Abstract
The varietal differences and the interrelationships in transition of the characters related to lodging resistance after the heading were examined in direct sowing on well-drained paddy field. Four varieties differing in lodging resistance (i.e. Koshihikari, Yamakogane, No^-rin 25 and Kanto^- 90) were grown at two seeding densities. On the other hand, the effects of nitrogen top-dressing at full heading stage on lodging resistance were examined. 1. The decrease of lodging resistance after heading could bc attributed to the decrease in breaking strength and to the raise of the centroid height of the culm. 2. The decrease in the breaking strength after the heading had closer relationship to the decrease in the strength of leaf sheath than that of culm. 3. In the short-culmed variety, the contribution rate of leaf sheath to the breaking strength amounted to about 50% and this value maintained during about 30 days after the heading. However, in the long-culmed variety, thc rate was in the range of 30% to 40% and in lodging susceptible variety 'Koshihikari', the rate decreased rapidly after the heading. 4. The relative amount of starch in the breaking internode (the lowest internode over 6 cm long) decreased rapidly after the heading in thc lodging susceptible variety. Positive correlation coefficient was found between the amount of starch in the culm and the number of living leaf sheath wrapping the internode. 5. After heading, the number of living leaf and leaf sheath in the long-culmed variety were much the same to those in the short-culmed variety. However, the breaking internode of the long-culmed variety was one internode lower than that of the short-culmed variety, therefore, both the contribution rate of leaf sheath to the breaking strength and the starch amount of the culm in the long-culmed variety were less than those in the short-culmed variety. 6. By the top-dressing of nitrogen at full heading stage, the leaves and leaf sheathes of the lower internode were made alive longer. Consequently, it is considered that this treatment showed positive effect to lodging resistance through thc maintenance of breaking strength.
