Photosynthetic Dynamic Variation of Switchgrass under Different Row Spacing in Loess Hilly gully Region Genetic diversity of Switchgrass(Panicum virgatum L. ) germplasms detected by SCoT Markers Effect of the pH value on switchgrass seedling growth and development in hydroponics Effect of Planting Row Distance and Nitrogen Rate on Switchgrass Yield and the Content of Crude Protein