High Temperature Injury of Ripening in Rice Plant : IV. Effect of high temperature on ¹⁴C-assimilation and translocation at an early ripening period

Abstract

The effects of high temperature (day-night, 35-30°C) upon photosynthesis and assimilate translocation to panicle were investigated by application of ¹⁴C0₂ to the whole plant at several ripening stages. 1) Translocation rate of ¹⁴C-assimilates into panicle was very low at 25-2O°C, leaving much ¹⁴C in the straw. Rising night temperature to 25°(25-25°C) increased the rate equivalent to those at 35-35°C and 35-30°C. Therefore, a great difference was found in the translocation rate between daily mean temperature of 25°(25-25°C and 2325-20°C). 2) At high temperature, ¹⁴C assimilated at an early ripening stage was found one week later in grains as mueh as 80% of total shoot ¹⁴C, but that assimilated two weeks after flowering remained much in the straw with a decreased amount in the grains. At outdoor natural temperature, one the contrary, the percentage of ¹⁴C found in the grains progressively increased as the ripening stage at which ¹⁴C was assimilated advanced until two weeks after flowering. 3) At high temperature, ¹⁴C assimilated at an early ripening stage translocated uniformly to all grains, but that assimilated two weeks after flowering moved little or much in the grains of upper or lower part of a panicle, respectively. Under outdoor condition, ¹⁴C assimilated at an early ripening stage moved much to the upper grains with a little accumulation in the lower grains. Thereafter it moved equally to all grains and still accumulated in upper or early flowered grains even at two weeks after flowering. 4) Just after ¹⁴C₂ application for half an hour, 90% or 80% of total ¹⁴C assimilated were contained in ethanol-soluble fraction in leaf-blade or leaf-sheath plus stem, respectively. 0ne week after ¹⁴C₂ application, however, much of ¹⁴C was found in ethanol-insoluble fraction in straw, especially so in leaf-blade. This tendency was little changed by temperature or ripening stage. In the grains, less ¹⁴C was found in ethanol-insoluble fraction at high temperature than at normal temperature. When ¹⁴C was applied at a middle ripening stage, the less ¹⁴C accumulation was found in starch and protein fraction at high temperature as compared with that at normal temperature, especially so in the lower grains of a panicle. 5) There was little difference in apparent photosynthesis between the plants ripened at high and normal temperatures as long as being deduced from ¹⁴C assimilation