全球变化知识资源中心

C-4 photosynthesis evolved when grasses migrated out of contracting forests under a declining atmospheric CO2 concentration ([CO2](a)) and drying climate around 30 million years ago. C-4 grasses are hypothesised to benefit from improved plant-water relations in open habitats such as savannas, giving advantages over C-3 plants under low [CO2](a). But experimental evidence in a low CO2 environment is limited, and comparisons with C-3 trees are needed to understand savanna vegetation patterns. To test whether stomatal conductance (g(S)) and CO2 assimilation (A) are maintained in drier soil for C-4 grasses than C-3 trees, particularly under low [CO2](a), we investigated photosynthesis and plant-water relations of three C-3 tree and three C-4 grass species grown at 800, 400 or 200 ppm [CO2](a) over moderate wetting-drying cycles. C-4 grasses had a lower soil-to-leaf water potential gradient than C-3 trees, especially at 200 ppm [CO2](a), indicating reduced leaf water demand relative to supply. Yet the dependence of g(S) and A on predawn leaf water potential (a measure of soil water availability) was greater for the C-4 grasses than trees, particularly under low [CO2](a). Our findings establish that g(S) and A are not maintained in drier soil for C-4 grasses compared with C-3 trees, suggesting that this mechanism was not prevailing in the expansion of C-4-dominated grasslands under low [CO2](a). This inherent susceptibility to sudden decreases in soil water availability justifies why C-4 grasses have not evolved a resistant xylem allowing operation under drought, but instead shut down below a water potential threshold and rapidly recover. We point to this capacity to respond to transient water availability as a key overlooked driver of C-4 grass success under low [CO2](a). A is available for this article.