The effects of parental CO2 and offspring nutrient environment on initial growth and photosynthesis in an annual grass

Seeds of Bromus madritensis ssp. rubens (red brome, an exotic annual grass in the Mojave Desert), from parents grown at three CO2 levels (360, 550, an d 700 mu mol mol(-1)), were grown in factorial CO2 (360, 550, and 700 mu mo l mol(-1)) and nutrient (zero addition, 1 : 40-strength, and 1 : 10-strengt h Hoagland's solution) environments to evaluate parental CO2 effects on off spring performance characteristics across a range of developmental environm ents. We evaluated growth rate, leaf nitrogen content, and photosynthetic g as exchange over a 3-wk period. Seedlings from elevated-CO2 parental seed s ources (2 x seedlings) had reduced growth rates compared with seedlings fro m ambient CO2-grown parents (AMB seedlings). As compared to 360, 550 and 70 0 mu mol mol(-1) CO2-stimulated relative growth rate (RGR) for most seedlin gs, the degree of stimulation was greatest for the AMB seedlings and least for the 2 x AMB seedlings. Instantaneous rates of photosynthesis mirrored t he pattern of RGR across the parental CO2 and seedling CO2 treatment combin ations. At 360 mu mol mol(-1) CO2, photosynthetic rates of seedlings were h alf that of 2 x AMB seedlings, but at 2 x AMB 700 mu mol mol(-1) CO2, their photosynthetic rates were not statistically different. Analysis of A-Ci re sponse curves indicates that 2 x AMB seedlings had reduced Rubisco activity compared with AMB seedlings, most likely as a result of less total nitroge n investment in leaves. AMB seedlings responded to low levels of nutrient i nput (1 : 40 Hoagland's solution) with increased growth rates and leaf nitr ogen content compared with zero nutrient addition. The 2 x AMB seedlings re quired the application of 1 : 10 Hoagland's before an increase in these two parameters, compared with zero nutrient addition. These results indicate t hat elevated CO2 affects Bromus offspring performance through changes in ad ult-seed-seedling nitrogen dynamics, such that reductions in photosynthesis and growth rates occur in successive generations. Species-specific allocat ion patterns that increase or decrease nitrogen allocation to seeds may enh ance or diminish the ability of subsequent offspring to respond to an eleva ted CO2 environment.