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