Reproductive allocation and seed production in Bromus madritensis ssp rubens at elevated atmospheric CO2

1. Two trends are consistent across the response of plant species to growth at elevated CO2: decreased leaf nitrogen content and increased photosynthe tic gas exchange. While both of these are very important to the understandi ng of plant and ecosystem responses to climate change, little research has evaluated the consequences of these patterns on reproductive allocation and seed production. 2. For this reason, Bromus madritensis ssp. rubens was grown in ambient (36 0 mu mol mol(-1)), x 1.5 ambient (550 mu mol mol(-1)) and elevated (700 mu mol mol(-1)) CO2 environments to compare the relationship between allocatio n to growth and reproduction as a function of CO2 growth environment. 3. There were no differences in final total biomass or reproductive mass be tween CO2 growth environments. There were significant decreases in reproduc tive mass per unit total mass and per unit vegetative mass, but not per uni t leaf surface area (LSA), as growth CO2 environment increased from 360 to 700 mu mol mol(-1) CO2. Despite these decreases, the number of seeds produc ed per unit LSA in elevated CO2 significantly increased as compared to ambi ent CO2. These results may be owing to a shift in allocation to greater inv estment in vegetative growth as compared to reproduction under elevated lev els of atmospheric CO2. 4. Prior to reproduction, there were no significant differences between CO2 treatments in carbon uptake by leaves. In contrast, plants grown in elevat ed CO2 did not show a decline in photosynthetic rate during seed filling, s uggesting that nitrogen may not have been re-translocated from leaves to se eds as apparently occurred in ambient plants. 5. Patterns measured here may partially explain the parental effect of CO2 environment exhibited in Bromus. Seeds produced from elevated parental CO2 growth conditions lead to seedlings that produce smaller leaves that are de layed in development and smaller roots as compared to structures produced b y seeds from ambient-grown parents. 6. Because the success of Bromus is partially owing to its ability to produ ce large numbers of viable seeds, these changes in reproductive allocation and subsequent seedling performance with respect to growth in an elevated C O2 environment may have impacts on community composition in the Mojave Dese rt.