Grass veins are leaky pipes: Vessel widening in grass leaves explain variation in stomatal conductance and vessel diameter among species

Authors: OCHELTREE, TROY - COLORADO STATE UNIVERSITY; Gleason, Sean

Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/26/2023
Publication Date: 11/14/2023
Citation: Ocheltree, T.W., Gleason, S.M. 2023. Grass veins are leaky pipes: Vessel widening in grass leaves explain variation in stomatal conductance and vessel diameter among species. New Phytologist. 241(1):243-252. https://doi.org/10.1111/nph.19368.
DOI: https://doi.org/10.1111/nph.19368

Interpretive Summary: Leaf vein networks can either be branching, where large veins diverge into smaller “water delivery” veins (e.g., sunflower leaf), or they can be parallel, where veins run the entire length of the leaf without branching (e.g., maize leaf). Although these two vascular designs are common throughout the plant kingdom, the benefits, costs, and requirements of these network architectures are largely unknown. One key feature all vascular networks is they need to efficiently transport water to the sites of evaporation (and photosynthesis) within the leaves. Given that the vessels (water-conducting “pipes”) of parallel-veined species do not branch, they must therefore lose water to transpiration along their entire lengths, and these volumetric water losses must be accounted for via changes in conduit volume. We hypothesized that vessel diameter in parallel-veined species should widen with distance from the leaf tip to account for: 1) increasing hydraulic resistance, and 2) water loss to transpiration. We found that parallel-vein species did indeed exhibit “faster” vessel widening than species with branching networks, and that the rate of this widening was also aligned with the maximum rate of transpiration. Our work supports the idea that natural selection has placed a premium on parallel vein networks designs that are optimized to provide high hydraulic conductance for the least expenditure in carbon.

Technical Abstract: Parallel vs branching leaf vein architecture is one of the most striking and consistent differences between monocotyledon (parallel) and eudicotyledon (branching) plants, yet the structural and physiological implications of these two different vascular designs are largely unknown. Given that parallel veins do not branch, they must supply water to the lamina along their entire lengths, i.e., “leakiness” must be a key feature of veins arranged in parallel. We hypothesized that vessel diameters within grass veins would widen at a rate that 1) minimizes increasing pathlength resistance, and 2) accounts for water volume lost to transpiration. We found that the rate of conduit widening (widening exponent), from tip-to-base, was larger than the widening exponent required to minimize pathlength resistance (0.45 vs. 0.22). Furthermore, variation in the widening exponent was positively correlated with maximal stomatal conductance (r2 = 0.20; p = 0.040) and net CO2 assimilation (r2 = 0.45; p < 0.001), suggesting that faster rates of conduit widening were associated with higher rates of water loss to gas exchange. This suggests that natural selection has favored parallel vein networks that are neither over-built nor under-built relative to transpiration requirements, resulting in the optimization of hydraulic conductance (~carbon income) and carbon expenditure.