Abstract: The flowering and boll-forming stage is the key yield determinant period of upland cotton. Short-duration water stress occurring during this stage significantly reduced cotton development and final productivity. Nitrogen application plays an important role in alleviating the adverse effects of dry soil on plant development. In order to explore effects of nitrogen on cotton photosynthesis under drought stress. The photosynthesis and chlorophyll fluorescence characteristics of cotton with different nitrogen application were examined under developing gradually drought stress during flowering and boll-forming stage. Leaf water potential, soil relative water content, net photosynthetic rate (P_n), stomatal conductance (G_s), and intercellular CO₂ concentration (C_i) were reduced, but the pigment contents of cotton leaf were increased under soil drought stress. The reduced degree of P_n was increased with increasing nitrogen levels. Under drought stress, P_n, G_s, C_i, Chl a content, Chl b content, Chl a+b content. and Car content in 240 kg N ha^-1 treatment was the highest among the three nitrogen levels. Drought stress caused a significant decline in the maximum photochemical efficiency of photosystem II (F_v/F_m), the quantum yield of electron transport (ΦPS II), the electron transport rate (ETR) and the photochemical quenching co-efficient (qP). These changes were accompanied by an increase in both the minimal fluorescence (Fo) and the non-photochemical quenching co-efficient (NPQ). F_v/F_m, ΦPS II, ETR, and qP were higher at 240 kg N ha^-1 level than at 0 and 480 kg N ha^-1 levels. The transpiration (Tr) of cotton was reduced and the temperature of leaf was enhanced under drought. The dry matter of cotton decreased under drought. Nitrogen application increased dry matter weight and augmented the water stress indexes. All results suggest that 240 kg N ha^-1 is the optimal nitrogen application rate under drought, deficient (0 kg N ^ha-1) and excessive (480 kg N ^ha-1) nitrogen application are of disadvantage to photosynthesis and chlorophyll fluorescence characteristics.

Key words: Cotton, Drought, Nitrogen levels, Photosynthesis, Chlorophyll fluorescence, Yield