Embryogenic Callus Formation and Plantlet Regeneration of Piragmites communis Photosynthesis and chlorophyⅡ fluorescence parameters of the reed (Phragmites communis)grown in the hydro-fluctuation belt of Three Gorges Reservoir Area Comparison of Plasma Membrane H+-ATPase Activity in Two Ecotypes of Reed (Phragmites communis) Leaves from Different Habitats