Effect of Combined Application of Nitrogen and Phosphorus on Photosynthesis Parameters at Grain-filling Stage and Grain Yield in Winter Wheat Effect of plastic film mulching on photosynthetic and fluorescence characteristics of corn leaf under different fertilization schemes at jointing stage Effect of exogenous carbon monoxide donor Hematin on photosynthesis parameters and chlorophyll fluorescence characteristics of Coptis chinensis seedlings under NaCl stress