Effects of Different Fruit Set Enhancement Treatments on Fruit Set Rate and Fruit Quality of the Self-Compatible Sweet Cherry Cultivar ‘Sunburst’ Under Protected Cultivation

Sweet cherries are an important temperate fruit crop, and protected cultivation has expanded rapidly in recent years. However, unstable fruit set under protected cultivation remains a major challenge, and the application of fruit set enhancement treatments has become common practice in commercial production to stabilize and improve fruit set performance. Various fruit set enhancement treatments, including biotic pollination and hormonal regulation, have been shown to influence source–sink relationships through different physiological pathways, thereby affecting fruit set rate and fruit quality. In this study, three fruit set enhancement treatments, namely hand pollination (HP), plant growth regulator treatment (PGRT), and bee pollination (BP), were applied, with an untreated control (CK) included for comparison, to evaluate their effects on fruit set rate and fruit quality in the self-compatible sweet cherry cultivar ‘Sunburst’ under protected cultivation. Both HP and PGRT significantly increased fruit set rate by 33.47 and 27.84 percentage points, respectively, compared with CK (71.68% and 66.05% vs. 38.21% in CK), and enhanced fruit firmness (86.50 g and 97.00 g vs. 70.10 g in CK) and raffinose content (0.53 mg g−1 FW and 0.32 mg g−1 FW vs. 0.15 mg g−1 FW in CK). However, PGRT significantly increased pedicel length (3.43 cm vs. 2.81 cm in CK) and gluconic acid content (3.44 mg g−1 FW vs. 2.02 mg g−1 FW in CK), while decreasing ascorbic acid content (0.23 mg g−1 FW vs. 0.34 mg g−1 FW in CK). Although BP resulted in the highest soluble solids content (16.68%) and total phenolic content (1.93 mg g−1), its fruit set rate remained low (42.06%) and was not significantly different from that of CK. Comprehensive evaluation indicated that, whilst ensuring adequate fruit set, hand pollination demonstrated superior overall performance among the evaluated treatments by maintaining desirable fruit appearance and balanced sugar–acid metabolism, serving as a practical, non-chemical, and environmentally safe supplementary fruit set enhancement strategy under the specific greenhouse conditions evaluated.