Mycorrhizal associations and phosphorus acquisition : from cells to ecosystems

The vast majority of terrestrial plants form some type of mycorrhizal association. In this chapter, attention is focused on the most common and widespread types: Arbuscular mycorrhizas and ectomycorrhizas. The former are well known to play key roles in plant uptake of soluble inorganic phosphorus (Pi), via the arbuscular mycorrhizal (AM) uptake pathway. This pathway involves cell types and Pi transporters that are different from those utilised by the direct, epidermal, plant uptake pathway, and recent studies have indicated considerable interplay between the two pathways. Although less attention has been paid to the roles of ectomycorrhizas in P uptake, it is increasingly clear that this symbiosis is involved not only in soluble Pi uptake but also in the solubilisation of insoluble Pi and the mobilisation of organic P via phosphatases. Hence, these mycorrhizas likely play significant roles in nutrient turnover in forest ecosystems. For both mycorrhizal types there is considerable variation in outcomes in terms of P uptake and plant growth, depending on the identity of plant and fungal symbionts. It is emphasised that experiments with single plants and fungi in pots are unlikely to be good predictors of outcomes in the field, due to such factors as complex plant-fungal assemblages, interplant competition and plant densities.