Resolving the contrasting leaf hydraulic adaptation of C 3 and C 4 grasses

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