Inter- and intragenotypic competition under elevated carbon dioxide in Arabidopsis thaliana

The consequences of elevated CO2 on plant growth have been well studied on individual plants. The response of a more complex system with several plants interacting is less understood-a situation that limits our capacity to predict the response of natural plant communities. In this study we analyzed the effect of CO2 enrichment on intergenotypic competition in Arabidopsis thaliana. Seeds of five: genotypes collected from different natural populations were used. Each genotype was cultivated in a pure stand and in a mixture with each of the other four genotypes in two CO2 conditions (ambient and elevated). At harvest time, genotype fitness was estimated by the number of fruits and seeds produced per plant. At current levels of CO2, genotypes performed better in a pure stand than in a mixture. Kin selection, associated with the low seed dispersal and autogamous reproductive regime of A. thaliana, is invoked to explain these positive responses among plants of similar genotype. Surprisingly, in a high-CO2 atmosphere (700 muL/L) the reverse situation was observed: plants performed better in mixtures than in pure stands. Positive frequency dependent selection under ambient CO2 concentration became negative under elevated CO2, which could lead more easily to the maintenance of genetic variation. This hypothesis was tested with a simple model of competition. At equilibrium, the simulation did not show coexistence among more genotypes under elevated CO2 than under ambient CO2 concentration. However, this study allows predictions about evolutionary trajectories under high CO2 conditions. In A. thaliana, genotypes that will maintain the most their ability to grow well in pure stand should be selected under increasing CO2.