Ms. 2106 SHORT COMMUNICATION Changes in rhizosphere populations of selected physiological groups of bacteria related to substitution of specific pairs of c h r o m o s o m e s in spring wheat Summary Rhizosphere population characteristics of two cultivars of spring wheat (Tritieum aestivum L. emend Thell.), Rescue (R) and Cadet (C), and the related chromosome substitution lines, C-R5B and C-R5D, were investigated. R e p l a c e m e n t of the chromosome pair, 5 B, in Cadet with 5 B from Rescue made m a n y of the rhizosphere microbial characteristics of C-R5 B similar to or the same as those in the rhizosphere of the donor parent, Rescue. In contrast, substitution of the functionally related chromosome pair, 5D, did not cause marked changes in the rhizosphere microbial population, demonstrating the specificity of the plant's control over factors governing the rhizosphere microbial environment.

The modification of the genotype of the Cadet and Rescue cultivars of spring wheat (Triticum aestivum L. emend. Thell) by disomic chromosome substitution altered the amount of plant nitrogen derived from dinitrogen fixation by the associated bacterium in a phytotron experiment. With the exception of the C-R5B line, inoculation of the parent Cadet or its substitution lines with either the bacillus C-11-25 or Azospirillum brasilense increased plant dry matter and the total N yield. Rescue lines were unaffected by inoculation unless genotypically altered by substitution of the 5B or 5D chromosome from Cadet. Different substitution lines reacted uniquely to inoculation with the specific bacteria: C-R2A and R-C2D promoted greater dinitrogen fixation by A. brasilense; C-R5D, R-C5B, and R-C5D promoted greater dinitrogen fixation by the C-11-25 bacillus. Both bacteria had high and identical levels of dinitrogen fixation in association with the C-R2D line; neither bacterium fixed N when grown in association with the C-R5B, Rescue, or R-C2A lines. Although the ability of spring wheat to induce dinitrogen fixation in associated bacteria is influenced by chromosomes 5B (which controls root rot reaction) and 5D, it does not appear to be directly related to reaction to common root rot.

No abstract

I h e s of a variety of common \\heat, S-615, rno~~osornic for chrornoson~es 111 and S \ ' I of homoeologous group 3 had culn~s less solid in the top internode than normal S-615. Monosomics of homoeologous group 5, namely, V, IX, and XVIII, were less solid in the bottom four internodes than S-615. These live chromosomes carry genes for solid stem in this variety. Monosomics X I X , X X , and X X I , the D-genome chromosomes of homoeologous groups 6, 2, and 7 respecti\-ely, were more solid than the normal check in both top and lower internodes, indicating that the missing chromosomes carry genes for hollow stem. Chromosome X I I I , a I~ornoeologue of X X , which in Chinese Spring has a gene for hollow stem, does not affect the amount of pith in the culm of S-615.The concept of the culm phenotype in a given environment resulting from an interaction of genes promoting pith development and those opposing it makes it possible to reconcile results of genetic experiments on solid stem in wheat that previously appeared contradictory.'Manuscript

The vernalization responses of 'Rescue', 'Cadet', 'Cypress', and selected chromosome substitution lines derived from these cultivars were measured by comparing days to ear emergence of vernalized and unvernalized plants under 24- and 16-h photoperiods. The genotype of 'Cadet' appears to be Vrn1Vrni1, vrn4vrn4, vrnxvrnx (where vrnx is an unidentified locus), and of 'Rescue' vrn1vrn1, Vrn3Vrn3, Vrn4Vrn4. 'Cypress' appears to carry Vrn4 and one or both of vrn1, and vrn3. Although some minor photoperiod responses were observed, no loci involved in major photoperiod responses were detected on chromosomes 5A, 4B, and 5B in 'Rescue' or 'Cadet' or on chromosome 5B in 'Cypress'. However, the duration of the basic vegetative phase appears to be controlled by a locus or loci on chromosome 5B, and chromosomes 5A and 4B may be involved in minor photoperiodic responses.Key words: Triticum aestivum, vernalization, photoperiod responses.