Abstract

Premise of research. Crassulacean acid metabolism (CAM), a photosynthetic pathway that evolved in response to aridity, is beneficial for plant species in arid and semiarid environments, with positive effects on plant fitness. However, since it implies a higher metabolic cost compared with that for C3 plants, its expression under moist conditions, where it might be less necessary, could have fewer positive or even negative effects on plant fitness. Variations of the effects of CAM expression under different moisture regimes have never been addressed. Our objectives were to determine whether there are such variations and, if so, their possible ecological and evolutionary implications. Methodology. We measured CAM expression (as nocturnal acidification, ΔH1) and moisture-related morphological leaf traits, estimating their impact on reproductive effort in two populations of Puya chilensis living under contrasting moisture regimes in a natural climatic gradient in central Chile. To evaluate the effects of the measured traits on fitness, a phenotypic selection analysis was performed. Pivotal results. Phenotypic selection analysis showed that ΔH1 was the only trait significantly associated with relative fitness in both populations. There is a significant “switch” in the direction of selection between both studied populations: directional selection on CAM was positive in the dry population and negative in the moist population. Conclusions. Results suggest that under moist regimes the expression of CAM by P. chilensis could be removed by natural selection. To our knowledge, this is the first report of a negative effect of CAM on plant fitness. Since CAM is not usually studied in moist environments, we believe this pattern might be more frequent than expected. Further research on this issue is needed, especially in the context of current climate change.