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With these improvements some major inconveniencies of the FDA-method of fluorescent staining for observation of soil organisms in situ could be overcome in as
all fungal structures could be observed - while still the metabolically active parts are stained exclusively by the FDA and thus can be located easily. With this level the
technique now allows the routine study of fungi inside the soil (SAXENA & LYSEK 1993; JENSEN 1994). Further - and desirable - advances are aimed to the differentiation
between the various fungal structures - e. g. conidia, inactive or resting spores, chlamydospores and sclerotia etc. Whether these can be obtained by application of
further fluorochromes is still under study.
The next aim is to extend it to all soil organisms or groups of organisms to allow complete studies of the soil
biotope. Since protozoa could be observed after FDA-fluorochromation already (SAXENA and JENSEN, personal communication) and nematodes partially in the FDA- or in
the triple-stained mounts, while bacteria can be stained with fluorochromes like acridine orange, this target seems to be inside easy or at least possible
reach (STRUGGER 1949; WYNN-WILLIAMS 1985).
Another aim for further development is the retardation of the photo-fading of FDA. The citifluor antifadant is of
some help, but longer lasting observations are still difficult if not impossible. The stable fluorescence of calcofluor white compensates only partial, since
the FDA-staining of the metabolically active regions would allow to follow alterations or developments - which are always connected to metabolism - better than with any
other substance.
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