One would expect that a comparison of literature from before and after Søndergaard and Laegaard's seminal paper in 1977 might reveal different conclusions from the same evidence. What is a bit more unexpected is the conflicting evidence generated long after most researchers recognized that mycorrhizae were an important part of aquatic plant community dynamics. Take, for example, a comparison of three papers written in the 1990s. Rickerl et al. (1994) examined South Dakotan wetland plants in July in dry soils (no surface water) and wet soils (at least 10 cm water depth) for mycorrhizal infection. Wetzel and van der Valk (1996) carried out a similar study, in June, in Iowa and North Dakota, and reported average water depth for each sampled site. Finally, Cooke and Lefor (1998) surveyed wetland plants in Connecticut, although water depth for each sampling site was not reported since this was an initial survey of 89 species of wetland plants. The results of these three studies alone point to the difficulty in obtaining a clear picture of the distribution of mycorrhizae in hydrophytes.
|Typha x glauca
|Rickerl et al.||24% dry
|Wetzel and van der Valk||-------||56.4% (1 cm water)||-------||-------||0.3% (43 cm water)||-------||-------|
|Cooke and Lefor||80% (October)||-------||30% (August)||0% (July)||Polygonum spp. 30% (October)||-------||-------|
Review articles are an essential tool in making sense of hydrophyte/mycorrhizal interactions, and generally report that mycorrhizal colonization is less common and less extensive in wetland plants and trees than in the more commonly studied agricultural and terrestrial systems. Many of the most useful summaries of mycorrhizal distribution in aquatic systems are quite recent, and, I suspect, still to come. Mukerji and Mandeep (1998) published one of the best syntheses of this information to date. Several trends can be taken from this article:
Mukerji and Mandeep further offer that wet habitats may inhibit mycorrhizae formation due to the lack of oxygen in submerged sediments, a result further supported by Keeley
(1980) and Read (1976). Changes in root exudates, which may be necessary signals for mycorrhizal formation, may alter mycorrhiza formation in wet soils. Root metabolism and
the formation of aerenchymous spaces in roots (Blom et al. 1996) may affect distribution of mycorrhizae. Mark Brundrett recently reported that mycorrhizae do not
colonize aerenchymous root spaces in wetland plants (see http://www.ffp.csiro.au/research /mycorrhizae/).
This conclusion is further supported by Cooke et al. (1993) and Cantlemos and Ehrenfeld (1999). Mycorrhizae seem to be more common at shallow soil depths in aquatic systems.
See Cantelmos and Ehrenfeld 1999 for a recent study. These findings suggest that mycorrhizae in hydrophytes may range anywhere from mutualistic to parasitic, and point to
the need for further research in this area.
Rickerl et al. (1994) reported greater numbers of mycorrhizal spores in wet soils than in dry soils, but a considerably greater extent of mycorrhizal infection in dry than in wet soils. In contrast, Rani and Mukerji (in Mukerji and Mandeep 1998) found fewer spores in wetter soils than in dry soils, a conclusion supported by A.G. Khan in several studies (1993a,b,c in Mukerji and Mandeep 1998). Cooke and Lefor (1998) wrote that the absence of spores in wetland soils do not rule out mycorrhizal infection of plants growing in these soils. The use of spore counts as an indication of VA mycorrhizal abundance is risky at best.
We know that mycorrhizae are present in hydrophytes, and that they are less common than in terrestrial systems. We also know that, in general, mycorrhizae can be parasitic or mutualistic, although I am not aware of any studies demonstrating parasitic mycorrhizal infection in hydrophytes (see Allen et al. 1989 for an example of parasitic mycorrhizal infection in terrestrial soils). Jon Keeley (1980) suggested that one reason for the difference in mycorrhizal infection rates between aquatic and terrestrial systems may be the added cost, to an aquatic plant, of providing mycorrhizae with oxygen. This argument, like most, is based on the assumption that mycorrhizae are aerobic. This is a fairly commonly stated view (Crawford 1992 and LeTacon et al. 1983 in Mukerji and Mandeep 1998, Read 1976). Some support for the mycorrhizae as obligate aerobes theory exists. Saif (1981 in Cooke at el. 1993) observed that Glomus macrocarpum infection increased with increasing partial pressure of oxygen in saturated soils. This supports Read's (1976) finding that mycorrhizae are most abundant around areas of plant roots that leak oxygen into the surrounding soil. However, this theory has also never been explicitly tested, as most recently pointed out by Cantelmos and Ehrenfeld (1999). Are mycorrhizae dependent on the presence of certain minimum levels of oxygen in aquatic sediments? Does this influence their distribution around hydrophyte roots?
With my advisor, Dr. Siobhan Fennessy of Kenyon College, I am beginning studies to determine whether or not mycorrhizae in aquatic systems are constrained by the boundaries of the oxidized rhizosphere around plant roots. Using an oxygen probe with a 300 µm diameter tip and the root chamber technique for tracking mycorrhizal infection using fluorescence microscopy (provided by Dr. Carl Friese at the University of Dayton), we plan to map the oxidized rhizosphere around Phalaris arundinacea, a wetland perennial grass that thrives in a range of moisture levels. Experiments completed this summer (as yet unpublished data from 1999) showed high levels of mycorrhizal infection in P. arundinacea in continuously flooded wetland soils. Plants inoculated with mycorrhizae (a mixture of Glomus clarum, G. claroideum, G. etunicatum, and G. intraradices) grew larger, had higher root:shoot ratios, and contained slightly higher amounts of phosphorus in shoot tissues than did control plants in flooded soils. Having found a plant species which develops mycorrhizal infection in unsaturated, saturated, and flooded soils, our next step is to investigate whether or not such infection occurs only in areas of soil directly surrounding P. arundinacea roots or extends away from the roots. We will measure oxygen concentrations at varying distances from the root and determine whether or not rate of mycorrhizal infection and abundance of external hyphae correspond to oxygen concentration in aquatic sediments. If mycorrhizal infection is not related to oxygen concentration, what other factor is limiting the distribution of mycorrhizae in aquatic sediments? Are mycorrhizae truly obligate aerobes, as suggested by the mycorrhizal literature? If oxygen concentration does limit the spread of mycorrhizal hyphae through soils, how can mycorrhizae reach beyond the zone of soil depleted of nutrients by plant roots to transport phosphorus and other nutrients to the host plant? Do hydrophytes expand their oxidized rhizosphere to accomodate mycorrhizae? If so, one would expect this added cost of the symbiosis to result in mycorrhizae that resembled parasites more than mutualists.
There is much to learn about mycorrhizae in aquatic plants, and some basic questions to answer before this symbiosis can be understood in aquatic systems. The outstanding exchange of information in this field, along with the the growing interest in wetlands and other hydrophyte habitats, will help us answer these questions. Only then can we begin to use mycorrhizae as growth enhancers and aids to restoration as is already being done in terrestrial systems, and to sort out the effects of mycorrhizae on community dynamics in aquatic plants. For more information, visit the up-to-date bibliographies at http://mycorrhiza.ag.utk.edu, or browse through the references listed in the credits section of this page. You can also contact Dr. Fennessy at email@example.com. Questions about this site should be directed to firstname.lastname@example.org.