Effect of Azospirillum brasilense inoculation on root morphology and respiration in tomato seedlings

Abstract

The level of Azospirillum brasilense strain Cd colonization in the rhizosphere of some vegetables was 104–105 colony-forming units (CFU) per root of one plant in 2-week-old plants inoculated with 5 × 108Azospirillum cells. Significant increases in root length (35%) and in top (90%) and root (50%) dry weight and total leaf area (90%) were observed in 18-day-old inoculated tomato plants compared with non-inoculated controls. An inoculum concentration of 1 × 108 to 5 × 108 CFU/ml stimulated the appearance of root hairs. Large numbers of bacteria (1 × 109 CFU/ml) caused asymmetrical growth of the root tip. In a petri dish system, Azospirillum (1 × 108 CFU/ml) increased root dry weight (150%), protein content (20%), respiration rate per root (70%) and the specific activity of malate dehydrogenase (45%–65%) over non-inoculated controls. The specific respiration rate, expressed as micromol of O2 per minute per milligram of dry weight of roots, was significantly lower in inoculated roots, suggesting that less energy was spent for accumulation of more dry material.