Effects of Arbuscular mycorrhizal fungi and phosphorus fertilization on the antioxidant activity of Stevia rebaudiana

The purpose of this study has been to evaluate the effect of mycorrhization and phosphorus fertilization on the antioxidant properties of S. rebaudiana (Bertoni) plants in a pot trial established in Pisa, Italy.
Different agronomic treatments were performed in the crop plantation: Three phosphorus doses (0, 25 and 50 mg of P2O5/kg of soil as triple superphosphate) were tested in plants with or without Arbuscular Mycorrhizal Fungi (AMF) inoculation, using Glomus intraradices as inoculum. In turn, leaves were harvested at 2 different periods: 89 and 123 days after transplanting (DAT).
Dried leaves aqueous extracts were analyzed for the total phenolic content through Folin-Ciocalteu method. The antioxidant capacity was evaluated by means of the oxygen radical absorbance capacity (ORAC) method, and the cellular antioxidant activity (CAA) assay. It was shown that the higher total phenolic content is obtained during the first harvest in which the plants were in the full phase of vegetative growth; the treatment with 50 mg P showed the highest polyphenols content (66.45 mg GAE/g).
This
research indicated that the supplementation with 25 mg of P gives the highest ORAC values, being in average 2182.15 μmol trolox equivalents/g stevia dry leaves. Similar results were observed in plants subjected to combined treatment with 25 mg P + AMF inoculation. In general, the ORAC values depended upon the harvest period, being higher in the 123 DAT plants when compared to 89 DAT plants. No significant correlation was found between ORAC values from the first and second harvest.
The CAA results for the analyzed treatments with and without AMF inoculation are not significantly different from each other (p > 0.05), ranging in average from 9.0 to 12.1 μmol quercetin equivalents/g stevia dry leaves, depending upon the phosphorus treatment. A strong significantly correlation was observed for CAA values between different harvest times (p< 0.001). The highest CAA mean value was obtained for the plants harvested 89 DAT (mean of all treatments: 13.2 μmol quercetin equivalents/g stevia dry leaves). Between the P2O5-supplemented plants, the treatment with 50 mg, gave the higher antioxidant potential by means of CAA (12.0 μmol quercetin equivalent/g stevia). This result is in agreement with that obtained with the combined treatment with 50 mg P2O5 supplementation + AMF inoculation (12.1 μmol quercetin equivalents/g stevia dry leaves).