Photosynthetic, hydraulic and biomass properties in closely related C-3 and C-4 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 C-4 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 C-3 and C-4 species. In this study, photosynthetic, hydraulic and biomass properties in a closely related C(3)Tarenaya hassleriana and a C(4)Cleome gynandra are compared. Light response curves, measured at 30 degrees C, showed that the C4C. gynandra had almost twice greater net assimilation rates than the C3T. hassleriana under each increasing irradiation level. On the contrary, transpiration rates and stomatal conductance were around twice as high in the C-3, leading to approximately 3.5 times higher WUE in the C-4 compared with the C-3 species. The C-3 showed about 3.3 times higher hydraulic conductivity, 4.3 times greater specific conductivity and 2.6 times higher leaf-specific conductivity than the C-4 species. The C-3 produced more vessels per xylem area and larger vessels. All of these differences resulted in different biomass properties, where the C-4 produced more biomass in general and had less root to shoot ratio than the C-3 species. These results are in support of our previous findings that WUE, and any changes that affect WUE, contribute to xylem evolution in plants.