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 (Aarea) may depend on higher vein density (Dv) and hydraulic conductance (Kleaf). However, the higher Dv of C4 grasses suggests a hydraulic surplus, given their reduced need for high Kleaf resulting from lower stomatal conductance (gs).

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 Kleaf in our common garden, but C4 grasses had higher Kleaf than C3 species in our meta-analysis. Variation in Kleaf depended on outside-xylem pathways. C4 grasses have high Kleaf : gs, which modeling shows is essential to achieve their photosynthetic advantage.

Across C3 grasses, higher Aarea was associated with higher Kleaf, and adaptation to aridity, whereas for C4 species, adaptation to aridity was associated with higher Kleaf : gs. 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.