Effects of rudimental stubble heights on the yield and source-sink characteristics of ratooning rice:a case study for two-line hybrid rice Peiliangyou 210 STUDY ON THE DAMAGE AND ACTION THRESHOLD OF WHITE-BACKED PLANTHOPPER TO RATOONING RICE Differences in Characteristics of Source,Sink,and Flow between Ratooning Rice and Its Same-Term Heading Main-Crop Rice Achievements and Developments of Ratooning Rice in South of China