Transcriptome and metabolome analyses reveal the molecular mechanisms of Rhododendron pulchrum under cadmium stress

Soil contamination by cadmium (Cd) poses a persistent threat to plant growth and ecosystem health. To clarify the physiological and molecular basis of Cd tolerance in Rhododendron pulchrum, a woody ornamental species, seedlings were grown for 30 d in soil amended with five Cd levels (nominal 0, 10, 50, 100 and 200 mg·kg⁻¹; measured 0.33, 6.07, 20.27, 64.52 and 126.54 mg·kg⁻¹), and growth, physiological traits, and the leaf transcriptome and metabolome were analyzed. Low Cd did not inhibit growth and was accompanied by increased ground diameter and total biomass, whereas high Cd reduced growth, lowered pigment contents, and increased lipid peroxidation. Cd was retained predominantly in roots. Tissue concentrations followed the order root > stem > leaf, the translocation factor remained below 0.38 in all treatments, and 50.60-71.71% of the Cd accumulated by the plant remained in the roots. Omics analyses showed that Cd-responsive genes and metabolites were enriched in the phenylpropanoid and flavonoid biosynthesis pathways; key genes of these pathways (CCR, CAD, POD and DFR) were upregulated in leaves, together with the accumulation of flavonols and flavan-3-ols, indicating activation of antioxidant metabolism and ROS scavenging in leaf tissues. These results show that R. pulchrum combines high Cd tolerance with root-dominated Cd retention, supporting its use for the phytostabilization of soils with low to moderate Cd contamination.