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 C-4 plants, should have altered xylem properties. To evaluate this hypothesis, we assessed the hydraulic and anatomical properties of 19 C-3 and C-4 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 C-4 than C-3 shrubs from the American west. In the Asian species, the C-3 Nitraria schoberi had similar and Halimodendron halodendron higher K-s values compared with three C-4 species. Leaf specific conductivity (K-L; hydraulic conductivity per leaf area) was 60%-98% lower in the C-4 than C-3 species, demonstrating that the presence of the C-4 pathway alters the relationship between leaf area and the ability of the xylem to transport water. C-4 species produced similar or smaller vessels than the C-3 shrubs except in Calligonum, and most C-4 shrubs exhibited higher wood densities than the C-3 species. Together, smaller conduit size and higher wood density indicate that in most cases, the C-4 shrubs exploited higher WUE by altering xylem structure to enhance safety from cavitation. In a minority of cases, the C-4 shrubs maintained similar xylem properties but enhanced the canopy area per branch. By establishing a link between C-4 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.