C photosynthesis and hydraulics in grasses.

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