PLANT GENOTYPE, MICRONUTRIENT FERTILIZATION AND TAKE-ALL INFECTION INFLUENCE BACTERIAL-POPULATIONS IN THE RHIZOSPHERE OF WHEAT

The relationship between micronutrient efficiency of four wheat (Triti cum aestivum L.) genotypes, tolerance to take-all disease (caused by G aeumannomyces graminis (Sacc.) Arx and Olivier var. tritici Walker), a nd bacterial populations in the rhizosphere was tested in soil fertili zed differentially with Zn and Mn. Plant growth was reduced by Mn or Z n deficiency and also by take-all. There was an inverse relationship b etween micronutrient efficiency of wheat genotypes when grown in defic ient soils and the length of take-all lesions on roots (efficient geno types had shorter lesions than inefficient ones). In comparison to the rhizosphere of control plants of genotypes Aroona and C8MM receiving sufficient Mn and Zn, the total numbers of bacterial cfu (colony formi ng units) were greater in the rhizosphere of Zn-efficient genotype Aro ona under Zn deficiency and in Mn-efficient genotype C8MM under Mn def iciency. These effects were not observed in other genotypes. Take-all decreased the number of bacterial cfu in the rhizosphere of fully-fert ilized plants but not of those subjected to either Mn or Zn deficiency . In contrast, the Zn deficiency treatment acted synergistically with take-all to increase the number of fluorescent pseudomonads in the rhi zosphere. Although numbers of Mn-oxidising and Mn-reducing bacteria we re generally low, take-all disease increased the number of Mn reducers in the rhizosphere of Mn-efficient genotypes Aroona and C8MM. Under M n-deficiency conditions, the number of Mn reducers in the rhizosphere increased in Aroona but not in C8MM wheat. The results suggest that ba cterial microflora may play a role in the expression of Mn and Zn effi ciency and tolerance to take-all in some wheat genotypes.