Dark panicle color and high panicle position increase spikelet temperature of rice (Oryza sativa L.)

Rice (Oryza sativa L.) quality and yield are degraded by high temperature, especially at the ripening stage after the heading of panicles. The effect is lethal when the panicle temperature (T_p) is excessively high; therefore, maintaining a low T_p is important to avoid deleterious impacts on the grains. Microclimatic factors and plant physiological elements determine the T_p. One determining factor is the color (or reflectance) of spikelets that constitute the panicle because it determines the absorption of shortwave radiation energy. An additional factor is the panicle position because it influences heat exchange by the wind and input energy from downward shortwave radiation. In this study, inter-strain differences in spikelet color and panicle height at heading were assessed. The T_p of strains differing in panicle color and panicle height were measured with thermocouples. In addition, to estimate the effect of each trait, we adopted a micrometeorological model. Panicle color was quantified using a hyperspectral sensor. Combining the spectral reflectance and spectral radiation, we assessed the effect of panicle color on T_p. The differences in panicle color and panicle position significantly affected T_p. The strain with a dark panicle had a maximum measured T_p about 1.8 °C higher than that of the strain with a light-colored panicle. The T_p of a strain with panicles at higher positions was up to 2.0 °C higher than that of a strain with panicles at lower positions. These relationships were consistent with the model estimates. When shortwave radiation was strong, the difference in T_p between strains showed a positive correlation, suggesting that the temperature difference was associated with shortwave radiation. Therefore, we concluded that rice strains with a brighter panicle color and low panicle position are less prone to deleterious impacts of high temperature because net radiation is reduced.