CO(2) availability influences hydraulic function of C(3) and C(4) grass leaves

Atmospheric CO(2) (c(a)) has increased since the last glacial period, increasing photosynthetic water use efficiency and improving plant productivity. Evolution of C(4) photosynthesis at low c(a) led to decreased stomatal conductance (g(s)), which provided an advantage over C(3) plants that may be reduced by rising c(a). Using controlled environments, we determined how increasing c(a) affects C(4) water use relative to C(3) plants. Leaf gas exchange and mass per area (LMA) were measured for four C(3) and four C(4) annual, crop-related grasses at glacial (200 µmol mol(−1)), ambient (400 µmol mol(−1)), and super-ambient (640 µmol mol(−1)) c(a). C(4) plants had lower g(s), which resulted in a water use efficiency advantage at all c(a) and was broadly consistent with slower stomatal responses to shade, indicating less pressure on leaf water status. At glacial c(a), net CO(2) assimilation and LMA were lower for C(3) than for C(4) leaves, and C(3) and C(4) grasses decreased leaf hydraulic conductance (K(leaf)) similarly, but only C(4) leaves decreased osmotic potential at turgor loss. Greater carbon availability in C(4) leaves at glacial c(a) generated a different hydraulic adjustment relative to C(3) plants. At current and future c(a), C(4) grasses have advantages over C(3) grasses due to lower g(s), lower stomatal sensitivity, and higher absolute water use efficiency.