Nonstomatal limitations are responsible for drought-induced photosynthetic inhibition in four C₄ grasses

• Here, the contribution of stomatal and nonstomatal factors to photosynthetic inhibition under water stress in four tropical C₄ grasses was investigated (Panicum coloratum, Bothriochloa bladhii, Cenchrus ciliaris and Astrebla lappacea). • Plants were grown in well watered soil, and then the effects of soil drying were measured on leaf gas exchange, chlorophyll a fluorescence and water relations. • During the drying cycle, leaf water potential (Ψ_leaf) and relative water content (RWC) decreased from c. -0.4 to -2.8 MPa and 100-40%, respectively. The CO ₂ assimilation rates (A) and quantum yield of PSII (Φ _PSII) of all four grasses decreased rapidly with declining RWC. High CO₂ concentration (2500 μl l^-1) had no effect on A or Φ_PSII at any stage of the drying cycle. Electron transport capacity and dark respiration rates were unaltered by drought. The CO ₂ compensation concentrations of P. coloratum and C. ciliaris rose sharply when leaf RWC fell below 70%. In P. coloratum, 5% CO₂ did not prevent the decline of O₂ evolution rates under water stress. • We conclude that inhibition of photosynthesis in the four C₄ grasses under water stress is dependent mainly on biochemical limitations.