynthetic products with a mycorrhizal symbiont

Stefan A.H. Geritz,* Mats Gyllenberg and Ping Yan

Rolf Nevanlinna Institute, Department of Mathematics, University of Helsinki, FIN-00014 Helsinki, Finland

e-mail: stefan.geritz@helsinki.fi


Questions: What fraction of photosynthetic products should a plant allocate to a mycorrhizal symbiont to maximize its own fitness?

Mathematical methods: Maximization of plant shoot biomass at equilibrium as a function of the fraction of photosynthetic carbon allocated to a mycorrhizal symbiont in a system consisting of two ordinary differential equations describing the growth of the plant and the symbiont.

Key assumptions: During growth, the plant maintains an optimal ratio of the rates at which carbon and nutrients become available by varying the ratio of its shoot and root biomass. Plant fitness is maximized by maximizing shoot biomass at equilibrium.

Conclusions: Sharing of photosynthetic products with a mycorrhizal symbiont is optimal if and only if the cost-efficiency of the mycorrhiza in terms of the rate of nutrient acquisition per unit of carbon cost is sufficiently large and in particular exceeds that of uninfected roots. If the cost-efficiency of uninfected roots is very low, then symbiosis with the mycorrhiza is obligatory for plant survival.

Keywords: mycorrhiza, optimization model, plant growth model, symbiosis

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