Two atrazine-tolerant barley mutants were isolated from atrazine-selection experiments performed on barley chloroplast-mutator plants. Genetic analysis demonstrated that atrazine tolerance was maternally inherited. Molecular characterization of the mutants was performed by PCR amplification of an internal fragment of the chloroplast gene psbA. The BstXI restriction patterns of the amplified fragments showed two bands in both tolerant barley mutants and only one in the atrazine-sensitive control. The 277-bp amplified fragments from the parental line and both atrazine-tolerant mutants were cloned and sequenced. Sequence analysis showed a single nucleotide substitution in both barley atrazine-tolerant mutants, i.e. A to G at the +790 position of the psbA gene-coding sequence. This point mutation corresponds to an amino-acid change of serine- to -glycine and creates a BstXI restriction site. Our results confirmed the conservative variability involved in atrazine tolerance which was previously reported for several other species. To our knowledge this is the first report on the obtention of atrazine-tolerant barley. This finding provides support to the hypothesis that, in addition to a wide variety of chlorophyll deficiencies, the barley chloroplast mutator genotype induces variability in other traits, which could include agronomically valuable mutants.

Monogenic lines derived by recombination from Buck Manantial wheat, a cultivar which has durable resistance, were used as hosts to detect Puccinia recondita tritici induced mutants for increased virulence. After treatments with ethyl methane sulphonate on clone 66 of P. recondita 9 types of mutants were obtained at approximate frequencies of 1 × 10−4 and host lines were grouped in 6 classes, No increase virulence was obtained against B. Manantial after 2 cycles of treatments, but different combinations of virulences were observed on monogenic lines derived from it. Simultaneity of occurrence of some mutational events suggests complexity of virulence genes in the pathogen. At least 4 genes for incompatibility are present in B. Manantial when confronted with clone 66 and 4 to 7 mutational points are recognized in the pathogen. The specific relationships tending to equate the number of genes in both organisms would not be a general rule. Durable resistance can be explained by a combination of several specific disease reaction genes for which the pathogen population has not been able to accumulate all the corresponding alleles for virulence.

Trisomics for chromosomes 1A and 6B of wheat backcrossed to euploids showed progeny with unexpected segregation frequencies (cytological and genetical) that can be attributed to preferential pairing between homologues. Recombination frequency between the centromere and a gene for mildew reaction of chromosome 1A showed an increase from the first to the second backcross that could be attributed to a positive correlation between homozygosity and the degree of pairing between homologous chromosomes. The use of wheat trisomics for genetic analysis is also discussed.Key words: preferential pairing, trisomic segregation, wheat cytogenetics, recombination frequency, pathogen reaction.

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