Effects of Combined Basal and Bud-Stage Foliar KH2PO4 Application on Vegetative Growth, Floral Performance and Nutrient Partitioning in Potted Hydrangea paniculata ‘Limelight’

Phosphorus (P) and potassium (K) are essential mineral nutrients for plant growth, development, and reproduction, and their supply levels and application methods may affect the growth, physiological performance, and flowering quality of ornamental plants. In this study, one-year-old potted cuttings of Hydrangea paniculata ‘Limelight’ were used in a two-factor completely randomized design. Four basal application rates of KH2PO4 (50, 100, 150, and 200 mg P2O5 kg−1 substrate) were combined with foliar application of 0.1% KH2PO4 at the bud stage. Root morphology, biomass allocation, photosynthetic characteristics, antioxidant physiology, flowering quality, and the accumulation and allocation of N, P, and K among plant organs were evaluated. Transcriptome sequencing and quantitative reverse transcription PCR (qRT-PCR) validation were also performed for representative treatments. The results showed that treatment 2KF, which combined basal application of 100 mg P2O5 kg−1 substrate with foliar application at the bud stage, exhibited a favorable balance between physiological performance and flowering traits. The maximum quantum efficiency of PSII (Fv/Fm) in leaves was 0.76, significantly higher than that in treatments 1KF and 4KF. The leaf soluble sugar content reached 13.12 mg g−1 fresh weight (FW), whereas the malondialdehyde (MDA) content remained relatively low. The root-to-shoot ratio was 1.56 ± 0.48, 38.1% higher than that of the control (CK) and higher than those of the other treatments. Transcriptome analysis identified 8142 differentially expressed genes (DEGs) between 2KF and the nitrogen-only control (CN), including 3386 upregulated and 4756 downregulated genes. These DEGs were primarily associated with nutrient transport, photosynthesis, carbon metabolism, antioxidant responses, and stress responses. In treatment 2KF, the expression of several genes associated with P and K transport was upregulated, whereas the expression of genes related to the phosphate starvation response was suppressed. The expression patterns of 12 candidate genes examined by qRT-PCR were generally consistent with the RNA-seq results. In conclusion, under the conditions of this experiment, basal KH2PO4 application combined with foliar application at the bud stage, particularly treatment 2KF, produced favorable overall responses in Hydrangea paniculata ‘Limelight’. These findings provide a reference for optimizing fertilization management in this ornamental cultivar.