RELATIONSHIP BETWEEN SEEDLING EMERGENCE RATE AND EMBRYO WEIGHT OF RICE UNDER LOW-TEMPERATURE AND SUBMERGED SOIL CONDITION

Rapid seed germination and higher seedling emergence under low-tempera ture and submerged soil conditions are desirable to stabilize the dire ct seeding cultivation of rice in temperate regions. This study was co nducted to clarify the mechanism of relationship between embryo weight and seedling emergence rate in submerged soil at 18 degrees C. The re sults were asfollows: Sixty-five percent of 158 rice cultivars (Oryza sativa L.) showed a higher emergence percent than 25%. Rice cultivars, which did not reach 25% emergence were mainly indica and upland rice cultivars. The averagefrequency distribution of the emergence coeffici ent was 0.8 in indict and 5.5 in japonica. A wide range of variation f or embryo weight was observedin both groups. The range of embryo weigh t was 0.28-0.76 mg in indica and 0.41-0.81 mg in japonica. There was n o significant correlation between embryo weight and emergence coeffici ent for the 158 rice cultivars under submerged soil conditions at 18 d egrees C. The seedling emergence of cv. Norin 6 and cv. Joshu were goo d under submerged soil conditions at 18 degrees C,even though their em bryo weights were light. Small embryo size with better emergence could be a suitable condition for breeding a rice cultivar with better qual ity. Onthe other hand, the embryo weight of one cultivar, Koshihikari, which was harvested on variable planting dates and ratoon crop ranged from 0.53 to 0.71 mg. There was no significant correlation between em bryo weight and emergence coefficient of cv. Koshihikari. This result may indicate that emergence in submerged soil conditions at 18 degrees C is regulated by some factor related to early growth rate than embry o weight.