Evolutionary divergence of potential drought adaptations between two subspecies of an annual plant: Are trait combinations facilitated, independent, or constrained?

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 resis...