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The effect of different inoculum densities and of varying methods of seed inoculation on the ability of the maize rhizobacterial strain Pseudomonas cepacia 526 (ATCC 53267) to colonize and multiply in the maize rhizosphere was studied in greenhouse and field trials. The amount of inoculum used had a significant effect on both the colonization and spread of P. cepacia on the roots and rhizosphere. However, even an inoculum as small as 30 bacteria seed−1 resulted in 105 bacteria g−1 dry wt root after 2 weeks of plant growth in a montmorillonite clay vertisol soil from Darling Downs, Australia. After vigorous washing, root macerates still yielded P. cepacia indicating its close association to the roots. P. cepacia strain 64, which elicited high pectinase activity, had the capacity to penetrate maize root mucilage, while strains with low pectinase activity did not. The seed-inoculated bacteria spread rapidly to the newly-formed root surfaces of the seedling as well as on the adventitious (prop) roots in mature plants. The basal part of the root was colonized to a greater extent than the root tip region. Several field trials, conducted in the U.S.A. and in Australia using P. cepacia strains 526, 406 (ATCC 53266) and 64, revealed that they can colonize maize roots under different pedo-climatic conditions and the type of inoculum (liquid or a peat-based) had no significant effect on their root colonization. The results indicated that for maize cultivars resistant to stalk rot, bacterial colonization of roots was highly variable, but this was not so for susceptible cultivars. |
