Exogenous Abscisic Acid Enhances Freezing Tolerance in Rhododendron obtusum ‘Fen Zhuanglou’ by Stabilizing Photosynthetic Pigments, Promoting Osmolyte Accumulation, and Activating Antioxidant Defense

Exogenous abscisic acid (ABA) enhances freezing tolerance, yet its dose-specificity and metabolic trade-offs in ornamental woody species remain poorly characterized. Here, we investigated the effects of exogenous ABA (0, 10, 30, 50 mg·L−1) on Rhododendron obtusum ‘Fen Zhuanglou’ seedlings exposed to −4 °C for 5 days. Based on phenotypic screening, 10 mg·L−1 ABA was identified as the hormetic optimum, whereas 50 mg·L−1 failed to alleviate injury. Under freezing stress, optimal ABA treatment (10 mg·L−1) reduced malondialdehyde (MDA) content by 23.5% compared to untreated controls, while partially rescuing superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities by 40.0%, 29.2%, and 35.2%, respectively. Notably, ABA triggered a metabolic trade-off: soluble protein declined by 22.98% while soluble sugar increased by 6.05%, suggesting a reallocation of metabolic resources. Furthermore, pigment profiling suggested impaired chlorophyll catabolism in unprotected leaves—chlorophyll a and xanthophylls accumulated to 7-fold and 43-fold the levels of ABA-treated leaves—whereas ABA-treated leaves retained a higher carotenoid fraction (7-fold). These findings establish that ABA-mediated freezing tolerance operates through a metabolic trade-off rather than universal preservation, providing an evidence-based protocol (10 mg·L−1) for ornamental rhododendron production in cold regions.