DOI 10.1007/s00442-004-1517-3 ECOPHYSIOLOGY

Abstract 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 (Ks) 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 Ks values compared with three C4 species. Leaf specific conductivity (KL; 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