CO₂ Fertilisation of Growth in Wild C₄ Grasses may be Amplified by Upregulated Photosynthesis

Rising atmospheric CO₂ is expected to have limited direct effects on C₄ photosynthesis because the carbon-concentrating mechanism maintains near-saturating CO₂ at Rubisco. Consequently, growth responses of C₄ plants to elevated CO₂ (eCO₂) are generally attributed to indirect improvements in plant water relations. However, it remains unclear whether sustained exposure to eCO₂ also induces changes in photosynthetic capacity that contribute directly to carbon gain. We examined the interactive effects of CO₂ concentration and soil water availability on photosynthesis, biochemical capacity, and growth in three African C₄ grasses grown under ambient (400 ppm) and elevated (550 ppm) CO₂. Elevated CO₂ increased net photosynthesis across species, with stronger responses under drought. Intrinsic water-use efficiency increased primarily through enhanced assimilation, while stomatal responses were contingent on water availability rather than driven directly by elevated CO₂. Importantly, eCO₂ induced species- and context-dependent increases in apparent photosynthetic capacity after prolonged treatment exposure. Megathyrsus maximus showed strong V_cmax increases under both water treatments and J_max upregulation under well-watered conditions, Heteropogon contortus exhibited biochemical upregulation primarily under drought, and Themeda triandra showed little evidence of acclimation. Growth responses closely mirrored these physiological patterns, occurring only in species exhibiting coordinated biochemical adjustment. These results indicate that C₄ responses to eCO₂ cannot be explained solely by stomatal water saving and provide evidence that species-specific changes in apparent photosynthetic capacity may contribute to sustained carbon gain, particularly under water limitation.