Strong functional divergence in the evolution of drought tolerance of the Coniferales

Background/Question/Methods

Water stress is one of the primary selective forces in plant evolution, causing plants of all ages to routinely die from acute water shortage. Specific adaptations in land plants have been linked to survival under water stress, but little is known about the adaptive process connecting plant function with drying climate change. 

Results/Conclusions

Here we present evidence from 40 species of conifers demonstrating that the evolution of drought resistant conifers follows two distinct pathways emerging from coordinated evolution of tissues involved water supply (xylem) and water loss (stomatal pores). We show that this remarkably simple pattern of drought evolution appears after a post-Cretaceous radiation of dry-adapted Pinaceae and Cupressaceae, both of which follow highly divergent functional trajectories to cope with low rainfall. Pinaceae are found to rely on high levels of the stomatal-closing hormone Abscissic Acid to close stomata tightly, while Cupressaceae evolved xylem tissues with extreme cavitation resistance. Our findings indicate how early woody plant systems evolved to cope with water shortage, highlighting the conifers as a functional link between the simple drought survival strategies seen in ferns and the diverse mechanisms seen in angiosperms.