The amylose content of the endosperm is a major factor affecting the stickiness of boiled rice. The response of the amylose content of low-amylose lines derived from several mutants to temperature during the ripening period was investigated. The amylose content decreased from the low temperature treatment to the high temperature treatment in each line or cultivar examined. The amylose content of the nonwaxy cultivar “Tsugaru-roman” ranged from 12% (high temperature treatment) to 23% (low temperature treatment). The ratio of the decrease of the amylose content to the increase of the temperature during the ripening period (Δ AM/ °C) ranged from 0.8%/°C to 1.1%/°C in “Tsugaru-roman”. The progeny lines of “Milky-queen” (harboring the wx-mq gene) and “Yamagata 84” (harboring the wx-y gene) showed lower values of Δ AM/°C, whereas the progeny lines of “74wx2N-1” displayed remarkable changes in the amylose content, compared to the nonwaxy cultivars. “Fukei 191”, from “NM391” (harboring the du(t) gene), showed a smaller decrease in the amylose content in 2001 and remarkable changes in the amylose content similar to those observed in the “74wx2N-1” progenies in 2002. The amylose content of “Tankei 2031” (low-amylose gene has not yet been identified) was about 3% lower than that of the nonwaxy cultivar in each treatment. The value was comparatively higher than that of other low-amylose lines. These varietal differences in the response of the amylose content to the temperature during the ripening period are considered to be controlled by the low-amylose genes harbored by each line.

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This study was conducted to elucidate the mode of inheritance of the occurrence of white-back and basal-white kernels associated with high temperature during the ripening period in rice. F₁ to F₃ generations from a cross between “Chiyonishiki”, which is sensitive to high temperature with a high incidence of the two types of such defective kernels, and “Koshijiwase”, which is resistant to high temperature with a low incidence of defects, were used as materials. The plants were exposed to high temperatures in a greenhouse during the period from heading to maturity. The number of heat-damaged kernels was counted to obtain the incidence rates of white-back and basal-white kernels for each plant. The incidence rates revealed differences in the model of inheritance between the two types of defective kernels, for example, low-incidence was dominant over high-incidence for the incidence of white-back kernels, while no dominance was found between low and high incidence for basal-white kernels. The values of heritability in a narrow sense for the incidence of white-back kernels and basal-white kernels were 0.350 and 0.259, respectively, in the F₂ generation. The values of heritability in a true sense for the incidence of white-back kernels and basal-white kernels were 0.536 and 0.411, respectively. Selection in later generations is recommended for the breeding of a variety with a low incidence of defective kernels even at high temperatures. Moreover, the incidence of white-back kernels showed a positive genetic correlation with that of basal-white kernels. Therefore the incidence of both types of defective kernels could be reduced concurrently. The incidence of white-back kernels failed to show a significant correlation with the thousand kernel weight, kernel length and dorsiventral diameter while that of basal-white kernels showed a negative genetic correlation with the kernel length and dorsiventral diameter. These results indicate that selection for reducing the deterioration of the grain quality and the weight decrease at high temperatures during the ripening period could be achieved.

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The colorimetric method assay for the trypsin inhibitor activity (TIA), using porcine pancreatic trypsin as enzyme and BAPA (Benzoyl-D, L-arginine-p-nitroanilide) as substrate was modified to become suitable for microplate analysis. Our method allows sweetpotato (Ipomoea batatas (L.) Lam) breeders to analyze rapidly a large number of samples using a smaller sample size. In the present study, this method was applied to determine the TIA of sweetpotato genotypes. The coefficient of determination (R²) of TIA with the partially purified trypsin inhibitor (SPTI) and the commercially available soybean trypsin inhibitor in a range of 0-1 μg was 0.974 and 0.999, respectively. A single microplate required approximately 30 min for sample preparation and 90 min for analysis. Sweetpotato roots of 8 cultivars and 199 breeding lines at the initial stages of a breeding program were analyzed for TIA. The TIA values of 8 cultivars ranged for 65 to 392 U/mg DW with an average value of 197 U/mg DW. On the other hand, The TIA values of the 199 lines ranged from 38 to 944 U/mg DW with an average value of 273 U/mg DW. These data revealed that TIA was considerably higher in the breeding lines than in the cultivars tested. In conclusion, the TIA range of the breeding lines was relatively wide and in some of the lines with a higher TIA value the threshold level may exceed that for pigs estimated by the conversion value of TIA to the amount of soybean trypsin inhibitor.

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