Abstract: Cotton is highly sensitive to temperature. High temperature usually causes the decline of net photosynthetic rate, resulting in cotton yield reduction at the boll stage. The experiment was carried out, using cotton cultivars Simian 3 and Siza 3, with similar genetic background in the half-open-top greenhouse in Pailou experiment station of Nanjing Agricultural University (118º50′E, 32º02′N). The results indicated that under the 2–3°C of increased canopy temperature (average boll period temperature was 33.5–35.2°C), the chlorophyll SPAD readings and net photosynthetic rate (P_n) of the functional leaf on the main stems reduced, while the stomatal conductance (G_s), intercellular CO₂ concentration (C_i) and transpiration rate (T_r) increased. Soluble protein content increased significantly while soluble sugar content, soluble amino acid content and C/N decreased. Malondialdehyde (MDA) content in cotton leaves increased, whereas the activities of peroxidase (POD) and catalase (CAT) of cotton leaves decreased. The variation ranges of related indices for Siza 3 were less than those for Simian 3. The results suggested that in the 2–3°C increased canopy temperature (average boll period temperature was 33.5–35.2°C) condition, the cotton plant suffered from serious temperature stress, then the plant photosynthesis was inhibited, the carbohydrates supplementary capacity was decreased in the leaf subtending the boll, and declination of the dry matter accumulation rate reached 20%. The cotton plant also suffered water deficiency stress in leaf but the reduction of the photosynthesis rate was mainly affected by non-stomatic restrictions, and it might be caused by the damage of cell membrane. The tolerance to the increased temperature is higher in Siza 3 than Simian 3.

Key words: Cotton, Boll period, Increased canopy temperature, Functional leaf, Leaf subtending the boll