Effects of Plant Growth Regulators on Plum (Prunus domestica L.) Grown on Two Rootstocks at Harvest and at the Postharvest Period

Plant growth regulators (PGRs), such as cytokinins (6-benzyladenine; BA) and gibberellins (GAs), are widely used in fruit production. This study focused on the plum cultivar “Čačanska rodna” (P. domestica L.) grafted on vegetative rootstock “WaVit” and generative rootstock Prunus cerasifera, with interstock Prunus spinosa. PGR treatments included 50 mg L−1 and 100 mg L−1 of BA and 200 mg L−1 of gibberellin A3 (GA3) and non-treated control. Pomological characteristics of plum fruits were examined at harvest, while physicochemical properties were analyzed at harvest and after 28 days of cold storage and 4 days of shelf life. GA3 and BA application changed the fruit morphological traits, improved skin strength, and increased carotenoid, anthocyanin and sucrose content while decreasing the titratable acidity at harvest. The beneficial effects of higher sucrose, anthocyanin and carotenoid levels persisted in all PGR-treated fruits after cold storage and shelf life. GA3-treated fruits had firmer flesh, stronger skin and higher total soluble solids (TSS) content, while in BA-treated plums, these effects were rootstock-dependent. The physical properties and chemical composition of plum fruit in the postharvest period suggest beneficial effects of the applied PGR treatments. Moreover, these chemical treatments might have prolonged the beneficial impact on fruit storability, nutritional profile and sensory properties. Based on our results, GA3 preharvest treatment can be included in standard cultivation practices within contemporary production systems of European plums not only to improve fruit quality at harvest but also to improve the storage potential and nutritional value, regardless of the rootstock used.