Free radicals in the fruit of three strawberry cultivars exposed to drought stress in the field

In strawberry fruits harvested after different periods of drought stress, no effects of drought or time of harvest were observed on the total free radical contents of frozen fruit of one cultivar (Fragaria ananassa cv. Symphony) or on the amounts of unstable free radicals reacting with the spin trap α-(4-pyridyl-1-oxide)-N-t-butylnitrone (4-POBN) in crushed fruits of a second cultivar (Fragaria vesca cv. Semperflorens). In contrast, with freeze-dried samples, distinct effects of drought stress were seen with two out of three varieties studied, and there were also differences between harvests. Over all samples, there was an inverse relationship between the free radical content of freeze-dried fruit and the water content of fresh fruit. The free radical levels were approximately ten times higher in the freeze-dried fruits, than in the equivalent frozen samples (expressed on a dry weight basis). Massive free radical generation, therefore, accompanies the freeze-drying process and the differences between stressed and control samples indicate an effect of drought stress on the levels of free radical scavenging compounds in the fruit.