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

Summary • 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 CO₂ 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 CO₂ 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. [ABSTRACT FROM AUTHOR]