Studies on the Physical Characteristics of Rice Seedling : VI. Effects of Light Intensity and Air Temperature

To obtain suitable seedlings for mechanical transplanting, effects of light intensity and air temperature for raising seedlings were examined on the physical characteristics of the seedlings (i.e., buckling moment, buckling stress, section modulus of basal part, buckling index and natural deflection ratio of seedling). 1. Shading (40-70% of sun light intensity) had unfavourable effects on the physical characteristics of the seedling. Seedlings grown in shade were (1) thinner in basal section, (2) breakable in basal part, and had (3) lower resistance to buckling and bending, which were due to unbalanced top growth compared with undeveloped basal part. It is considered that the results mentioned above are because of the insufficient dry matter production caused by shading. 2. Two types were observed in the effects of air temperature to the physical characteristics of seedling. One was the indirect effect caused by growth difference, and another was the direct effect observed on the seedlings at the same leaf age. Remarkable difference was observed among the seedlings grown during same period under various air temperature conditions (day temperature 15-30°C). In the case of the seedlings grown during same period, gradual growth under lower temperature resulted in (1) a thinness in basal section and (2) a reduction in buckling resistance of basal part, but it resulted in also (3) an increment of structural rigidity in basal part and (4) higher resistance to buckling and bending, which were due to the well developed basal part with erect and short foliage. 3. In the case of the seedlings at the 4th leaf age, 24-30°C of day temperature (with 18°C of night temperature) was (1) thicker in basal section with (2) an increment in structural rigidity, and had (3) higher buckling resistance in basal part. This is due to the high dry matter production which is favourable under higher temperature condition than 20°C. The resistance to bending of seedling increased under lower temperature than 24°C, because of the well developed leaf sheath. Hence, rather lower temperature than 24°C might be adaptable for general improvement of the physical characteristics of seedling. 4. The suitable light intensity and air temperature for the physical characteristics were also favourable for raising seedlings with good physiological characteristics. By settlement of the suitable environment condition mentioned above, it is possible to obtain vigorous seedling suited for mechanical transplanting.