The presence and possible ecological significance of mycorrhizae of the submersed macrophyte,Vallisneria americana

Atypical fungal vesicles and arbuscules were found within the roots of the submersed macrophyteVallisneria americana collected at the tidal fresh headwaters of the Chesapeake Bay (Susquehanna flats) in July 1991 and 1992, suggesting the presence of a myocrrhizal association. In order to determine whether the presence of the fungus facilitates phosphorus uptake and plant growth,V. americana cores were placed in separate pots in an aquatic greenhouse and were given one of the following treatments: control, fungicide (Captan) application, or fungicide plus phosphate enrichment. Fungicide addition resulted in significantly decreased shoot elongation rates and chlorophylla production; phosphate enrichment plus fungicide restored plant growth to control levels. Low nitrogen in plant tissues of fungicide treatment groups suggests nitrogen uptake may also be promoted by the fungal association. A second laboratory experiment withV. americana grown from turions demonstrated the negative effects of the fungicide are only evident on plant growth when fungal infection is present, indicating the fungicide was not directly toxic to the macrophyte, but acted by disrupting a mycorrhizal relationship. This study supports the hypothesis that mycorrhizae are important in nutrient acquisition and growth ofVallisneria in an estuarine environment.