Variation in carbon isotope discrimination in relation to plant performance in a natural population of Cryptantha flava

Few studies of phenotypic selection have focused on physiological traits, especially in natural populations. The adaptive significance of plant water-use efficiency, the ratio of photosynthesis to water loss through transpiration, has rarely been examined. In this study, carbon isotopic discrimination, Δ, an integrated measure of water-use efficiency, was repeatedly measured in juveniles and adults in a natural population of the herbaceous desert perennial Cryptantha flava over a 4-year period and examined for plasticity in Δ, consistency between years in values of Δ, and evidence for selection on Δ phenotypes. There was significant concordance in Δ values among the 4 years for adult plants and significant correlations in Δ values measured in different years for juveniles and adults combined. The wettest year of the study, 1998, proved an exception because Δ values that year were not correlated with Δ values in any other year of the study. Consistency in Δ measured on the same plants in different years could indicate genotypic variation and/or consistency in the water status of the microhabitats the plants occupied. Two forms of plasticity in Δ were also evident; mean seasonal values were correlated with precipitation the preceding autumn, and Δ values also declined with plant size, indicating increasing water-use efficiency. Phenotypic selection was evident because in the first year of the study juvenile plants that would survive until year five averaged lower Δ values than did those that failed to survive. During the driest year, 2000, Δ was significantly negatively correlated with adult plant size, measured as the number of leaf rosettes, but the negative relationship between Δ and the number of flowering stalks, a more direct measure of fitness, was not significant. These results suggest that the direction of phenotypic selection on Δ changes as plants grow.