Influence of rhizobial inoculation on seedling vigor and germination of lettuce

Rhizobia are known by their ability to fix nitrogen in symbiosis with legumes, but they are also capable of promote the growth of non-legume, mainly due to indoleacetic acid production (IAA). In this research, it was aimed to select rhizobia producers of IAA and evaluate the effect of different levels of this hormone over the germination and initial development of lettuce seedlings. Four isolates of Bradyrhizobium sp. and one isolate of Rhizobium leguminosarum biovar trifolii were grown during four days in yeast manitol medium enriched with tryptophan. After that period, the production of IAA was evaluated and the isolates were inoculated in lettuce seeds. The isolate TV-13, of R. leguminosarum biovar trifold, produced 171.1 mu g mL(-1) of IAA, causing damages to lettuce seedlings. On other hand, Bradyrhizobium sp. isolates produced between 1.2 and 3.3 mu g mL(-1) of IAA and improved seedlings vigor. In order to verify if these results were due to IAA concentrations, other two assays were carried out, in which lettuce seeds were imbibed in TV-13 cultures with or without tryptophan or in increasing dosages of synthetic IAA. The isolate TV-13 grown in the presence of tryptophan caused progressive damages to lettuce seedlings development, fact that did not occur in the absence of tryptophan. It was also verified a delay inn germination of seeds exposed to high levels of synthetic IAA. The results show the influence of IAA on germination parameters, so that the inoculation of lettuce seeds with rhizobia that produce low amounts of IAA is a recommended practice.