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

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Abstract
• 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 (Ψleaf ) 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 (ΦPSII ) of all four grasses decreased rapidly with declining RWC. High CO2 concentration (2500 µl l-1 ) had no effect on A or ΦPSII 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.