Doubled haploid (DH) lines together with a cotyledon bioassay were employed for the molecular analysis of resistance to the blackleg fungus Leptosphaeria maculans in the Australian Brassica napus cultivars Shiralee and Maluka. We used bulked segregant analysis to identify 13 RAPD and two RFLP markers linked to the resistance phenotype and mapped these markers in the segregating DH population. Our data suggest the presence of a single major locus controlling resistance in the cultivar Shiralee, confirming our previous results obtained from Mendelian genetic analyses. In addition, preliminary mapping data for the cultivar Maluka also support a single locus model for resistance and indicate that the resistance genes from 'Shiralee' and 'Maluka' are either linked or possibly identical. The molecular markers identified in this study should be a useful tool for breeding blackleg resistant varieties using marker-assisted selection, and are the essential first step towards the map-based cloning of this resistance gene.

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The effects of position and age of leaves on CO2 exchange rate (CER) are described for a single-cross corn (Zea mays L.) hybrid ('Harrow 691') grown at 10-h and 20-h photoperiods. The effect of leaf age is also described for barren plants grown at a 10-h photoperiod.CER of newly matured leaves increased from leaf 3 to leaf 6 (10 h) or 8 (20 h). The rates were not significantly different for leaves 6 to 13, but were lower for leaf 14, at 10 h; while the rate for leaf 10 was lower than for leaf 8 but not different from that for leaves 11–15, at 20 h.CER declined with leaf age, but the rate of decline was reduced after pollination at both 10 h and 20 h. The stomatal resistance changed little for a period of 4 to 5 weeks following silking. The decline in CER of all leaves studied for barren plants was smooth, with the rate being unaffected in the postsilking period; in these plants changes in stomatal resistance closely reflected the decrease in photosynthetic rates.The results emphasize that the CER of newly matured leaves was lower for leaves produced in the early stages of ontogeny than for those maturing later, and that the pattern of decline with age in photosynthetic activity varied considerably amongst those leaves that would have been contributing assimilates to the developing ear.

InterspeciWc hybrids were produced from reciprocal crosses between Brassica napus (2n = 38, AACC) and B. oleracea var. alboglabra (2n = 18, CC) to introgress the zero-erucic acid alleles from B. napus into B. oleracea. The ovule culture embryo rescue technique was applied for production of F 1 plants. The eVects of silique age, as measured by days after pollination (DAP), and growth condition (temperature) on the eYciency of this technique was investigated. The greatest numbers of hybrids per pollination were produced under 20°/15°C (day/night) at 16 DAP for B. oleracea ($) £ B. napus crosses, while under 15°/10°C at 14 DAP for B. napus ($) £ B. oleracea crosses. Application of the ovule culture technique also increased the eYciency of BC 1 (F 1 £ B. oleracea) hybrid production by 10-fold over in vivo seed set. The segregation of erucic acid alleles in the selfpollinated backcross generation, i.e. in BC 1 S 1 seeds, revealed that the gametes of the F 1 and BC 1 plants carrying a greater number of A-genome chromosomes were more viable. This resulted in a signiWcantly greater number of intermediate and a smaller number of high-erucic acid BC 1 S 1 seeds.

The Brassica B-genome species possess many valuable agronomic and disease resistance traits. To transfer traits from the B genome of B. carinata into B. napus, an interspecific cross between B. napus and B. carinata was performed and a doubled haploid (DH) population was generated from the BC2S3 generation. Successful production of interspecific DH lines as identified using B-genome microsatellite markers is reported. Five percent of DH lines carry either intact B-genome chromosomes or chromosomes that have deletions. All of the DH lines have linkage group J13/B7 in common. This was further confirmed using B. nigra genomic DNA in a fluorescent in situ hybridization assay where the B-genome chromosomes were visualized and distinguished from the A- and C-genome chromosomes. The 60 DH lines were also evaluated for morphological traits in the field for two seasons and were tested for resistance to blackleg, caused by Leptosphaeria maculans, under greenhouse conditions. Variation in the DH population followed a normal distribution for several agronomic traits and response to blackleg. The lines with B-genome chromosomes were significantly different (p < 0.01) from the lines without B-genome chromosomes for both morphological and seed quality traits such as days to flowering, days to maturity, and erucic acid content.