RESEARCH APPROACHES TO STUDY THE FUNCTIONING OF VESICULAR-ARBUSCULAR MYCORRHIZAS IN THE FIELD

Approaches suitable for the study on the functioning of vesicular-arbu scular mycorrhizas (VAM) in the field are discussed, with emphasis on those VAM-specific processes which may influence P nutrition of crops. These processes are related to hyphal P transport in particular, and conventional techniques based on comparisons between VAM-colonized pla nts and uncolonized controls are therefore of limited value in such st udies. Novel methods are described, which make use of a two-compartmen t principle and isotopes to directly measure the P transport by the VA M mycelium under controlled conditions and in the field. The principle of isotopic equilibrium may be used for indirectly quantifying the re lative amounts of P supplied by hyphae and by roots of VAM plants. The hyphal contribution to P uptake may also be assessed by comparing hyp hal uptake of P-32 from a hyphal compartment with uptake of P-33 from a corresponding compartment containing both roots and hyphae. These me thods are suitable for a functional characterization of VAM fungi and for identifying constraints to the functioning of VAM.