A bump in the node: the hydraulic implications of rhizomatous growth

PREMISE
Rhizomatous growth characterizes numerous taxa among vascular plants. While abundant information exists on nutrient sharing and demography, the question of how these metameric organisms move water through their bodies remains largely unstudied. Moreover, we lack an understanding of the evolutionary implications of rhizomatous growth across vascular plants. Here, we examine these questions by investigating the effects of rhizomatous growth and vascular construction on whole-plant hydraulic function.

METHODS
In five terrestrial fern species with diverse vascular construction, we used microCT and bright-field microscopy to examine basic vascular construction across nodes. These data were integrated with measurements of stomatal conductance under rooted and uprooted conditions to relate vascular construction and hydraulic architecture to leaf water status.

RESULTS
Similar to phytomers of woody seed plants, nodal regions in rhizomatous ferns are areas of hydraulic resistance. While the investigated species are somewhat hydraulically integrated along their rhizomes, hydraulic conductivity drops at nodes and stomatal conductance declined sharply when nodes were locally uprooted. Together, our data suggest that nodes are chokepoints in axial water movement along the rhizome.

CONCLUSIONS
Nodal chokepoints decrease hydraulic integration between phytomers. At the same time, chokepoints may act as ‘safety-valves’, hydraulically localizing each phytomer—potentially decreasing embolism and pathogen spread—suggesting a potential tradeoff in the principal construction of the fern rhizome. Moreover, we propose that shoot-borne roots (homorhizy) and the prostrate habit of rhizomatous ferns decrease the hydraulic and structural burdens that upright plants typically incur. The absence of these hydraulic and structural demands may be one reason many rhizomatous plants lack, or have minimally developed, secondary xylem.