Dissecting possible correlations between leaf functional traits and heavy metal accumulation in two contrasting mangrove species across tidal gradients

The leaves of mangroves are a key component of the plant biomass, and they have a prominent function in heavy metal accumulation from contaminated sediments, which modulates their functional traits. However, it remains unclear how mangrove leaf heavy metals coordinate with leaf traits at different tidal levels. Thus, two species, exotic Sonneratia apetala and native Bruguiera sexangula, were selected to perform a qualitative study that shed light on the mechanisms underlying mangrove leaf functional traits and heavy metal dynamics (As, Pb, Cd, Cr, and Cu) across different tidal levels. The results showed that with increasing tidal levels individual leaf weight and individual leaf area increased in B. sexangula but decreased in S. apetala. The B. sexangula showed a markedly higher capacity for heavy metal accumulation, sequestering metals in the spongy mesophyll, contrasting with S. apetala, which concentrated metals in the palisade mesophyll. Both species peaked in leaf heavy metal concentrations at mid-tidal levels, a phenomenon linked to specialized leaf structural adjustments, increased phytohormone levels (ZR, JA-Me, IAA and ABA), and amplified detoxification mechanisms, including reduced glutathione, non-protein thiols, glutathione-S-transferase, flavonoids, tannins, and anthocyanins, which were facilitated by acidic pH and Fe plaque deposition on roots. Partial least squares path modeling further suggested that phytohormones influenced metal accumulation indirectly by altering leaf anatomical features and boosting detoxification. These results highlight the importance of phytohormones' regulatory role in determining the variation of heavy metals in both species, which has significant implications for applications in using and selecting mangrove plants for phytoremediation.