Effect of Foliar Biostimulant Application on Bioactive Compounds and Antioxidant Capacity in Blueberry (Vaccinium corymbosum L.)

Blueberries are rich in bioactive compounds and antioxidants whose contents can be significantly affected by pre-harvest agronomic practices. Thus, using natural biostimulants like Ecklonia maxima (EM) extract and glycine betaine (GB) is a promising strategy to improve blueberry quality. The effect of foliar pre-harvest application of EM and GB on bioactive compounds and antioxidant capacity (AC) of ‘Duke’ and ‘Draper’ blueberries was investigated in 2022 and 2023. Blueberries treated with GB had higher polyphenol contents and AC by the ABTS•+ and CUPRAC methods, particularly at higher doses. Vitamin C in ‘Duke’ was decreased by both doses of GB and low-dose EM + GB by 10–30% over two years, whereas in ‘Draper’, both doses of GB increased vitamin C in 2023 by 40–80%. High GB and EM + GB doses improved the AC of ‘Duke’ blueberries by 7–12% (CUPRAC) in both seasons. In 2022, the high dose of EM increased the levels of polyphenols in both cultivars by 14–21% and AC by ABTS•+ and CUPRAC in cv. ‘Draper’. As a result, blueberries’ nutritional value and AC can be enhanced by biostimulants, whose application must be adjusted according to cultivar and dose to optimize their use.