Responses to Different Magnesium Supply Treatments in the Mature Leaves of Cunninghamia lanceolata Seedlings: Morphological, Physiological, and Structural Perspectives

(1) Background: Repeated planting cycles and monoculture practices have led to widespread magnesium (Mg) deficiency in Chinese fir (<i>Cunninghamia lanceolata</i>) plantations. To gain clarity on how different Mg concentrations affect seedling growth and physiology, we designed the following experiment. (2) Methods: One-year-old seedlings were exposed to three Mg concentration treatments: High (HM), Medium (MM), and Low (LM). Their responses were evaluated in terms of growth traits, photosynthetic activity, and chloroplast structure. (3) Results: Both HM and LM significantly affected leaf development, with LM having the strongest impact. LM disrupted chloroplast structure, causing thylakoid membrane rupture, mitochondrial damage, accumulation of osmiophilic granules, and increased spacing between chloroplasts and cell walls. LM also impaired photosynthesis, lowering the net photosynthetic rate (Pn) and peroxidase (POD) activity, while increasing malondialdehyde (MDA) levels. Leaf growth was reduced, as shown by smaller leaf area and lower biomass. In contrast, HM temporarily enhanced some physiological traits, including intercellular CO₂ concentration (Ci), transpiration rate (Tr), leaf dry matter content (LDMC), and ATPase activity, though it also reduced Fv/Fo compared to MM. (4) Conclusions: Both high and low Mg concentration negatively affected photosynthesis, with Mg deficiency causing the most severe damage. These findings highlight the importance of managing soil Mg levels to maintain healthy growth and productivity in <i>C. lanceolata</i> plantations.