The Evolution Of Optimal Water Transport In Land Plants

The successful colonization of land by plants occurred over 450 million years ago with the first true tracheophytic plants appearing 30 million years later.  Because water loss imposes threat to life terrestrial surface, plants have evolved a sophisticated network of vascular tissue delivering water and carbon compounds throughout the plant body.  Much work has shown that plant body size and resource acquisition scales with water transport, but only recent models have been sufficiently constrained to account for the diverse roles that xylem plays in plant structure (mechanical support of the canopy) and function (water transport). No such modeling efforts have been directed at primitive plants such as ferns and lycophytes, with oldest extant plant lineages.  The goal of this project is to characterize the structure-function relationships of non-woody plant xylem, and develop models to develop a deeper understanding of the evolution of plant water transport.  Recently acquired data on Huperzia squarrosa, a primitive vascular plant ancestral to ferns, indicate that xylem arrangement and hydraulic function may operate under similar constraints as in more evolved woody plants (i.e. angiosperm trees). Specifically, conduit size and distribution along the main axis change in a manner expected based on known 'conduit packing rules' developed from woody angiosperms and conifers. This suggests that strong selection pressure act on the evolution of effective resource distribution (water delivery) in plants, thereby optimizing the xylem network in primitive taxa.  Future work will examine the more complex hydraulic architectures of terrestrial and tree ferns, and monocots such as bamboo.