Effect of spring flooding on endophyte differentiation, nitrogenase activity, root growth and shoot growth in Myrica gale

Summary Spring flooding was investigated as a possible limiting factor in the development of nitrogenase activity, root growth, and shoot growth in Myrica gale. Dormant, one year old Myrica gale plants were placed in a greenhouse in early April and given three treatments: control (not flooded), flooded-water (flooded with water to 2.5 cm above the soil level) and flooded-peat (flooded with water-saturated peat to 4.0cm above the soil level). Nitrogenase activity was absent at budbreak but appeared concurrently with the differentiation of vesicles by the Frankia sp. endophyte. Flooding delayed the onset of nitrogenase activity, substantially reduced the specific nitrogenase activity of the nodules, and also severely limited the production of the new nodule biomass. Consequently by 67 days past budbreak nitrogenase activity was much greater in the control plants (5.55 ± 0.42/µmol C₂H₄ /plant. h; x̅ ± SE; N = 9) than in the flooded-water (1.18 ± 0.29) and flooded-peat (0.15 ± 0.05) plants. Production of new secondary roots was substantially reduced in the flooded plants but adventitious roots were rapidly produced along the flooded portion of the stem in the better aerated zone near the surface. New nodules formed on several adventitious roots by 67 days indicating that the plants are able to replace their largely nonfunctional deeply flooded nodules with new nodules in the aerobic zone. Initially shoot growth was unaffected by flooding but by 67 days the flooded plants had substantially less leaf biomass, lower leaf and stem nitrogen concentrations, and less total shoot nitrogen content than the control plants.