Resolving the paradox of leaf vasculature in C4 grasses: Does high vein density drive high hydraulic conductance and photosynthesis?

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Alec Baird, Jessica Pasquet-Kok, Christine Vuong, Yu Zhang and Lawren Sack, Department of Ecology & Evolutionary Biology, University of California Los Angeles, Los Angeles, CA, Samuel H. Taylor and Colin P. Osborne, Department of Animal and Plant Sciences, University of Sheffield, Sheffield, United Kingdom, Teera Watcharamongkol, Kanchanaburi Rajabhat University, Kanchanaburi, Thailand, Christine Scoffoni, Department of Biological Sciences, California State University, Los Angeles, CA, Erika J Edwards, Ecology and Evolutionary Biology, Yale University, New Haven, CT

Background/Question/Methods
The remarkable global ecological dominance and contribution to high terrestrial productivity of grasses is largely due to the abundance of grass species using C4 photosynthesis (> 40%). The C4 photosynthesis syndrome is represented by CO2 concentrating mechanisms (CCMs), associated with specialized leaf anatomies that drive high photosynthetic rates and water use efficiencies especially under high temperatures or low CO2. In C3 eudicotyledons, higher leaf hydraulic conductance (Kleaf) and photosynthetic rate result from higher vein length per leaf area (vein density). Yet such mechanism holds a paradox for C4 grasses as they have higher vein density to provide bundle sheath volume for their carbon concentrating mechanism, but lower maximum stomatal conductance (gs) and transpiratory demand than C3 grasses. Whether or not this higher vein density in C4 grasses provides a higher Kleaf remains untested. To address this paradox we disentangle the contrasting hydraulic adaptation of C3 and C4 grass species and its coordination with photosynthetic rate for common garden-grown species sampled across the phylogeny, with a diverse array of C4 origins, and leaf structure and function (n = 11 C3, 16 = C4).
Results/Conclusions
On average, C3 and C4 grass species were similar in Kleaf. The higher vein density of C4 grasses did not confer higher Kleaf; first, the higher vein density was counteracted by smaller veins with fewer conduits, and, more importantly, the pathways outside the xylem were the main determinant of Kleaf for C3 and C4 grasses. The similar Kleaf but lower gs of C4 than C3 grasses resulted in higher Kleaf/gs, representing greater hydraulic supply to demand. Modeling indicated that this hyper-efficient water transport is essential in the success of C4 grasses, necessary to maintain open stomata during transpiration and thereby to realize their biochemical advantage and achieve higher photosynthetic rates than C3 grasses in moist soil and moderate drought. This hydraulic hyper-efficiency also can explain the decoupling of C4 gas exchange from rainfall across native ranges, and supports the expectation of a C4 advantage in drying climates under rising CO2 concentrations. These findings highlight the key role of leaf hydraulic diversity in determining grass leaf physiology, and their importance for clarifying their evolution and biogeography and for breeding climate-change-ready crops.