A complete set of disomic substitution lines have been developed in the tetraploid wheat cultivar Langdon (Triticum turgidum L. var. durum). These aneuploid lines each have a pair of durum wheat homoeologues replaced by a pair of D-genome chromosomes transferred from 'Chinese Spring' hexaploid wheat. They can be used to determine the chromosomal location of genes, to transfer chromosomes from one cultivar or line of tetraploid wheat to another, to study the cytogenetics of tetraploid wheat, to determine gene linkages, and to identify chromosomes involved in translocations. Their phenotypic characteristics, their cytogenetic behavior, and suggested methods for their use are described.Key words: cytogenetics, monosomic, chromosome transmission, telosomic, chromosome substitution.

A genetic suppressor of resistance to stem rust, Puccinia graminis Pers.:Pers. f. sp. tritici Eriks. & E. Henn., of wheat (Triticum aestivum L.) is on the long arm of chromosome 7D. Mutation or inactivation of the suppressor gene would allow expression of the suppressed resistance genes already present in hexaploid wheat or of resistance genes transferred to wheat from diploid or tetraploid relatives. Our objective was to induce a mutation of the suppressor gene and describe its inheritance. Seed treatment of the hexaploid wheat cultivar Canthatch (CTH) with ethyl methanesulfonate produced 15 stem rust‐resistant mutants. The mutant lines were tested for reaction to 13 races of stem rust, and crossed with CTH and 7D aneuploids of CTH and ‘Chinese Spring’ (CS). Parental, F1, F2, and F3 seedlings were tested for reaction to race CRL‐QSH of stem rust. Similarity among the 15 mutants in reactions to 13 races of stem rust indicated that each of the mutants had a similar genetic alteration resulting in an inactivated or deleted suppressor gene. The suppressor gene mutations showed incomplete dominance and monogenic inheritance in crosses with CTH. One mutant was ditelosomic 7DS and lacked the suppressor gene. Allelism tests indicated that the mutated gene(s) in the other 14 mutants were at or near the same locus on 7DL. Crosses between the mutants and CS monosomic 7D or CS ditelosomic 7DS indicated that CTH may have three recessive genes for resistance that were inhibited by the suppressor. The mutant lines may permit the use of the suppressed genes for stem rust resistance in hexaploid wheat and related species to breed hexaploid wheat cultivars with improved resistance.

Each year the greenbug (Schizaphis graminum Rondani) causes losses in wheat production having an estimated value of several million dollars. A new source of resistance to greenbug toxin has been found in PI 268210 (Triticum tauschii (Coss.) Schmal 2n=14). The cultivar ‘Langdon’ (T. turgidum L. durum group 2n=28) was used as female in a cross with PI 268210, and an amphiploid with 2n=42 chromosomes was obtained. The amphiploid was crossed with two hexaploid wheat lines, Kt‐C and ‘Little Club’*9/WRT238 (T. aestivum L. aestivum group 2n=42). F2 segregations indicated that resistance to greenbug toxin in the amphiploid was conditioned by a single dominant gene. Chromosome pairing between the D‐genome chromosomes of the amphiploid and the D‐genome chromosomes of the T. aestivum lines appeared to be normal and chromosomal interchanges were not detected.

A non-lethal chlorophyll deficient mutation was induced by use of the chemical mutagen ethyl methanesulfonate. Chloroplasts from the control and mutant plants were found to be very similar ultrastructurally. Thylakoid membrane volume was only slightly greater in plastids from the control as compared with plastids from the mutant. The chlorophyll content of the mutant was reduced by over 60%. This decrease in chlorophyll was not accompanied by a similar decrease in electron transport. Uncoupled electron transport rate based on a unit chlorophyll basis was nearly twice as great for mutant chloroplasts as for control plastids. However, electron transport rate based on a unit membrane volume was similar in mutant and control plants. At high irradiance the relative quantum requirement of the control and mutant was similar when expressed on membrane volume.

The supernumerary spikelet (SS) trait of durum wheat (Triticum turgidum L.), including the ramified and four-rowed spike traits, is characterized by an increased number of spikelets per spike. Chromosomal location of the SS gene(s) was determined by crossing the ramified spike line PI349056 to the set of 'Langdon' D-genome disomic substitution lines. Double monosomic F1 plants were backcrossed to PI349056 and the testcross F1 plants were classified for chromosome pairing and spike type. Segregation for spike type was observed in the testcross F2. Data indicated that the major SS gene was located on chromosome 2A. Subsequent crosses with the 'Langdon' 2A telosomics indicated that the major SS gene was located on the short arm of chromosome 2A. Segregation of the testcross F2 indicated that a minor SS gene was located on chromosome 2B. Results also indicated that inhibitors of SS may be located on the D-genome chromosomes and an additional experiment was designed to test this hypothesis. Eight D-genome monosomic addition lines were developed by backcrossing PI349056 from one to three times to plants containing D-genome univalents. The test populations contained two cytological types of plants, disomics having 14 pairs of durum chromosomes and D-genome monosomic additions having 14 pairs of durum chromosomes plus a D-genome monosome. Comparison of these two types of plants indicated that chromosome 2D (from 'Chinese Spring' wheat) had a strong inhibitor of SS expression.Key words: Triticum, branched spike, ramified spike, four-rowed spike, cytogenetics.