Effects of Temperature during Ripening Period on Seed Coat Color of Azuki Bean

Abstract

Seed coat color is an important character of azuki bean appearance as is seed size. Two experiments were conducted to clarify the effects of temperature during the ripening period on seed coat color of an azuki bean cultivar, Erimoshouzu. Three kinds of pods in different ripening stages were treated with 28 combinations of two treatments; temperature (5, 15, 30, 40°C) and duration (2, 4, 6, 8, 10, 12, 14 days). Seed coat color was expressed by an XYZ color system. The effect of temperature on seed coat color was apparent in immature pods but was small in fully ripened pods. As temperature rose, the dominant wave length of hue increased, but the Y value decreased. Excitation purity of chroma in fully ripened pods decreased as temperature rose, but that in immature and partially ripened pods increased as the temperature rose up to 30°C, then decreased at 40°C. In a field experiment, the mean temperature, 10 days after the young pods became white in appearance, had a high positive correlation with a dominant wave length(r=0.913), and negative relationships with a Y value (r=-0.983) and excitation purity (r=-0.893). These results indicate that when the temperature during ripening period is high, seed coat color becomes darker since the dominant wave length becomes longer, and Y value and excitation purity become small. Estimation formulas for seed coat color at harvest time were proposed. y_D (dominant wave) = 0.7947x + 591.3, Y_Y (Y value) =-0.3452x + 12.64 and Y_E (excitation purity) =-0.5507x + 44.4, where x is the mean temperature (measured by using the average temperature of 30 years) for ten days after the young pods became white in appearance.