The effect ofAzospirillum brasilenseand auxin on root morphology in seedlings ofSorghum bicolor × Sorghum sudanense

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Abstract
The purpose of this work was to examine the effect of Azospirillum brasilense inoculation on root development in the hybrid Sorghum bicolor × Sorghum sudanense, and the possible involvement of auxin in this effect. Scanning electron photographs of inoculated roots revealed an increase in root diameter and in the density and length of root hairs. Inoculation of roots with 108 colony‐forming units (cfu) per mL caused asymmetric growth of the root tips and shortened the distance between the first root hair formed and the root tip. In non‐inoculated roots or roots inoculated with 105 cfu mL−1 the distance was longer. Experiments with suitable concentrations of indoleacetic acid produced changes in root morphology and root hair formation comparable to those produced by inoculation with Azospirillum. These results suggest a possible involvement of auxin in the effect of Azospirillum on root morphology. Root elongation and the rate of root elongation were inhibited by inoculation of germinated seeds with 108 cfu mL−4. IAA at concentration of 10−6 M also inhibited root elongation and the rate of root elongation. In both cases, inhibition began 24 h after tteatment. Maximum decrease in the rate of root elongation was attained after 60 h. Similar effects on root elongation were obtained with a nif ‐ mutant of Azospirillum, indicating that nitrogen fixation is not involved in this effect on root morphology. Azospirillum stimulated the appearance of lateral roots.

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