Effect of plant growth temperature on antioxidant capacity in strawberry

The influence of four day/night growing temperature combinations (18/12, 25/12, 25/22, and 30/22 °C) on phenolic acid, flavonol, and anthocyanin content and their antioxidant activities against peroxyl radicals (ROO^•), superoxide radicals (O₂^•-), hydrogen peroxide (H₂O₂), hydroxyl radicals (OH^•), and singlet oxygen (¹O₂) in fruit juice of Earliglow and Kent strawberry (Fragaria × ananassa Duch.) cultivars was studied. Pelargonidin-based anthocyanins such as pelargonidin 3-glucoside (291.3-945.1 μg/g fresh wt.), pelargonidin 3-rutinoside (24.7-50.9 μg/g fresh wt.), and pelargonidin 3-glucoside-succinate (62.2-244.0 μg/g fresh wt.) were the predominant anthocyanins in strawberry fruit juice. The content of cyanidin-based anthocyanins, cyanidin 3-glucoside and cyanidin 3-glucoside-succinate, was much lower than that of pelargonidin-based anthocyanins. Strawberry growth in high temperature conditions significantly enhanced the content of p-coumaroylglucose, dihydroflavonol, quercetin 3-glucoside, quercetin 3-glucuronide, kaempferol 3-glucoside, kaempferol 3-glucuronide, cyanidin 3-glucoside, pelargonidin 3-glucoside, pelargonidin 3-rutinoside, cyanidin 3-glucoside-succinate, and pelargonidin 3-glucoside-succinate in strawberry juice. Plants grown in the cool day and cool night temperature (18/12 °C) generally had the lowest phenolic acid, flavonols, and anthocyanins. An increase in night temperature from 12 to 22 °C, with the day temperature kept constant at 25 °C, resulted in a significant increase in phenolic acid, flavonols, and anthocyanins. These conditions also resulted in a significant increase in antioxidant capacity. The highest day/night temperature (30/22 °C) yielded fruit with the most phenolic content as well as ROO^•, O₂^•-, H₂O₂, OH^•, and ¹O₂ radical absorbance capacity. Fruit of Kent cv. strawberry had higher values of phenolic acid, flavonols, anthocyanins, and antioxidant capacities than fruit of Earliglow cv. strawberry under all temperature regimes.