Silicon alleviates aluminum stress via distinct mechanisms in two Eucalyptus species differing in Al tolerance

Silicon (Si) is an environmentally friendly and non-toxic additive that can mitigate aluminum (Al) toxicity in plants. Eucalyptus , a widely cultivated tree species in southern China, faces growth limitation and Si deficiency under Al stress. However, the role of Si in reshaping plant functional-elemental networks under Al stress remains unclear. Hence, we conducted a pot experiment on Al-sensitive Eucalyptus ( E. tereticornis ) and Al-tolerant Eucalyptus ( E. urophylla ), with two Al levels (0 and 4.5 mM) and four Si levels (0, 0.5, 1.0 and 1.5 mM). Results showed that Al stress significantly reduced plant biomass, functional traits, photosynthesis, and multi-element accumulation in both species ( P < 0.05), with E. tereticornis more adversely affected. Silicon addition, particularly at 0.5 mM, markedly alleviated Al-induced growth inhibition by promoting biomass accumulation, improving functional traits, enhancing photosynthesis and water use efficiency, and increasing the uptake of essential nutrients (e.g., C, N, P, K, Si, Ca, Mg, Mn, Zn, and Fe). Notably, Si addition had different effects on Al accumulation: it reduced Al accumulation in E. tereticornis but increased it in E. urophylla . Network analysis further suggested that E. tereticornis primarily relied on Si-induced external Al exclusion by enhancing the uptake of competing cations (e.g., K + , Ca 2+ and Mg 2+ ), whereas E. urophylla adopted internal Si-mediated detoxification by promoting Al and Si co-accumulation and the formation of low-toxicity Al-Si complexes, thereby enhancing Al immobilization. Consequently, these findings highlight the distinct Si-mediated strategies for Al stress in two Eucalyptus species, providing novel insights into the Si alleviation mechanism under Al stress.