Transport of Phosphorus and Carbon in VA Mycorrhizas
The evolutionary success of VA mycorrhizas reflects the unique combination of a superior biotrophic mode of fungal carbon acquisition and the ability of the living plant to absorb nutrients, especially phosphorus, from the fungus. Improved plant growth in
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JAKOBSEN!
1 Introduction The evolutionary success of VA mycorrhizas reflects the unique combination of a superior biotrophic mode of fungal carbon acquisition and the ability of the living plant to absorb nutrients, especially phosphorus, from the fungus. Improved plant growth in response to mycorrhization is closely linked to this bidirectional exchange of C and P between the symbionts, an exchange which is strongly influenced by plant and fungal genotypes and by the environment. Interfungal variation in the achievement of plant growth responses to colonization by VA-mycorrhizal fungi was first demonstrated more than two decades ago (Gilmore 1971; Mosse 1972), but mechanisms reponsible for such variation remain largely unidentified. The management of VA mycorrhizas towards maximum benefit in plant production should rely on a thorough identification of those structural and physiological components which determine symbiotic efficiency (Smith and Gianinazzi-Pearson 1988). The practical relevance is obvious, not only as criteria for the selection of inoculant fungi (Abbott et al. 1992), but also for predicting the potential benefit from indigenous populations of fungi (Abbott and Robson 1991b). Novel methods for studying hyphal nutrient transport have advanced our understanding of the functioning of VA mycorrhizas (Jakobsen 1994). The present chapter identifies some fungal determinants of importance to P transport and some aspects of the carbon balance of VA mycorrhizas are discussed. Growth responses to mycorrhiza formation are discussed in relation to the symbiotic exchange of P and C, in particular in relation to variations between fungal isolates.
1 Plant Biology Section, Environmental Science and Technology Department, Ris0 National Laboratory, 4000 Roskilde, Denmark
A. Varma/B. Hock (Eds.) Mycorrhiza
© Springer-Verlag Berlin Heidelberg 1995
298
I. Jakobsen
2 Functional Characterization of VA-Mycorrhizal Fungi Our ability to functionally characterize VA mycorrhizas has now moved beyond the initial simple comparisons of growth and nutrient uptake between mycorrhizal and non-mycorrhizal plants. Present techniques involving compartmentation of growth systems and use of tracer isotopes allow for routine measurements of hyphal nutrient transport , a key process in mycorrhizal functioning . We can now directly select inoculant fungi with a high capacity for hyphal nutrient transport or fungi with contrasting transport patterns for use in more detailed physiological studies. Early studies of hyphal nutrient transport over short time intervals employed different petri dish systems with a physical barrier, which could be passed by hyphae but not by roots (Hattingh et al. 1973; Pearson and Tinker 1975; Rhodes and Gerdemann 1975; Cooper and Tinker 1978, 1981). Larger-scale systems involving the separation of a soil-filled hyphal compartment by means of a root-impermeable mesh were first introduced by Schiiepp et al. (1987) and different modifications have later been used in numerous studies of hyphal nutrient transp
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