"Aplicação da radiação por feixe de elétrons como agente esterilizante de microorganismos em substrato turfoso"

Abstract

Inoculation of root nodule bacteria into legume seeds such as soybean [Glycine max. (L.)], common bean (Phaseolus vulgaris L.) and forrage pasture has been effective and convenient as this simple procedure may introduce effective strains of Bradyrhizobium/Rhizobium into agricultural soils without a past history of successful cropping systems with the legume hosts. Peat-based substrates previously sterilized have been used for decades as bacteria carrier, protecting them from the prevailing harsh conditions in tropical soils and ensuring their survival with nutrient and protection against the soil antagonists. The Brazilian Government requires that all peat-based substrates must be gamma-sterilized from a cobalt-60 (60 Co) source, prior the introduction of the root nodule bacteria into the package. The recommendation is for a dose up to 50 kGy for an effective suppression of pathogens and saprophytes, in order to avoid competition among the substrate microbiota. Recently, the use of the electron beam (EB) accelerator has shown to be a new alternative for peat pre-sterilization, as this technique may promote reactive free-radicals which are efficient to suppress microbial contaminants. This fast technology is considered more environment and ecology friendly-sound than gamma radiation (γ). The disvantage of not reaching higher depth than gamma rays from 60 Co must be considered, and attempts of optimizing the technique are crucial. This study compared both methods by using increasing rates of radiation by 60 Co by the EB method-0, 10, 20, 30, 40 e 50 kGy in a commercial peat used for inoculants. Experimental data from days 7, 14, 21 and 28 days (growth period) and 150, 180 and 210 days (storage period) indicated high numbers of the strain Rhizobium tropici CM-01, labelled with gusA + (Study 1) and celB + (Study 2) from both eat-sterilizing techniques, reaching values above the minimum of 1x10 8 cells g-1 peat. At high rates, above 40 kGy, and after long incubation periods (ex. after 150 days), EB method was more efficient to suppress actinobacteria, one serious antagonist for rhizobia. Strain CM-01 celB + , data for the period of bacterial growth confirmed the efficiency of the method even at rates as low as 10 kGy.