Photosynthetic pathway alters xylem structure and hydraulic function in herbaceous plants

Plants using the C 4 photosynthetic pathway have greater water use efficiency (WUE) than C 3 plants of similar ecological function. Consequently, for equivalent rates of photosynthesis in identical climates, C 4 plants do not need to acquire and transport as much water as C 3 species. Because the structure of xylem tissue reflects hydraulic demand by the leaf canopy, a reduction in water transport requirements due to C 4 photosynthesis should affect the evolution of xylem characteristics in C 4 plants. In a comparison of stem hydraulic conductivity and vascular anatomy between eight C 3 and eight C 4 herbaceous species, C 4 plants had lower hydraulic conductivity per unit leaf area ( K L ) than C 3 species of similar life form. When averages from all the species were pooled together, the mean K L for the C 4 species was 1.60 × 10 −4 kg m −1 s −1 MPa −1 , which was only one‐third of the mean K L of 4.65 × 10 −4 kg m −1 s −1 MPa −1 determined for the C 3 species. The differences in K L between C 3 and C 4 species corresponded to the two‐ to three‐fold differences in WUE observed between C 3 and C 4 plants. In the C 4 species from arid regions, the difference in K L was associated with a lower hydraulic conductivity per xylem area, smaller and shorter vessels, and less vulnerable xylem to cavitation, indicating the C 4 species had evolved safer xylem than the C 3 species. In the plants from resource‐rich areas, such as the C 4 weed Amaranthus retroflexus , hydraulic conductivity per xylem area and xylem anatomy were similar to that of the C 3 species, but the C 4 plants had greater leaf area per xylem area. The results indicate the WUE advantage of C 4 photosynthesis allows for greater flexibility in hydraulic design and potential fitness. In resource‐rich environments in which competition is high, an existing hydraulic design can support greater leaf area, allowing for higher carbon gain, growth and competitive potential. In arid regions, C 4 plants evolved safer xylem, which can increase survival and performance during drought events.