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-C-4 intermediacy in Heliotropium (sensu lato), a genus with over 100 C-3 and 150 C-4 species. CO2 compensation points (Gamma) and photosynthetic water-use efficiencies (WUEs) were intermediate between C-3 and C-4 values in three species of Heliotropium: Heliotropium convolvulaceum (Gamma = 20 mu mol CO2 mol(-1) air), Heliotropium racemosum (Gamma = 22 mu mol mol(-1)) and Heliotropium greggii (Gamma = 17 mu mol mol(-1)). Heliotropium procumbens may also be a weak C-3-C-4 intermediate based on a slight reduction in Gamma (48.5 mu mol CO2 mol(-1)) compared to C(3)Heliotropium species (52-60 mu mol 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 mu mol mol(-1)); however, no significant differences in stomatal conductance were observed between the C-3 and C-3-C-4 species. We also assessed the response of Gamma to variation in O-2 concentration for these species. Heliotropium convolvulaceum, H. greggii and H. racemosum exhibited similar responses of Gamma to O-2 with response slopes that were intermediate between the responses of C-3 and C-4 species below 210 mmol O-2 mol(-1) air. The presence of multiple species displaying C-3-C-4 intermediate traits indicates that Heliotropium could be a valuable new model for studying the evolutionary transition from C-3 to C-4 photosynthesis.