Resolving the contrasting leaf hydraulic adaptation of C3 and C4 grasses

Summary Grasses are exceptionally productive, yet their hydraulic adaptation is paradoxical. Among C 3 grasses, a high photosynthetic rate ( A area ) may depend on higher vein density ( D v ) and hydraulic conductance ( K leaf ). However, the higher D v of C 4 grasses suggests a hydraulic surplus, given their reduced need for high K leaf resulting from lower stomatal conductance ( g s ). Combining hydraulic and photosynthetic physiological data for diverse common garden C 3 and C 4 species with data for 332 species from the published literature, and mechanistic modeling, we validated a framework for linkages of photosynthesis with hydraulic transport, anatomy, and adaptation to aridity. C 3 and C 4 grasses had similar K leaf in our common garden, but C 4 grasses had higher K leaf than C 3 species in our meta‐analysis. Variation in K leaf depended on outside‐xylem pathways. C 4 grasses have high K leaf : g s , which modeling shows is essential to achieve their photosynthetic advantage. Across C 3 grasses, higher A area was associated with higher K leaf , and adaptation to aridity, whereas for C 4 species, adaptation to aridity was associated with higher K leaf : g s . These associations are consistent with adaptation for stress avoidance. Hydraulic traits are a critical element of evolutionary and ecological success in C 3 and C 4 grasses and are crucial avenues for crop design and ecological forecasting.