The functional significance of C₃-C₄ intermediate traits in Heliotropium L. (Boraginaceae): Gas exchange perspectives

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.