POTENTIAL FOR SELECTION ON PLANTS FOR WATER-USE EFFICIENCY AS ESTIMATED BY CARBON-ISOTOPE DISCRIMINATION

Water-use efficiency is thought to be related to plant performance and natural selection for plants in arid habitats, based on a general expectation that increased water-use efficiency is associated with decreased carbon gain and biomass accumulation. Using leaf carbon isotope discrimination (Delta) to determine integrated water-use efficiency, we estimated genetic variance for, and examined the relationships among Delta, biomass, and gas exchange characters for full-sibling families of the woody shrub, Chrysothamnus nauseosus, grown from seed collected at Tintic, Utah. In both well-watered greenhouse and common garden experiments, and water-limited common garden experiments, there were significant family differences for Delta, biomass, and morphological characters, indicating a potential for genetic change in response to selection. However, estimates of broad-sense heritabilities for Delta were low, indicating that the rate of change in response to selection would be relatively slow. This was consistent with the large amount of phenotypic plasticity observed for Delta as it differed with water treatment and year in the garden experiment. Phenotypically, aboveground biomass and Delta were negatively correlated within the well-watered treatments (i.e., more water-use efficient plants were larger), not correlated within the water-limited treatment, and positively correlated for combined well-watered and water-limited garden treatments, suggesting that variation in both photosynthetic capacity and stomatal limitation contribute to the variation in Delta. In contrast to the phenotypic correlations, genetic correlations for biomass and Delta were consistently negative within each treatment, and selection for higher water-use efficiency through low Delta for C. nauseosus plants in this population would tend to shift populations toward larger plants. For C. nauseosus, increased water-use efficiency is not necessarily associated with decreased carbon gain.