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summary Red alder (Alnus rubra Bong.) is a nitrogen-fixing pioneer tree species of the Pacific Northwest of North America. We investigated the response of different seed sources of red alder to elevated atmospheric CO2 and to varied levels of water stress. Seeds were stratified, germinated and grown for up to 147 d under ambient (350 //.I I−1) or elevated (700 ftl I−1) CO2. There were no significant interactions of seed source latitude with either treatment, although seedlings from more northerly sources were larger. Elevated CO2 and low moisture stress resulted in larger plants with more leaf area; effects of the two factors appeared additive. Effects of both factors on biomass allocation, including root: shoot ratios, were small or nonsignificant. Elevated CO2 decreased specific nitrogenase activity and generally increased photosynthesis (A) and stomatal conductance (g). The ratio A: g, potential water use efficiency, also increased when plants were under water stress. Elevated CO2 appears to improve drought tolerance in red alder. Overall, these results indicate that red alder would benefit in total plant growth from increased ambient CO2 and could tolerate changes in precipitation.

SUMMARY Inoculated and uninoculated Alnus rubra Bong, seedlings were grown for 47 days in atmospheres containing ambient (350 μ1 CO2 1−1) and elevated (650 μl CO2 1−1) levels of CO2, with and without combined nitrogen (20 mg 1−1) supplied as ammonium nitrate. Five plants from each treatment were harvested 15, 30, and 47 days after exposure to CO2 treatments began. Evidence for the presence of a positive feedback loop between nitrogen fixation and photosynthesis was observed in nodulated plants growing at elevated CO2. These plants had greater whole-plant photosynthesis and nitrogenase activity, leaf area and nitrogen content, as well as nodule and plant dry mass, relative to nodulated plants grown at ambient CO2 and non-nodulated plants grown at both CO2 levels. This feedback may be an important way in which the potential carbon drain of nitrogen fixation on the host plant could be compensated; increased nitrogen availability resulting in stimulated leaf area growth and whole-plant photosynthesis. The relative amount of dry mass allocated below ground decreased for all seedlings over time, and the amount allocated above ground increased. This shift in allocation occurred slowly and at a constant rate in non-nodulated plants and more rapidly and abruptly when plants were nodulated. The proportion of dry mass allocated below ground was consistently greater in non-nodulated plants. Dry mass allocation to the stem was increased when combined nitrogen was applied and was greatest in nodulated plants grown at high CO2. Dry mass partitioning among other organs was not directly affected by nodulation, CO2 enrichment, or other treatment interactions.

SUMMARY Frankia strains ArI5, AvcIl, AvsI2, CpI1 and MpI1, previously isolated from nodules of actinorhizal plants, were tested in association with various actinorhizal plants to determine differences in nitrogen-fixing effectiveness. Alnus incana ssp. rugosa, Alnus rubra, Alnus viridis ssp. crispa, Comptonia peregrina and Myrica gale plants, grown in nitrogen-free water culture, were inoculated with the cultured Frankia strains. Significant differences in rates of acetylene reduction were found among strains which suggested that endophytes were not fully compatible with some or all of the host plants tested. A pattern of compatibilities among hosts and endophytes was developed to explain differences in rates of acetylene reduction.

Summary The acetylene reducing activity of Alnus rubra nodules is maximal in 15 to 30% oxygen. Within the nodule, the large number and size of many of the intercellular spaces between the uninfected cells in the outer cortex facilitates the diffusion of air through the lenticeb to the infected cells in the mid-cortex. Access of air to the infected cells, which are arranged in longitudinal bundles separated by one or more lines of starch-containing uninfected cells and which run at an angle away from the stele towards the nodule apex, is restricted by the reduced number and size of air spaces in this region. The cytoplasm of the infected cells shows high activity of peroxidase and o-diphenol oxidase, but the lack of correlation between the activity of these enzymes and nitrogenase activity suggests that they are not involved in the control of access of oxygen to the endophyte. It is suggested that one role of nodule peroxidase is the ignification of the infected cells associated with cellular senescence.