Disentangling evolutionary, environmental and morphological drivers of plant anatomical adaptations to drought and cold in North American herbaceous plants

Background/Question/Methods Understanding what determine plants ability to survive drought and cold is crucial for predicting how plants may respond to ongoing climate change. Plant survival strategies are usually characterized by morphological and physiological adaptations, while their underlying anatomical settings are largely unknown. Woody angiosperms have repeatedly evolved small water transporting conduits and large storage parenchyma tissues at colder or drier places to cope with freezing- and drought-induced damages. However, whether these adaptations are also valid for herbaceous dicots remains unclear.

Results/Conclusions We explored how stem anatomical variations in c. 900 herb species dominating in the western USA (Sonora, Mohave, Great Basin, Great Plains, Rockies) are driven by elevation and soil moisture via control over aboveground plant stature and belowground root and rhizome systems. The elevation related cooling controls the conductive system through reduced vessel diameter and extended assimilatory and storage tissues with more chlorenchyma and less sclerenchyma around vessels. The soil moisture deficit, on the other hand, determines stabilization structures by promoting species with thicker epidermis and deep roots in drier habitats. Observed trends of decreasing vessel sizes and lignification rate with elevation supports the existing knowledge that narrower vessels and extensive parenchyma assist plants to grow in cold environments by avoiding freezing-induced cavitation. Our results bring novel information on ecological drivers influencing the evolution of anatomical adaptations in North American plants. Distinct plant communities, covering elevations from 86 below sea level to 4348 m above sea level, harbor unrelated species with different evolutionary histories that have converged towards similar anatomical structures.