C4 photosynthesis and hydraulics in grasses

center dot The anatomical reorganization required for C-4 photosynthesis should also impact plant hydraulics. Most C-4 plants possess large bundle sheath cells and high vein density, which should also lead to higher leaf capacitance and hydraulic conductance (K-leaf). Paradoxically, the C-4 pathway reduces water demand and increases water use efficiency, creating a potential mismatch between supply capacity and demand in C-4 plant water relations. center dot Here, we use phylogenetic analyses, physiological measurements, and models to examine the reorganization of hydraulics in closely related C-4 and C-3 grasses. center dot The evolution of C-4 disrupts the expected positive correlation between maximal assimilation rate (A(max)) and K-leaf, decoupling a canonical relationship between hydraulics and photosynthesis generally observed in vascular plants. Evolutionarily young C-4 lineages have higher K-leaf, capacitance, turgor loss point, and lower stomatal conductance than their C-3 relatives. By contrast, species from older C-4 lineages show decreased K-leaf and capacitance. The decline of K-leaf through the evolution of C-4 lineages was likely controlled by the reduction in outside-xylem hydraulic conductance, for example the reorganization of leaf intercellular airspace. center dot These results indicate that, over time, C-4 plants have evolved to optimize hydraulic investments while maintaining the anatomical requirements for the C-4 carbon-concentrating mechanism.