Photosynthetic pathway alters hydraulic structure and function in woody plants

Xylem structure and function is proposed to reflect an evolutionary balance between demands for efficient movement of water to the leaf canopy and resistance to cavitation during high xylem tension. Water use efficiency (WUE) affects this balance by altering the water cost of photosynthesis. Therefore species of greater WUE, such as C₄ plants, should have altered xylem properties. To evaluate this hypothesis, we assessed the hydraulic and anatomical properties of 19 C₃ and C₄ woody species from arid regions of the American west and central Asia. Specific conductivity of stem xylem (K_s ) was 16%–98% lower in the C₄ than C₃ shrubs from the American west. In the Asian species, the C₃ Nitraria schoberi had similar and Halimodendron halodendron higher K_s values compared with three C₄ species. Leaf specific conductivity (K_L ; hydraulic conductivity per leaf area) was 60%–98% lower in the C₄ than C₃ species, demonstrating that the presence of the C₄ pathway alters the relationship between leaf area and the ability of the xylem to transport water. C₄ species produced similar or smaller vessels than the C₃ shrubs except in Calligonum, and most C₄ shrubs exhibited higher wood densities than the C₃ species. Together, smaller conduit size and higher wood density indicate that in most cases, the C₄ shrubs exploited higher WUE by altering xylem structure to enhance safety from cavitation. In a minority of cases, the C₄ shrubs maintained similar xylem properties but enhanced the canopy area per branch. By establishing a link between C₄ photosynthesis and xylem structure, this study indicates that other phenomena that affect WUE, such as atmospheric CO₂ variation, may also affect the evolution of wood structure and function.