INTER- AND INTRAGENOTYPIC COMPETITION UNDER ELEVATED CARBON DIOXIDE IN ARABIDOPSIS THALIANA

The consequences of elevated [CO.sub.2] 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 [CO.sub.2] 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 [CO.sub.2] 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 [CO.sub.2], 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-[CO.sub.2] atmosphere (700 [micro]L/L) the reverse situation was observed: plants performed better in mixtures than in pure stands. Positive frequency-dependent selection under ambient [CO.sub.2] concentration became negative under elevated [CO.sub.2], 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 [CO.sub.2] than under ambient [CO.sub.2] concentration. However, this study allows predictions about evolutionary trajectories under high-[CO.sub.2] conditions. In A. thaliana, genotypes that will maintain the most their ability to grow well in pure stand should be selected under increasing [CO.sub.2]. Key words: Arabidopsis thaliana; elevated [CO.sub.2] and competition among genotypes; fitness; genotype; global change, implications of elevated [CO.sub.2]; intraspecific competition; kin selection; selection, frequency dependent.