Abstract: The effects of water-retaining agent (60 kg·hm^-2) and nitrogen fertilizer (0, 225, and 450 kg·hm^-2) on the leaf photosynthetic characteristics, chlorophyll content, and water utilization of winter wheat at jointing and grain-filling stages were studied under field conditions. In all treatments, the net photosynthetic rate, stomata conductance, intercellular CO₂ concentration, water use efficiency, and chlorophyll content were greater at grain-filling stage than at jointing stage. Under nitrogen fertilization but without water-retaining agent application, the water use efficiency (WUE) of single leaf at jointing stage increased with increasing nitrogen fertilization rate, while the net photosynthetic rate, stomata conductance, intercellular CO₂ concentration, and transpiration rate decreased after an initial increase. The chlorophyll content was the highest under 225 kg·hm^-2 nitrogen fertilization. In the treatments of water-retaining agent application, the intercellular CO₂ concentration decreased with increasing nitrogen application rate, but the net photosynthetic rate, transpiration rate, and WUE increased. The application of water-retaining agent or its combination with nitrogen fertilization increased the chlorophyll content, but excessive nitrogen fertilization had lesser effects. At grain-filling stage, applying nitrogen fertilizer alone significantly increased the net photosynthetic rate and WUE, but decreased the stomata conductance, intercellular CO₂ concentration, and transpiration rate. The chlorophyll content increased with increasing nitrogen application rate. After applying water-retaining agent and with the increase of nitrogen fertilization rate, the photosynthetic rate and WUE decreased after an initial increase, while the intercellular CO₂ concentration and transpiration rate were in adverse but still lower than those without water-retaining agent application. The stomata conductance increased with increasing nitrogen fertilization rate. The chlorophyll content increased significantly under the application of water-retaining agent, but somewhat decreased under the combined application of water-retaining agent and nitrogen fertilizer. The application of both water-retaining agent and nitrogen fertilizer increased the 1000 grain mass, grain yield, and water production efficiency of winter whe at significantly, with the best effect in the treatment of water-retaining agent with 225 kg·hm^-2 nitrogen fertilization.

Key words: water-retaining agent, nitrogen, photosynthetic characteristics, chlorophyll, water use efficiency (WUE), sub-low temperature, melon, melatonin, nitrogen metabolism, osmotic adjustment substance.