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Evolutionary divergence of potential drought adaptations between two subspecies of an annual plant: Do some constraints need to be broken?

Premise Whether mechanisms of drought adaptation 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 question in two subspecies of Clarkia xantiana (Onagraceae), California winter annuals that diverged approximately 65,000 years ago and that are adapted, partly by differences in flowering time, to native ranges that differ in precipitation. Methods In these subspecies and in F5 recombinant inbred lines (RILs) from a cross between them we scored drought-adaptation traits related to phenology (times to seed germination and to flowering) and tissue water relations (measures of succulence; pressure-volume curve parameters), in common environments. Results Subspecies differed distinctly. The one native to more arid environments had some trait values associated with drought adaptation (e.g., early flowering and high succulence) but had higher osmotic potential at full turgor and lost turgor at higher water potential, indicating poorer tolerance of dehydration. Traits that differed between subspecies exhibited substantial genetic variation, with broad-sense heritability from 0.09 (stem succulence) to 0.43 (time to flowering). The genetic correlation structure suggests facilitated evolution of some trait combinations that might enhance drought adaptation (e.g., high succulence plus low turgor loss point), but the subspecies exhibit some trait combinations that do not follow genetic correlations. Conclusions As lineages diverged in their potential to escape drought by early flowering, other traits diverged as well. Genetic architecture might facilitate some correlated evolutionary responses to drought, but particular trait combinations also can evolve despite apparent genetic constraints.