Antioxidant properties of garlic as affected by selenium and humic acid treatments

Garlic is an important bulb vegetable because of its medicinal and nutritional values. Selenium, an important trace element, and humic acid, a soil amendment, can positively affect garlic's nutritional values. To evaluate the impact of selenium and humic acid on antioxidant activity and phenol, flavonoid and allicin contents in garlic, Na2SeO4 solution was sprayed in concentrations of 0, 10, 20 and 30 g Se/mL, and humic acid was used in fertigation at rates of 0, 10 and 20 kg/ha. Results showed the applied treatments had positive effects on the total antioxidant activity in garlic. The application of low concentrations of Se with moderate amounts of humic acid caused the highest antioxidant activity, as did the application of high Se concentrations with no humic acid. Humic acid at the rate of 10 kg/ha contributed more to the lower total phenol content than did the 20 kg/ha (H-20) rate. The Se treatment decreased flavonoid content, and the control plants (Se-0) had the highest amount of flavonoids. The results regarding sulphur and allicin contents were greatly similar in that they were both at their maximum level in the Se0H20 treatment. Se-0 led to the highest allicin content, and its difference from other Se treatments was significant (P < 0.01). A strong negative correlation was found between selenium concentration and allicin content (R-2 = 0.881). The findings showed that humic acid positively affects most tested traits, but negative or no effects were seen for the Se treatment. Therefore, if the production of Se-enriched garlic is the aim, a decrease in some nutritional values must be accepted.