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These results agreed with the finding by George (1967), who reported that mature grains of several varieties in the U.S. showed dormancy at 30°C but not at around 10°C. The optimum temperature for germination increased after ripeness. This was confirmed in the germination percentages in grains imbibed at 20°C in which PHS-tolerant cultivars did not germinate until the full-ripe stage (Table 1). However, the degree of dormancy would be dependent on the temperatures during grain development, especially near maturity. Among the PHS-tolerant cultivars, genetic variation would appear as a result of the sensitivity to low temperature, as simulated in the 12°C incubation experiment (Table 1). If grains matured at low temperature and high humidity, the development of grain dormancy might reduce at both the dough-ripe and full-ripe stages.

Fig. 1 shows the effects of ABA treatment on the germination of the grains imbibed at 20°C at the dough-ripe and full-ripe stages. The germination was completely inhibited by ABA in most of the cultivars at the dough-ripe stage. The grains at the full-ripe stage were less sensitive to the inhibition effect of ABA, which was especially notable in the PHS -susceptible cultivars. For example, the PHS -susceptible Chihokukomugi germinated in ABA solution. By contrast, the germination of almost all the PHS-tolerant cultivars was still inhibited by ABA at the full-ripe stage.

Fig. 2 shows the effects of ABA treatment on the germination of the grains imbibed at 12°C at the dough-ripe and full-ripe stages. The inhibition effect of ABA on germination decreased with an increased in the germination percentage in water, and there were significant correlations between the germination in water and ABA solution (r= 0.607, P is less than 0.01, in the winter wheat; r=0.901, P is less than 0.01, in spring wheat at the dough-ripe stage, r= 0.795, P is less than 0.01, in the winter wheat; r=0.667, P is less than 0.01, in the spring wheat at the full-ripe stage; Figs. 1 and 2).

Most PHS-susceptible cultivars germinated in the ABA solution as well as in water, whereas the tolerant cultivars, which germinated in water, were still inhibited by ABA. This result is in agreement with finding by Noda and Kanzaki (1988) who reported that the embryo sensitivity to the ABA of PHS-tolerant cultivars is maintained even after its dormancy disappears and can be estimated by applying ABA to whole grains that nearly lack dormancy. However, some of the PHS-tolerant cultivars germinated in ABA solution as well as in water, whereas most of the tolerant cultivars maintained low germinability in ABA solution. This was observed in the germination percentages of PHS-tolerant Lancer and RIA137 with only 20-30% germination, while Satanta and Norin61 were up to 75-100% at both grain-filling stages (Fig. 2).

In the present study, grain germinability and the inhibition effect of ABA on germination were evaluated at different incubation temperatures to determine genetic differences for low temperature germinability. When grains harvested at the dough-ripe stage, corresponding to physiological maturity, were incubated at high and low temperatures, most of the PHS-tolerant cultivars retained lower germinability in water and maintained ABA sensitivity at both temperatures. However, several PHS-tolerant cultivars germinated in ABA solution as well as in water at a low temperature.

As mentioned earlier, Osanai and Amano (1993)demonstrated the effectiveness of recurrent selection for low temperature germinability. Their genetic lines, designated 'OS' and 'OW', expressed more potent tolerance to PHS than the most tolerant cultivars in this experiment (Osanai and Amano 1996, 1999). Those lines received attention as breeding materials to develop cultivars that are more tolerant to PHS than Lancer and Kitakei 1354 because, when considering production under cool moist conditions at maturity in northern Japan, especially in Hokkaido, a high level of grain dormancy is desirable to prevent PHS damage.

For the enhancement of PHS-tolerance in wheat, it would be required to increase ABA sensitivity at lower temperatures during grain development. As revealed in Figs. 1 and 2, a strong correlation between low temperature germinability and ABA sensitivity was detected in this experiment. Hence we propose that it would be significant to improve sprouting tolerance with ABA sensitivity at low temperatures.

Acknowledgments

We thank Drs. Y. Amano and H. Miura for their critical reading of the manuscript and many comments.

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