Influence of arbuscular mycorrhizal fungi, cultivation systems and postharvest parameters in the concentration of steviosides and development of Stevia Rebaudiana (BERT.) Bertoni / Influencia de fungos micorrizicos arbusculares, sistemas de cultivo e parametros pos-colheita na concentração de esteviosideos e desenvolvimento de Stevia rebaudiana (BERT.) Bertoni

Edilberto Princi Portugal

Stevia rebaudiana, a shrub mostly interesting for its production of steviosides, was the object of this work. In regard to its productive chain, further researches must be done, not only in agronomical aspects, but also in the utilization of mycorrhizal fungal species, as well as in the post-harvest processes, namely the drying one. The present work aimed at the identification of an efficient arbuscular mycorrhizal fungal species and its influence in the development and concentration of steviosides in stevia plants grown in hydroponic and aeroponic systems; to select an adequate substrate for the development of mycorrhizal fungi; to evaluate drying systems in steviosides rates and to determine stevia?s isotherms of sorption. For the mycorrhizal fungi selection seven species were tested, five from a germplasm bank and two were isolated from their own stevia culture. Five assays in pots with sterilized soil were carried out in a greenhouse. The total dry mass of the aerial parts and of the leaves, the rate and accumulated quantity of steviosides in the leaves as well as the mycorrhizal colonization were determined. Nine commercial substrates were tested for the rate of colonization of the root system of an indicating gramineous plant (Brachiaria humidicola). Stevia plants that were inoculated (or not) with the mycorrhizal fungus Glomus intraradices were cultivated in three different growing systems, field, aeroponic and hydroponic. Three harvests were carried out in different periods and the total dry mass of the branch and leaves sections, contents and accumulated steviosides in the leaves and mycorrhizal colonization were determined. Three processes for drying leaves were tested: by sunlight, gas dryer and electrical stove with forced air circulation, at 45oC. The processes were evaluated in relation to the steviosides rate of the leaves. The equilibrium moisture content for stevia leaves was done at 30oC and 45oC, while the isotherms of sorption were tested for the best fit by mathematical models of BET, BET linear, Peleg, Gab, Halsey, Langmuir, Oswin, Henderson and Chung and Pfost. The Glomus intraradices species was the most efficient for the stevia development and accumulation of steviosides. The Rendmax Citrus substrate without manure rendered the best conditions for the mycorrhizal colonization in the gramineous roots. Among the growing systems studied, hydroponics turned out to be the least efficient for stevia cultivation. Aeroponics brought the highest production in terms of dry mass from the aerial parts, while in the field a larger accumulated quantity of steviosides was obtained. No difference was observed between the stove and gas drying processes at any of the harvesting periods. The steviosides rate from the leaves dried by sunlight was always inferior, coming to a loss of 20% in relation to the others. The best fit models were GAB and HALSEY. The BET LINEAR fitted by 45oC

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