AN ECOLOGICAL VIEW OF THE FORMATION OF VA MYCORRHIZAS

In spite of the major advances in understanding the functioning of sym bioses between plants and arbuscular mycorrhizal fungi, details of the ecology of mycorrhizal fungi are not well documented. The benefits of the association are related to the timing and extent of colonization of roots, and fungi differ in their contribution to plant growth and p resumably to soil aggregation. Knowledge of the processes that lead to successful colonization of roots by beneficial fungi at appropriate t imes for the host plants will form the basis of guidelines for soil ma nagement to maximize the benefits from the symbiosis. Fungi differ in the manner and extent to which they colonize roots. They also differ i n their capacity to form propagules. The importance of hyphae, spores and propagules within living or dead mycorrhizal roots also differs am ong species and for the same species in different habitats. The relati onships between colonization of roots and propagule formation, and bet ween propagule distribution and abundance and subsequent mycorrhiza fo rmation, for different fungi in field environments, are not well under stood. Methods for quantifying mycorrhizal fungi are not especially su itable for distinguishing among different fungi within roots. Conseque ntly, the dynamics of colonization of roots by different fungi, within and between seasons, have been little studied. Research is required t hat focuses on the dynamics of fungi within roots as well as on change s in the abundance of propagules of different fungi within soil. Inter actions between fungi during the colonization of roots, the colonizati on of soil by hyphae and sporulation are all poorly understood. Withou t knowledge of these processes, it will by difficult to predict the li kely success of inoculation with introduced fungi. Such knowledge is a lso required for selecting soil management procedures to enhance growt h and survival of key species within the population. The relative tole rance of various fungi to perturbations in their surroundings will pro vide a basis for identifying those fungi that are likely to persist in specific environments. The processes that influence mycorrhizal fungi in field soils can be identified in controlled studies. However, grea ter emphasis is required on studying these processes with mixed popula tions of fungi. The role played by diversity within populations of myc orrhizal fungi is virtually unexplored.