The legacy of C4evolution in the hydraulics of C3and C4grasses

The anatomical reorganization required for optimal 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 hydraulic conductance (K leaf ) and capacitance. Paradoxically, the C 4 pathway reduces water demand and increases water-use-efficiency, creating a potential mis-match between supply capacity and demand in C 4 plant water relations. We use phylogenetic analyses, physiological measurements, and models to examine the reorganization of hydraulics in closely-related C 4 and C 3 grasses. Evolutionarily young C 4 lineages have higher K leaf , capacitance, turgor-loss-point, and lower stomatal conductance than their C 3 relatives. In contrast, species from older C 4 lineages show decreased K leaf and capacitance, indicating that over time, C 4 plants have evolved to optimize hydraulic investments while maintaining C 4 anatomical requirements. The initial “over-plumbing” of C 4 plants disrupts the positive correlation between maximal assimilation rate and K leaf , decoupling a key relationship between hydraulics and photosynthesis generally observed in vascular plants.