The role of sulphur on the content of total polyphenols and antioxidant activity in onion (Allium cepa L.)

Sulphur is one of the most important elements in plants. Sulphur also positive influences on growth functions (nitrogen metabolism, enzyme activity and protein and oil synthesis), yield and quality bulbs of onion. In this experiment was watched the progress of making the total polyphenols content in different levels of fertilizer sulphur in onion during vegetation. In the work we watched also the influence of sulphur fertilization on the antioxidant activity. Sulphur was added in amounts 7.3 mg.kg-1 S, 11 mg.kg-1 S and 14,6 mg.kg-1 S. Samples of fresh onion we collected at the beginning, in the middle and in the end of vegetation period and we prepared an extract: 50 g cut onion extracted by 100 ml 80% ethanol according sixteen hours. These extracts were used for experiment. The content of total polyphenols was estimated by using Folin-Ciocalteau reagent. The absorbance was measured at 765 nm of wave length against blank. Antioxidant activity was measured using a compound DPPH˙ (2.2-diphenyl-1-picrylhydrazyl) at 515.6 nm in the spectrophotometer. Our values of total polyphenolic content during vegetation period  were in range from 508.16 ±27.59 mg.kg-1 to 638.2 ±12.84 mg.kg-1. The highest content of total polyphenolics was recorded at the end of the vegetation period (638.32 ±12.84 mg.kg-1) in III. sampling (incorporation of sulphur in quantity of 14.6 mg S.kg-1soil). This increase was statistically significant (P-value = 2.10-3). The lowest content of total polyphenols was measured in  the middle of vegetation period  (415.41 ±13.32 mg.kg-1) in II. sampling (incorporation of sulphur in quantity of 11 mg S.kg-1soil). Another indicator that has been evaluated and compared was the influence of sulphur fertilization on the antioxidant activity. The highest values of antioxidant activity were recorded at the end of vegetation period in all variants. The lowest value of antioxidant activity (25.41 ±7.67%) was measured in three sampling in II.variant.