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 C 4 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 CO 2 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% CO 2 did not prevent the decline of O 2 evolution rates under water stress. "¢ We conclude that inhibition of photosynthesis in the four C 4 grasses under water stress is dependent mainly on biochemical limitations.