Photosynthetic pathway influences xylem structure and function in Flaveria (Asteraceae)

Higher water use efficiency (WUE) in C-4 plants may allow for greater xylem safety because transpiration rates are reduced. To evaluate this hypothesis, stem hydraulics and anatomy were compared in 16 C-3, C-3-C-4 intermediate, C-4-like and C-4 species in the genus Flaveria. The C-3 species had the highest leaf-specific conductivity (K-L) compared with intermediate and C-4 species, with the perennial C-4 and C-4-like species having the lowest K-L values. Xylem-specific conductivity (K-S) was generally highest in the C-3 species and lower in intermediate and C-4 species. Xylem vessels were shorter, narrower and more frequent in C-3-C-4 intermediate, C-4-like and C-4 species compared with C-3 species. WUE values were approximately double in the C-4-like and C-4 species relative to the C-3-C-4 and C-3 species. C-4-like photosynthesis arose independently at least twice in Flaveria, and the trends in WUE and K-L were consistent in both lineages. These correlated changes in WUE and K-L indicate WUE increase promoted K-L decline during C-4 evolution; however, any involvement of WUE comes late in the evolutionary sequence. C-3-C-4 species exhibited reduced K-L but little change in WUE compared to C-3 species, indicating that some reduction in hydraulic efficiency preceded increases in WUE.