Resolving the contrasting leaf hydraulic adaptation of C₃ and C₄ grasses

Grasses are exceptionally productive, yet their hydraulic adaptation is paradoxical. Among C₃ 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 C₄ 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 C₃ and C₄ 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. C₃ and C₄ grasses had similar K_leaf in our common garden, but C₄ grasses had higher K_leaf than C₃ species in our meta-analysis. Variation in K_leaf depended on outside-xylem pathways. C₄ grasses have high K_leaf : g_s, which modeling shows is essential to achieve their photosynthetic advantage. Across C₃ grasses, higher A_area was associated with higher K_leaf, and adaptation to aridity, whereas for C₄ 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 C₃ and C₄ grasses and are crucial avenues for crop design and ecological forecasting.