The functional significance of C3–C4intermediate traits inHeliotropiumL. (Boraginaceae): gas exchange perspectives

Abstract

We demonstrate for the first time the presence of species exhibiting C₃–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 C₃–C₄ intermediate based on a slight reduction in Γ (48.5 µmol CO₂ mol⁻¹) compared to C₃Heliotropium 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 CO₂ 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 O₂ 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.