Temporal Variation in the Characteristics of Gas Exchange and Chlorophyll Fluorescence in Lucerne A regional test of alfalfa varieties in the Longdong area Analysis of Longdong Wild Alfalfa Based on Peroxidase Isozymes Zymogram Genetic specificity in Native Medicago sativa L. cv. Longdong Spatial-temporal distribution of the ecological service value and NPP of cultivated land on Longdong Loess Plateau