Forest ecosystems play an important role in environmental protection and maintaining ecological balance. Understanding the physiological mechanisms of tree species response to aluminum (Al) toxic is crucial to reveal the main causes of plantation decline in acid rain area. As an important afforestation tree species in tropical and subtropical areas, Eucalyptus has high economic value and plays crucial ecological roles. However, continuous fertilization and acid precipitation can exacerbate soil acidification and increase soil active Al, which has a significant negative impact on Eucalyptus growth. Hence, species and genotypes with high Al resistance are required to solve the problem of Al toxicity of acidic soils for sustainable forest production. In this study, E. urophylla was better adapted to Al stress than E. grandis or E. tereticornis; its high Al resistance was attributed to greater antioxidant enzyme activity and non-enzymatic antioxidant content, and a lower degree of membrane lipid peroxidation than E. grandis or E. tereticornis. The differences in adaptability among the three pure species were attributed to their distinct habitats. Eucalyptus urophylla × E. grandis inherited the outstanding adaptability to Al stress from its maternal species (E. urophylla), indicating that Al tolerance is highly heritable and can be selected in Eucalyptus breeding. Our results indicated that the response of Eucalyptus to Al stress may fluctuate according to the time under stress, and might be related to dynamic changes in ROS elimination and accumulation.

Under acidic conditions, aluminum (Al) toxicity is an important factor limiting plant productivity; however, the application of phosphorus (P) might alleviate the toxic effects of Al. In this study, seedlings of two vegetatively propagated Eucalyptus clones, E. grandis × E. urophylla ‘G9’ and E. grandis × E. urophylla ‘DH32-29’were subjected to six treatments (two levels of Al stress and three levels of P). Under excessive Al stress, root Al content was higher, whereas shoot and leaf Al contents were lower with P application than those without P application. Further, Al accumulation was higher in the roots, but lower in the shoots and leaves of G9 than in those of DH32-29. The secretion of organic acids was higher under Al stress than under no Al stress. Further, under Al stress, the roots of G9 secreted more organic acids than those of DH32-29. With an increase in P supply, Al-induced secretion of organic acids from roots decreased. Under Al stress, some enzymes, including PEPC, CS, and IDH, played important roles in organic acid biosynthesis and degradation. Thus, our results indicate that P can reduce Al toxicity via the fixation of elemental Al in roots and restriction of its transport to stems and leaves, although P application cannot promote the secretion of organic acid anions. Further, the higher Al-resistance of G9 might be attributed to the higher Al accumulation in and organic acid anion secretion from roots and the lower levels of Al in leaves.

As an efficient and high-quality additive in agriculture and forestry production, silicon (Si) plays an important role in alleviating heavy metal stress and improving plant growth. However, the alleviating effect of aluminum (Al) toxicity by Si in Eucalyptus is still incomplete. Here, a study was conducted using two Al concentrations (0 and 4.5 mM) with four Si concentrations (0, 0.5, 1, and 1.5 mM) to investigate plant growth, tolerance and antioxidant defense system in four Eucalyptus species (Eucalyptus tereticornis, Eucalyptus urophylla, Eucalyptus grandis, and Eucalyptus urophylla × Eucalyptus grandis). The results showed that the stress induced by 4.5 mM Al increased oxidative damage, disturbed the balance of enzymatic and non-enzymatic antioxidant systems, and negatively affected plant growth and tolerance quality in the four Eucalyptus species. However, the addition of 0.5 mM and 1 mM Si alleviated the effects of Al toxicity on plant growth and improved plant growth quality by strengthening stress tolerance. Besides, adding Si significantly facilitated the synergistic action of enzymatic and non-enzymatic antioxidant defenses, increased the removal of reactive oxygen species, reduced lipid peroxidation, and oxidative stress, and promoted the phytoremediation rate of the four Eucalyptus species by 18.7 ~ 34.8% compared to that in the absence of Si. Silicon can alleviate the effect of Al toxicity by enhancing the antioxidant capacity and improving plant growth and tolerance quality. Hence, the application of Si is an effective method for the phytoremediation of Eucalyptus plantations in southern China.

On acidified soil, the growth of Eucalyptus is seriously restricted by aluminum (Al) stress. Therefore, breeding Eucalyptus species with excellent Al tolerance, developing the genetic potential of species, and improving tolerance to Al stress are important for the sustainable development of artificial Eucalyptus forests. By observing the occurrence and distribution of the main reactive oxygen species (ROS) and reactive nitrogen species (RNS) in root tips of Eucalyptus seedlings under Al stress, this study analyzed change in the growth and physiological indexes of Eucalyptus seedlings under Al stress. The antioxidant enzymes activities of the root tips of different Eucalyptus species induced by Al stress resulted in different ROS and RNS contents, ultimately resulting in differing degrees of membrane lipid peroxidation. In addition to suppressions of root relative elongation and root activity, the accumulations of soluble sugar, soluble protein, and proline can be used as indicators of Al sensitivity in Eucalyptus species. This may be an important determinant of the differences in Al tolerance among Eucalyptus species. The accumulation of ROS and RNS in the roots of E. grandis and E. tereticornis resulted in severe oxidative and nitrification stress. The tolerance of E. urophylla and E. urophylla × E. grandis to Al stress was stronger than that of E. grandis and E. tereticornis. Differences in Al toxicity tolerance were related to long-term selection of the original habitat of the species; moreover, the Al tolerance was hereditary. Eucalyptus urophylla × E. grandis had stronger Al tolerance than its parents, which is indicative of heterosis. These results provide theoretical support for the breeding of tree species in areas with acidic soil. In addition to suppressions of root relative elongation and root activity, the accumulations of soluble sugar, soluble protein, and proline can be used as indicators of aluminum (Al) sensitivity in Eucalyptus species. The Al tolerance of E. urophylla × E. grandis was better than that of both parents, which may have been due to heterosis.

To improve the quality of Eucalyptus grandis × E. urophylla seedlings, L9(3⁴) orthogonal tests on nitrogen supplementation (10, 15, and 20 g/plant), phosphorus supplementation (3, 6, and 9 g/plant), and potassium supplementation (6, 9, and 12 g/plant) were conducted to select the most effective combined measurement. The results revealed that Eucalyptus had the highest values for total biomass (737.20 ± 32.74 g/plant), specific leaf area (196.73 ± 23.16 cm²/g), soluble sugar content (8.33 ± 0.75%), protein content (1.16 ± 0.19 g/L), and photosynthetic rate (20.02 ± 1.67 μmol/m²/s) under different combined conditions. Combination effects were observed under nitrogen, phosphorus, and potassium supplementation. Nitrogen was the most influential factor in the quality of the Eucalyptus seedlings, followed by phosphorus and potassium. We suggest that a combination of 15 g/plant nitrogen, 9 g/plant phosphorus, and 6 g/plant potassium produces the highest quality Eucalyptus seedlings.