Abstract: This research aimed to investigate the photosynthetic responses of Cynodon dactylon and Hemarthria altissima under different flooding conditions and to identify a suitable planting pattern. On April 29, 2016, a pot experiment was set up with a replacement series design in which H. altissima (H) and C. dactylon (C) were planted at seven ratios (H₀C₁₂, H₂C₁₀, H₄C₈, H₆C₆, H₈C₄, H₁₀C₂, and H₁₂C₀) under three water treatments, including normal growth water condition (CK), flooding at 10 cm above the soil surface (SF) and flooding at 2 m above the soil surface (TF). The flooding and density treatments had significant (P<0.05) effects on net photosynthetic rate (P_n) of H. altissima and C. dactylon. Under the CK water treatment, the P_n of H. altissima with the H₂C₁₀ ratio was greater than that with the monoculture, and the P_n of C. dactylon significantly decreased under the mixed intercropping conditions when compared with that under the monoculture conditions (P<0.05). Under the SF treatment, the P_n of H. altissima with the H₈C₄ planting ratio was greater than that at other planting densities, while the P_n of C. dactylon in monoculture was lower than that achieved under mixed intercropping (P<0.05). Compared with the CK treatment, the P_n of H. altissima significantly decreased (P<0.05) under the TF water treatment regime. Taking photosynthetic efficiency of the two species into consideration, the results suggested that a mixed intercropping pattern with a planting ratio of H₂C₁₀ should be adopted in high-altitude areas where no water flooding occurs, and the H₈C₄ ratio should be adopted in areas where the soil surface is flooded. The monoculture of H₁2C₀ is the optimal choice for low-altitude areas where complete flooding often takes place.

Key words: Hemarthria altissima, Cynodon dactylon, photosynthetic characteristics, water stress, density