Aluminum Alleviates Low-Phosphorus Stress in Camellia japonica by Promoting Growth, Enhancing Antioxidant Defense, and Improving Phosphorus Status

High aluminum (Al) availability and phosphorus (P) deficiency often coexist in acidic soils. Suitable Al concentrations promote growth in Camellia japonica, but its response to Al under P deficiency remains unclear. To examine this response under controlled Al and P supplies, two-year-old C. japonica ‘Shiliuhong’ plants were grown hydroponically at pH 4.0 for 8 weeks under two P levels (0.025 and 0.100 mmol·L−1) and four Al concentrations (0, 0.25, 0.50, and 1.00 mmol·L−1; n = 5 for each measurement). We measured growth, leaf physiological traits, and acid phosphatase (APase) activity. Total Al and P concentrations were determined in leaves and roots. At 1.00 mmol·L−1 Al, shoot and root dry weights increased by 151.6% and 210.4%, respectively, under low P and by 175.7% and 255.5%, respectively, under normal P, relative to the corresponding Al-free treatments. Longest root length increased from 1.78 to 10.32 cm under low P and from 3.02 to 13.58 cm under normal P. Al increased chlorophyll and soluble sugar contents, superoxide dismutase (SOD) and peroxidase (POD) activities, and ascorbate (AsA) and glutathione (GSH) contents, while lowering H2O2, O2−, and thiobarbituric acid-reactive substances (TBARS). Under low P, 1.00 mmol·L−1 Al reduced these oxidative-damage indicators by 23.8–56.0%, while root APase activity and root total P concentration increased by 66.8% and 156.5%, respectively. In the tested hydroponic system, C. japonica plants supplied with Al under low P showed better growth, less oxidative damage, and improved P status.