Effect of Ratio Source to Sink on Percentage of Head Milled Rice and Chalky Rice of Combinations of Mid-season Hybrid Rice in the South-East Districts of Sichuan Province under High Temperature and Summer Drought THE EFFECTS OF CULTIVATION DENSITY ON THE PERCENT OF HEAD MILLED RICE AND SOURCE TO SINK RATIOS OF MID_SEASON HYBRID RICE IN EASTERN AND SOUTHERN SICHUAN PROVINCE EFFECT OF SUPER SPARSE CULTIVATION ON HEAD MILLED RICE PERCENTAGE AND CHALKINESS IN HYBRID RICE VARIETIES IN THE EASTERN AND SOUTHERN DISTRICTS OF SICHUAN PROVINCE