Manipulation of flooding and arbuscular mycorrhiza formation influences growth and nutrition of two semiaquatic grass species

1. Two semiaquatic grasses, Panicum hemitomon Schultes and Leersia hexandra Schwartz, were grown for 12 weeks in sterilized soil in experimental mesoc osms, with and without the addition of arbuscular mycorrhizal (AM) fungal i noculum (as nonsterilized soil), under the following rooting-zone flood reg imes: waterlogged (W), free-draining (D), beginning W and ending D (W-D), a nd beginning D and ending W (D-W). The purpose of the experiment was to det ermine whether these controlled water regimes affected both colonization of wetland grasses by AM fungi and the effects of the colonization on various plant parameters. 2. Water regime, addition of inoculum, and their interaction were highly si gnificant effects on total and proportion of root length colonized by AM fu ngi. Trends were very similar for the two grass species. Colonization was l ess and plants smaller in the W and W-D than in the D and D-W treatments. T he viability of mycorrhiza at the end of the experiment, as measured by vit al staining techniques, was not affected by changes in water level. 3. Colonized plants in all water level treatments showed an improvement in phosphorus (P) nutrition over noncolonized plants. Colonized grasses of bot h species gained consistently more P per plant and had greater tissue P con centrations, with the greatest P concentration in the most heavily colonize d plants (from the D and D-W treatments). 4. The effect of flooding on the mycorrhizal association depended largely o n the extent to which the association was already established when the floo ding occurred. Flooding reduced the initiation of colonization either direc tly or indirectly, but once the fungi were established in the roots they we re able to maintain and expand with the growing root system.