Effects of fruit thinning on ascorbate–glutathione cycle metabolism in black currants (Ribes nigrum L.)

To elucidate mechanisms regulating ascorbic acid (AsA) biosynthesis and accumulation in the fruit and leaves of black currants, AsA and the activities of key enzymes in the ascorbate–glutathione (AsA–GSH) cycle were measured from fruit set to fruit ripening during fruit thinning treatments of three common commercial black currant cultivars that differed in their AsA levels: ‘Risager’ (low), ‘Brodtrop’ (medium) and ‘Adelinia’ (high). Treatments were 50% fruit reduction (50% of total fruit set) by hand, control was no thinning. Fruit thinning treatment significantly increased AsA content in fruit of all three cultivars from weeks 2 to 8, significantly decreased AsA content in leaves from weeks 3 to 8. Dehydroascorbate reductase and monodehydroascorbate reductase activities in fruit and leaves had a similar pattern, increasing during week 2, rose until they peaked in week 4. Ascorbate peroxidase activity in fruit in the thinning treatment was slightly lower than in the control. Fruit thinning was shown to be a good model to test AsA biosynthesis regulation and accumulation in black currants. The results from our study provided strong evidence that AsA–GSH cycle involved in AsA synthesis and accumulation in fruit.