Alleviation of Shade Stress in Chinese Yew (Taxus chinensis) Seedlings with 5-Aminolevulinic Acid (ALA)

5-aminolevulinic acid (ALA) is a novel regulator that can promote plant growth, nitrogen uptake, and abiotic stress tolerance. Its underlying mechanisms, however, have not been fully investigated. In this study, the effects of ALA on morphology, photosynthesis, antioxidant systems, and secondary metabolites in two cultivars of 5-year-old Chinese yew (Taxus chinensis) seedlings, ‘Taihang’ and ‘Fujian’, were examined under shade stress (30% light for 30 days) using different doses of ALA (0, 30, and 60 mg/L). The findings from our study show that shade stress significantly reduced plant height, stem thickness, and crown width and increased malondialdehyde (MDA) levels. However, the application of 30 mg/L ALA effectively mitigated these effects, which further induced the activity of antioxidant enzymes under shade stress, resulting in the activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) being increased by 10%, 16.4%, and 42.1%, and 19.8%, 20.1%, and 42% in ‘Taihang’ and ‘Fujian’, respectively. It also promoted their role in the absorption, conversion, and efficient use of light energy. Additionally, the use of 30 mg/L ALA caused a significant increase in the concentration of secondary metabolites, including polysaccharide (PC), carotenoid (CR), and flavonoids (FA), with increases of up to 46.1%, 13.4%, and 35.6% and 33.5%, 7.5%, and 57.5% in both yew cultivars, respectively, contributing to nutrient uptake. With ALA treatment, the yew seedlings showed higher chlorophyll (total chlorophyll, chlorophyll a and b) levels and photosynthesis rates than the seedlings that received the shade treatment alone. To conclude, the application of 30 mg/L ALA alleviated shade stress in yew seedlings by maintaining redox balance, protecting the photorespiratory system, and increasing organic metabolites, thus increasing the number of new branches and shoots and significantly promoting the growth of the seedlings. Spraying with ALA may be a sustainable strategy to improve the shade-resistant defense system of yew. As these findings increase our understanding of this shade stress response, they may have considerable implications for the domestication and cultivation of yew.