Inducibility of cyclic electron transport is linked to the transition from C3–C4 to C4 photosynthesis in Flaveria

The evolutionary convergence of complex biological features offers valuable insights into the interplay among environmental factors, organismal traits, and evolutionary outcomes. C4 photosynthesis exemplifies an adaptive syndrome derived from ancestral C3 photosynthesis in response to environmental stresses. In this study, we investigated the inducibility of C4 photosynthetic characteristics in various Flaveria species, including one of the youngest C4 species on Earth, under low CO2 conditions (100 ppm). Species used include F. robusta (proto-Kranz type), F. sonorensis (type I C3-C4), F. linearis (clade B C3-C4), F. ramosissima (type II C3-C4), and F. trinervia (C4). After 4 weeks of low CO2 treatment, F. sonorensis exhibited the highest inducibility of C4-related traits, as evidenced by enhanced chloroplast content in bundle sheath cells, reduced CO2 compensation point of photosynthesis (Г), increased apparent maximum carboxylation rate of Rubisco, and elevated cyclic electron transport. Conversely, F. linearis and F. ramosissima, despite possessing more preexisting C4-related traits, demonstrated less induction of C4-related features, with no significant enhancement of cyclic electron transport observed. These results indicate that environmental stresses can induce C4-related characteristics in C3-C4 intermediate species. Furthermore, an inducible cyclic electron transport may represent a critical precondition for the evolutionary transition from C3-C4 photosynthetic metabolism to a C4 type.