Two different genotypes of Pinus massoniana seedlings (aluminum-resistant FJ5 and aluminum-sensitive GD20) were used, the effects of different exogenous malic acid (0.00, 0.01, 0.02, 0.04, 0.08, 0.16 mmol·L⁻¹) on the growth attributes of P. massoniana seedlings treated by the Al³⁺ concentration of 0.8 mmol·L⁻¹ were studied, to provide a basis for the growth in acidified soil. In our experiment, the seedling growth was inhibited by Al³⁺ treatment. After treatment with a low concentration of exogenous malic acid, the activities of antioxidant enzymes in leaves were enhanced, the concentrations of hydrogen peroxide (H₂O₂), superoxide (O₂⁻·), malondialdehyde (MDA) and osmotic adjustment substances were reduced accordingly. GD20 exhibited more severe changes compared with FJ5. The larger ones of the contribution rates of the indices in principal component analysis were H₂O₂, Glutathione Reductase (GR). These results indicated that Al³⁺ with high concentration inhibits the growth of P. massoniana. Malic acid could effectively alleviate the toxicity, and the mitigation effect on the aluminum-sensitive species, which genotype is more sensitive to the response of Al³⁺ toxicity, was more effective than that on the aluminum-resistant. How to select and cultivate more resistant species, by using the main parameter (H₂O₂ and GR), is worthy in further study.

Interpreting leaf nitrogen (N) allocation is essential to understanding leaf N cycling and the economy of plant adaptation to environmental fluctuations, yet the way these mechanisms shift in various varieties under high temperatures remains unclear. Here, eight varieties of pecan (Carya illinoinensis [Wangenh.] K. Koch), Mahan, YLC10, YLC12, YLC13, YLC29, YLC35, YLJ042, and YLJ5, were compared to investigate the effects of high temperatures on leaf N, photosynthesis, N allocation, osmolytes, and lipid peroxidation and their interrelations. Results showed that YLC35 had a higher maximum net photosynthetic rate (P_max) and photosynthetic N-use efficiency (PNUE), while YLC29 had higher N content per area (N_a) and lower PNUE. YLC35, with lower malondialdehyde (MDA), had the highest proportions of N allocation in rubisco (P_r), bioenergetics (P_b), and photosynthetic apparatus (P_p), while YLC29, with the highest MDA, had the lowest P_r, P_b, and P_p, implying more leaf N allocated to the photosynthetic apparatus for boosting PNUE or to non-photosynthetic apparatus for alleviating damage. Structural equation modeling (SEM) demonstrated that N allocation was affected negatively by leaf N and positively by photosynthesis, and their combination indirectly affected lipid peroxidation through the reverse regulation of N allocation. Our results indicate that different varieties of pecan employ different resource-utilization strategies and growth–defense tradeoffs for homeostatic balance under high temperatures.

Aluminum (Al) toxicity induced by soil acidification severely threatens the growth of Pinus massoniana and forest ecosystem stability. Although melatonin (MT) can alleviate heavy metal toxicity in plants, how it coordinates Al tolerance resources within a cross-scale defense network remains elusive. We show that exogenous MT and Al stress synergistically stimulate endogenous MT biosynthesis, which then acts as a signaling hub driving a hierarchical, two-tier defense network. In the dominant physical defense, endogenous MT inhibits pectin methylesterase (PME) activity (-24.37% versus the Al-treated group), maintains high pectin methylesterification (83.51%), and reduces total pectin content (-40.06%), diminishing Al³⁺-binding sites in the cell wall and restricting Al influx into the protoplast at the source. In the subordinate biochemical defense, MT specifically upregulates the ascorbate–glutathione (AsA–GSH) cycle, compensating for the breakdown of the primary antioxidant system (superoxide dismutase and catalase) under severe Al stress. Partial least squares path modeling and topological analyses confirmed that cell wall physical exclusion plays the predominant role in disrupting the Al toxicity chain and alleviating membrane damage. Our findings bridge the theoretical gap in cross-scale hierarchical Al defense in woody plants and provide an MT-targeted strategy for restoring degraded vegetation in acidic soil regions.