1972 Volume 41 Issue 4 Pages 402-408
Under the rice cultivation, in which nitrogen supply to rice plants is restricted during the middle growth stage (the stage is from 70 to 90 in leaf number index), the leaf color of the plants turns into pale green at stage of about 25-30 days before heading, and at the same time, carbohydrates are much accumulated in rice plant as a result of the restricted nitrogen supply. Consequently, as reported in the previous paper, the low temperature resistance which is most sensible at the reduction division stage is considerably increased by the nitrogen restriction treatment. It order to ascertain whether the nitrogen restriction treatment in the middle growth stage increases the resistance of rice plants to some diseases and that to wind damages or not, the authors carried out on inoculation test of three kinds of diseases and a wind damage test. An inoculation test was carried out by following ways on the 15th day before heading: Blast; fungi were sprayed and incubated for 24 hrs. under the conditions of 30°C and 100% relative humidity. Sheath blight; a section of infected sheath was inserted between the sheath and the culm of the 4th leaf from above on each stem, and the inoculated plants were incubated under the same conditions as in blast. Bacterial leaf blight; a needle injection method was applied for all the uppermost fully expanded leaves. A wind damage test was conducted under the condition of wind velocity 8 m/s. for 5 hrs. at or just after heading stage in a wind tunnel. Results obtained may be summarized as follows. 1. The resistances to diseases. Blast; the number of infected leaves on the 14th day and 50th day after inoculation was much less on the plants on which nitrogen supply was restricted at the middle growth stage than on the control plants. bacterial leaf blight; the percentage of infected leaves to inoculated ones and the total length of lesions per infected leaf were also much less on nitrogen restricted plants than the control plants. Sheathe blight; similarly, the infection degree and development of lesions were much smaller on nitrogen restricted plants than the control plants. 2. The resistance to wind damage. Nitrogen restricted plants showed high resistance to wind damages, which could be expressed by the percentage of ripened grains, and the weight and thickness of grains. 3. From the above mentioned results, it can be deduced that the resistance to diseases at or after reduction division stage and the resistance to wind damage at or just after heading are increased by the restriction of nitrogen supply at the middle growth stage, and that the one reason why nitrogen restricted plants are highly resistant to some diseases and wind damages, can probably been ascribed to the accumulation of carbohydrates in plants as a result of nitrogen restriction.
