Effects of iron on the resistance physiology of <italic>Cunninghamia lanceolata</italic> under coupled stress of phosphorus deficiency and aluminum toxicity

[Objective] To elucidate the regulatory mechanisms underlying iron(Fe)meditated aluminum(Al)tolerance of Cunninghamia lanceolata under phosphorus(P)deficiency and aluminum stress, and to provide references for improving Al tolerance of C.lanceolata through nutrient management strategies.[Methods] A sand culture method was employed in this study with four treatments, including control(CK), aluminum stress(Al), P deficiency coupled with Al stress with Fe addition(-P+Al+Fe), and P deficiency coupled with Al stress without Fe addition(-P+Al-Fe), to investigate the effects of Fe on resistance physiology and cell wall components in root tips of C.lanceolata under P deficiency and Al stress conditions, using the superior genotype material of C.lanceolata seedlings YX11.[Results] We found that Al treatment significantly increased the contents of malondialdehyde(MDA), hydrogen peroxide(H2O2)and superoxide anion(O2-)in the root tips of C.lanceolata seedlings compared with CK, and the -P+Al+Fe treatment further increased MDA, H2O2 and O2- contents, while the -P+Al-Fe treatment significantly decreased MDA, H2O2 and O2- contents compared with the -P+Al+Fe treatment, and no significant differences were observed between -P+Al-Fe and Al treatments(P&gt;0.05).Compared with CK, a significant increase in antioxidant activities was found under different stress treatments, and the antioxidant enzyme activities in the root tips of the -P+Al-Fe treatment were all significantly higher than those in the -P+Al+Fe treatment(P&lt;0.05).Additionally, we also found that the contents of reduced glutathione and citric acid of C.lanceolata in the -P+Al-Fe treatment were all significantly higher than those in the -P+Al+Fe treatment(P&lt;0.05).Compared with CK, Al treatment significantly increased the pectin content in root cell wall and enhanced pectin methylesterase activity, which in turn decreased the degree of cell wall pectin methylesterification; the -P+Al+Fe treatment further intensified these processes, while the opposite trend was also observed in the -P+Al-Fe treatment.[Conclusion] In summary, under combined P deficiency and Al toxicity stress, Fe-mediated changes in the composition and properties of pectin promoted the accumulation of Fe and Al in roots, thereby leading to oxidative damage in roots.Meanwhile, the inhibition of key antioxidant enzyme activities and the decrease in citric acid content in roots jointly diminished the detoxification capacity of C.lanceolata roots, thus aggravating stress-induced oxidative damage.