Effect of Different Plant Growth Regulators and Their Combinations on Fruit Quality and Screening of Core Quality Evaluation Metrics in Mango

Mango is an economically vital tropical fruit, but commercial production is often compromised by unstable fruit development and inconsistent quality under conventional farming practices. Plant growth regulators (PGRs) are widely applied in mango production in Hainan Province and significantly influence fruit quality attributes, including fruit size, total soluble solids (TSSs), and titratable acidity (TA). This study aimed to elucidate the effects of PGRs on fruit enlargement and quality in ‘Guifei’ mango (Mangifera indica L.), identify the optimal PGR application regime, and preliminarily screen core indicators for mango fruit quality evaluation. A completely randomized design was adopted, with three mango trees per treatment as biological replicates. Different concentrations of thidiazuron (TDZ), gibberellic acid (GA3), and 6-benzyladenine (6-BA) were sprayed individually or in combination on 8–10-year-old mango trees at 20, 35, and 50 days after full bloom. Fruit morphological characteristics and quality attributes were then determined. Coefficient of variation analysis, Pearson correlation analysis, and principal component analysis were used to screen core fruit quality evaluation indicators, and Duncan’s multiple-range test was applied for mean separation to distinguish statistically significant differences from merely numerical trends. Among the individual PGR treatments, 16 mg/L TDZ, 100 mg/L GA3, and 100 mg/L 6-BA exhibited the most favorable balance between fruit enlargement and quality maintenance. Among the combined treatments, GA3 (100 mg/L) + 6-BA (100 mg/L) + TDZ (16 mg/L) exhibited the best overall performance, effectively promoting fruit enlargement while maintaining relatively stable fruit quality. Four indicators, namely, average single fruit weight, TSS, TA, and vitamin C content, were preliminarily identified as core indicators for evaluating ‘Guifei’ mango fruit quality. These findings provide a scientific basis for optimizing PGR application regimes and evaluating fruit quality in high-quality ‘Guifei’ mango production in Hainan.