Photosynthetic, hydraulic and biomass properties in closely related C3 and C4 species.

In plants, most water is absorbed by roots and transported through vascular conduits of xylem which evaporate from leaves during photosynthesis. As photosynthesis and transport processes are interconnected, it was hypothesized that any variation in water transport demand influencing water use efficiency (WUE), such as the evolution of C4 photosynthesis, should affect xylem structure and function. Several studies have provided evidence for this hypothesis, but none has comprehensively compared photosynthetic, hydraulic and biomass allocation properties between C3 and C4 species. In this study, photosynthetic, hydraulic and biomass properties in a closely related C3 Tarenaya hassleriana and a C4 Cleome gynandra are compared. Light response curves, measured at 30°C, showed that the C4 C. gynandra had almost twice greater net assimilation rates than the C3 T. hassleriana under each increasing irradiation level. On the contrary, transpiration rates and stomatal conductance were around twice as high in the C3 , leading to approximately 3.5 times higher WUE in the C4 compared with the C3 species. The C3 showed about 3.3 times higher hydraulic conductivity, 4.3 times greater specific conductivity and 2.6 times higher leaf-specific conductivity than the C4 species. The C3 produced more vessels per xylem area and larger vessels. All of these differences resulted in different biomass properties, where the C4 produced more biomass in general and had less root to shoot ratio than the C3 species. These results are in support of our previous findings that WUE, and any changes that affect WUE, contribute to xylem evolution in plants.