Symbiosis with arbuscular mycorrhizal fungi (AM) may affect the nutritional  status of a plant, especially regarding phosphorus (P) and carbon (C). Since the AM draws carbon from the host plant, the effect of AM on plant growth is dependent on the cost-benefit relationship of the symbiosis.

We analysed the effect of phosphorus nutrition and mycorrhiza on the level of leaf cell growth. We performed a kinematic analysis to (i) assess the contribution of nutrient status- and size- related factors at determining the rate of leaf growth (Kavanová et al. 2006a), and to (ii) analyse the effects of phosphorus status on cell division and cell growth in expanding leaves (Kavanová et al. 2006b). The leaf elongation rate of the grass Lolium perenne plants growing at different levels of soluble phosphorus supply or in symbiosis with the AM fungus Glomus hoi was tightly correlated with the phosphorus status of the leaf growth zone. Further, the presence of AM did not affect the carbohydrate concentration in the growth zone, showing that AM did not limit leaf growth by decreasing carbon availability. The changes in the leaf growth rate in plants differing in their phosphorus status were due to changes in the rates of cell division and mitotic growth (affecting thus the cell flux out of the division zone) and postmitotic cell expansion (affecting thus the final cell length).

At the whole plant level, we disentangled phosphorus status-dependent and -independent effects of AM (Glomus hoi) on the components of plant growth: morphology and assimilation rates, in perennial ryegrass (Lolium perenne). First, we assessed phosphorus response functions of plant morphological components in plants with similar size (Grimoldi et al. 2005). Further, we investigated the AM effects on carbon economy by comparing nonmycorrhizal and mycorrhizal plants of similar phosphorus content (Grimoldi et al. 2006). 13CO2/12CO2 steady-state labelling was used to trace all photosynthate assimilated during one photoperiod, and a respiratory 13CO2/12CO2 exchange system to assess dark respiration rates. The relationships between relative phosphate uptake rate, leaf and plant morphology were identical in mycorrhizal and nonmycorrhizal plants. Beneficial effects of mycorrhizal symbiosis were mainly mediated by adjustments in leaf morphology, which were largely dependent on AM effects upon phosphorus capture. When ontological and nutritional effects were accounted for, AM increased below-ground costs, which were not compensated by increased photosynthesis rates.