Premise: Whether drought-adaptation mechanisms tend to evolve together, evolve independently, and/or evolve constrained by genetic architecture is incompletely resolved, particularly for water relations traits besides gas exchange. We addressed this issue in two subspecies of Clarkia xantiana (Onagraceae), California winter annuals that separated approximately 65,000 years ago and are adapted, partly by differences in flowering time, to native ranges differing in precipitation.

Methods: In these subspecies and in recombinant inbred lines (RILs) from a cross between them we scored traits related to drought adaptation (timing of seed germination and of flowering; succulence; pressure-volume curve parameters) in common environments.

Results: The subspecies native to more arid environments (parviflora) exhibited slower seed germination in saturated conditions, earlier flowering, and greater succulence, likely indicating superior drought avoidance, drought escape, and dehydration resistance via water storage, respectively. The other subspecies (xantiana) had lower osmotic potential at full turgor and lower water potential at turgor loss, implying superior dehydration tolerance. Genetic correlations among RILs suggest facilitated evolution of some trait combinations and independence of others. Where genetic correlations exist, subspecies differences fell along them, with the exception of differences in succulence and turgor loss point. In that case, subspecies difference overcame genetic correlations, possibly reflecting strong selection and/or antagonistic genetic correlations with other traits.

Conclusions: Clarkia xantiana subspecies’ differ in multiple mechanisms of drought adaptation. Genetic architecture generally does not seem to have constrained the evolution of these mechanisms, and it may have facilitated the evolution of some of trait combinations.

These life-history and water-relations data come from plants in common environments in cultivation, in two temporal blocks. Taking wild-collected seeds from two parent populations, one of each named subspecies of Clarkia xantiana (Onagraceae), and greenhouse-generated seed from 14 F6 recombinant inbred lines of a cross between those populations, we sowed seeds on saturated germination blotting paper in a growth chamber, noting each seed's time to germination. Upon germination, we transplanted seedlings into artificial medium in containers and followed survivors to their date of first flowering, recording days elapsed. Hydrating and then harvesting plants on their date of first flower, we measured weight-based tissue succulence (saturated water contents of stems and leaves) and used pressure-volume analysis with a pressure-chamber to estimate several additional water relations traits. We used the spreadsheet tool of Sack and colleagues (Sack, L., J. Pasquet-Kok, and PrometheusWiki Contributors. 2011. Leaf pressure-volume curve parameters. PrometheusWiki. Website http://prometheuswiki.org/tiki-index.php?page=Leaf+pressure-volume+curve+parameters.) to estimate pressure-volume parameters from raw curves. Some individual plants' data we removed from the data set, to meet accepted standards of pressure-volume analysis and eliminate obvious errors (e.g., plants estimated to increase in weight as they dehydrated).

Missing values appear in the data table where it was not possible to estimate particular variables. The file "ReadMe_Burnette_and_Eckhart_AJB" explains the meanings of each variable.