We demonstrate for the first time the presence of species exhibiting C 3-C4 intermediacy in Heliotropium (sensu lato), a genus with over 100 C3 and 150 C4 species. CO2 compensation points (Γ) and photosynthetic water-use efficiencies (WUEs) were intermediate between C3 and C4 values in three species of Heliotropium: Heliotropium convolvulaceum (Γ = 20 μmol CO2 mol-1 air), Heliotropium racemosum (Γ = 22 μmol mol-1) and Heliotropium greggii (Γ = 17 μmol mol-1). Heliotropium procumbens may also be a weak C 3-C4 intermediate based on a slight reduction in Γ (48.5 μmol CO2 mol-1) compared to C 3Heliotropium species (52-60 μmol mol-1). The intermediate species H. convolvulaceum, H. greggii and H. racemosum exhibited over 50% enhancement of net CO2 assimilation rates at low CO 2 levels (200-300 μmol mol-1); however, no significant differences in stomatal conductance were observed between the C3 and C3-C4 species. We also assessed the response of Γ to variation in O2 concentration for these species. Heliotropium convolvulaceum, H. greggii and H. racemosum exhibited similar responses of Γ to O2 with response slopes that were intermediate between the responses of C3 and C4 species below 210 mmol O 2 mol-1 air. The presence of multiple species displaying C3-C4 intermediate traits indicates that Heliotropium could be a valuable new model for studying the evolutionary transition from C3 to C4 photosynthesis.
All Science Journal Classification (ASJC) codes
- Plant Science