Synergistic roles of zinc and boron in enhancing growth, stress physiology, and heavy metal tolerance in Brassica rapa L.

Abstract Heavy metal contamination in soil impairs plant growth and disrupts cellular functions. Brassica rapa L. exhibits phytoremediation potential owing to its strong metal tolerance and accumulation capacity. This study evaluated the effects of boron (B) and zinc (Zn), applied alone or together at 10–25 mg kg −1 , on growth, biochemical traits, and heavy-metal stress tolerance in B. rapa L. The result established B + Zn > B > Zn. Co-application of B and Zn at 15–20 mg kg −1 significantly enhanced growth and tolerance to metal-induced stress compared with individual treatments ( p < 0.05). At 20 mg kg −1 B + Zn, key biochemical markers, including chlorophyll, sugar, phenolic, carotenoid, protein, and proline levels, were significantly higher compared with individual treatments. Elemental analysis and PCA showed that the optimal 15–20 mg kg −1 B + Zn dose promoted the uptake of essential elements such as Fe and Mn. Simultaneously, reduced toxic metals accumulation (Cd and Pb). Conversely, the 25 mg kg −1 treatment clustered with elevated toxic metals, with Cd showing the highest accumulation, followed by Cr and Pb. The weak linear correlation ( R 2 < 0.1 ) observed across parameters indicates that these physiological benefits arise from complex, non-linear regulation. These findings underscore the critical role of precise micronutrient synergy in enhancing heavy-metal tolerance mechanisms in crops, and they are vital for optimizing phytoremediation and ensuring food safety in metal-contaminated agroecosystems.