| Discussion This investigation indicates that allelic variation probably exists at the Kr1 and Kr2 loci of the non-crossable varieties, Hope and Cappelle-Desprez. The similar study of FALK & KASHA (1983) using the group 5 chromosome substitution lines of Hope, Atlas 66 and Cheyenne into Chinese Spring also showed that the chromosomes carrying crossability genes differed in potency depending on the donor variety (FALK & KASHA 1983). Although other modifier genes on the substituted chromosome could have led to variation in the expression of the Kr genes, FALK & KASHA (1983) interpreted the results as possible evidence for the existence of a multiple allelic series at the Kr loci and this is also suggested by the data described here. One method of overcoming the incompativility caused by the crossability genes is to backcross the recessive crossability gene, kr1, from a crossable variety such as Chinese Spring, into the non-crossable variety (SNAPE & SIMPSON 1980). If this technique is to be used, it is important to ascertain the degree of allelic variation at the crossability loci of the recipient variety. In the case of Cappelle-Desprez, for example, the dominant alleles Kr1 and Kr2 are equally effective and consequently the substitution of kr1 of Chinese Spring into Cappelle-Desprez would make Cappelle-Desprez crossable, but only at a very low level. From this study, the third Kr allele, Kr3, of both Hope and Cappelle-Desprez appeared to be incapable of significantly reducing the crossability of Chinese Spring. In other studies, Chinese Spring (Hope 5D) showed a slight but again statistically non-significant reduction in seed setting ability, relative to Chinese Spring, in pollinations with rye (RILEY & CHAPMAN 1967) and H. bulbosum (SNAPE et al. 1979). The absolute values of the crossability of the substitution lines for Hope chromosomes 5A, 5B and 5D with H. bulbosum vary between the studies made by FALK & KASHA (1983), by SNAPE et al. (1979) and that described here. The highest seed set values were obtained by FALK and KASHA (1983) of 28, 4 and 50% for the three substitution lines respectively. The results obtained here were slightly lower; being 15.3, 2.5 and 29.9% seed set respectively. The lowest levels of seed set were obtained by SNAPE et al. (1979), of 4.9, 0.0 and 9.8%. These differences may reflect differences in the presence or absence of post-pollination applications of gibberellic acid (GA) or other environmental influences. In both the present investigation and in that of FALK & KASHA (1983) post-pollination GA applications were made one day after pollination and for three consecutive days after pollination respectively. The higher seed sets obtained by FALK & KASHA (1983) may reflect the greater number of GA applications. The lower seed sets obtained by SNAPE et al. (1979) may result from the lack of a post-pollination GA application, since previous investigations have shown that the application of GA has a stimulatory effect on pollen tube growth (LARTER & CHAUBEY 1965), the frequency of fertilization (SITCH & SNAPE 1986) and seed set (SITCH 1984). |
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