FIGURE 3.28 Effect of arbuscular mycorrhizal infection on shoot nitrogen content of plants of Spartina cynosuroides. Experimental conditions were: mycorrhizal infection (clear bars; N=un-inoculated, Y=inoculated) added phosphorus (hatched bars; H=3.10 mg PO -P/l, L=0.31 mg PO-P/l) water salinity (solid bars; F=no salt added, S=7 ppt salt) tidal inundation (stippled bars; B=bottom level, M=middle level, U=upper level). Data from McHugh and Dighton (unpublished).

In the 1960s Bjorkman (1960) discovered that by injecting radioactive carbon into the stem of a pine tree he could detect the isotope in an adjacent Monotropa plant. Monotropa is an achlorophyllous plant that was thought to be parasitic on the roots of trees, as it could not manufacture its own carbohydrates by photosynthesis. He also noted that there were ectomycorrhizal associations of the roots of both the Monotropa plant and the tree. He therefore suggested that there could be a carbohydrate exchange between plants by the mycorrhizal connection between them. The concept of interconnectedness

of plants within the ecosystem by mycorrhizal bridges has been a subject of research that has been dotted with examples of inconclusive results and skepticism. More recently however there have been a number of good research projects that have shown the presence of interplant transfer of both carbon and nutrients between plants connected by mycorrhizal “bridges”. Subsequent to Bjorkman’s work, Read et al. (1985) demonstrated that the 14C label applied to pine tree seedlings was preferentially transferred to neighboring unlabeled pine trees seedlings rather than to neighboring plant species associated with arbuscular mycorrhizae. From this finding, they suggested that there was strong evidence that there was movement of carbon between the donor plant (labeled with the 14C isotope) and the recipient plants that shared the same ectomycorrhizal associations. The study of Finlay and Read (1986a, b, c) showed unidirectional transfer of phosphorus from the ectomycorrhizal mycelium into the host tree, but no translocation from a radio-labeled host tree into adjacent unlabeled trees. This suggests that the interplant transfer of nutrients may depend upon the strengths of sources and sinks of nutrients in the whole system. In the experimental chambers used for this study, it was assumed that there was little difference in source and sink strengths among neighboring plants but a general demand for phosphorus by all plants growing within the chambers.