Seventeen winter (9 red and 8 white) and 13 spring (8 red and 5 white) wheat cultivars and breeder's lines were used. As listed in Table 1, they have different PHS phenotypes when grown in Hokkaido (Kuwabara et a!. 1996). For example, Lancer, RL4137, and Satanta are classified as PHS-tolerant, whereas Chihokukomugi, Verry S, and Haruyutaka are classified as PHS-susceptible. The materials were grown in the standard manner at the Memuro test field, Hokkaido National Agricultural Experiment Station. The winter wheats were sown in middle September, 1998, and the spring wheats, in late April, 1999.

Spikes of these cultivars were sampled at two grain filling stages: (1) the dough-ripe stage: 35-40% water content, with physiological maturity; and (2) full-ripe stage, when the water content is 25-30%, which occurred 7 days after the dough-ripe stage. To identify grain developmental stages, the classification system of Noda et al. (1994) and Kuwabara et al. (1996) was used, and the water content was measured from 30 days post anthesis (DPA) to 45 DPA. Collected spikes were dried in a forced-air oven at 35°C for 2 days and water content was reduced to less than 15%. Grains from primary and secondary florets of the central spikelets of spikes were then gently hand -threshed and stored in a freezer at -20°C, a storage temperature that is known to maintain the dormancy level (Mares 1983). Germination tests were carried out within 2 months of harvest .

Grains were surface-sterilized in sodium hypochlorite (5% available chlorine) for 20 mm and immediately washed several times with distilled water. Duplicates of 50 grains were placed on sterilized filter papers wetted with 6 ml of distilled water or 50 muM ABA in Petri dishes. The dishes were incubated at two different temperatures, 12°C and 20°C, for 7 days in the dark and the number of germinated grains was scored. Following Mares (1984), germination was defined as occurring with the appearance of a 2 mm shoot and 3 distinct seminal roots. The incubation temperatures chosen in this experiment are representative to either keep grain dormancy or break it (Bewley and Black 1994), and the lowest incubation temperature is almost equal to the minimum summer temperature in the major wheat production areas of Japan.

Results and discussion

Germination percentages of grains collected at the dough-ripe and full-ripe stages and imbibed at 20°C are given in Table 1. PHS tolerance was evaluated using a rain simulator for three seasons (Kuwabara et al. 1996). Germination test at 20°C of the grains of the dough-ripe stage indicated that PHS-tolerant cultivars showed lower level of germination than the susceptible cultivars. At the full-ripe stage the germination percentages increased in almost all the cultivars. However, most of the PHS-tolerant cultivars still showed a lower percentage of germination than the susceptible cultivars, although some of the tolerant cultivars, such as Norin 8, Norin 17, Retcital and CI-1, germinated as well as susceptible cultivars. The results agreed with the finding by Kuwabara et al. (1996), which indicated that the old cultivars, such as Norin 8, showed a high percentage of sprouting at the full-ripe stage, while the new cultivars, such as Taisetsukomugi, maintained high level of tolerance to sprouting at the full-ripe stage as well as the dough -ripe stage.

Germination percentages of the grains imbibed at 12°C are also shown in Table 1. The cultivar variation in germination percentage was smaller at 12°C than in 20°C not only at the full-ripe stage but also at the dough-ripe stage. The germination percentages of many PHS-tolerant cultivars at 12°C were as high as those of susceptible cultivars. This tendency was strong at the full-ripe stage. Several PHS-tolerant cultivars, Clarks Cream, RL4137, Zenkoujikomugi, and AUS 1408, however, still maintained a relatively low germination percentage at 12°C compared with other cultivars at the full-ripe and dough-ripe stage.