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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 x 10(-4) kg m(-1) s(-1) MPa-1, which was only one-third of the mean K-L of 4.65 x 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.