CO2 availability influences hydraulic function of C3 and C4 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.