The process that the evolution from C3 towards C4 photosynthesis is driven

Compared to the plants with C3 photosynthesis, the plants with C4 photosynthesis have higher light, water, and nitrogen use efficiencies. Though only 3% of higher plants have C4 photosynthesis, C4 photosynthesis contribute about 23% of terrestrial gross primary productivity. Currently, it remains elusive how the procedure of the evolution from C3 towards C4 photosynthesis is driven. Here, Flaveria genus, the chief model for C4 evolutionary researches, was used to study this driving through a combined analysis of ecological, physiological and biochemical data. From proto-Kranz to C4-like, results showed that precipitation was negatively associated with the activity of C4 metabolic core enzyme and C4 flux, respectively. With increased vein density, an increased temperature and precipitation were observed during the earlier evolutionary stage. Our study indicated that decreasing precipitation and increasing temperature accompanied the transition from C3 to C2 photosynthesis and further transition from C2 to developing C4 photosynthesis. There was a positive relationship between CO2 compensation point, stomatal density and precipitation, supporting the notion that decreasing precipitation enhances photorespiration through reducing stomatal conductance during the evolution. As the driving force from proto-Kranz to C4-like, our study highlights that drought dominated rather than heat during C4 evolution. This study suggests that the driving force between certain stages during the evolution towards C4 photosynthesis are different and increased water availability should be needed in the initial phase.