Exponential and stationary cultures of Azospirillum brasilense and Bradyrhizobium japonicum differentially promote early growth of soybean and maize seedlings.

CASSAN, FABRICIO; TORRES, DANIELA; RIBAS, AINALEN; PENNA, CLAUDIO; LUNA, VIRGINIA

Tarragona

Congreso; 20th International Conference on Plant Growth Substances; 2010

International Plant Growth Substances Association

Inoculants are complex formulations that combine selected microorganisms and biologically active molecules released during their growth, namely auxins, gibberellins and cytokinins. Azospirillum brasilense and Bradyrhizobium japonicum are the most successful bacterial species used as inoculants in Argentina and in many other countries. Thus, they are two of the most studied microorganisms in relation to their capacity to promote plant growth and to produce phytohormones in culture media, especially during the exponential growth phase. The aim of this work was to test the hypothesis that pure cultures of B. japonicum strain E109 and A. brasilense strain Az39 in exponential (EP) or stationary (SP) growth phase, could determine a differential response on early plant growth and development and this fact could be attributed, at least partly, to concentration and balance of phytohormones in the culture medium. Soybean (Glycine max L.) and maize (Zea mays L.) seeds were inoculated with agronomical doses of B. japonicum E109 or A. brasilense Az39 cultures in EP or SP. Previous to inoculation, supernatants from incubations in EP and SP and non-inoculated media (as control) were evaluated for indol-3-acetic acid , gibberellic acid and zeatin production. Inoculated seeds were stored 8 days in a germination chamber according to ISTA (International Seed Test Association) recommendations and seedlings were then cultured in a growth chamber over 21 days under controlled photoperiod, temperature and humidity. Our results suggest that both E109 and Az39 strains possess similar capacities to promote germination and early seedling growth in maize and soybean, and that such capacities would not depend exclusively on the bacterial cells, but also on the concentration and balance of phytohormones released into the medium. This ?hormonal effect of inoculation? is considered as the first response for the early growth promotion in inoculated seeds.