Both sulfur (S) supply and legume-rhizobium symbiosis can significantly contribute to enhancing the efficiency of phytoremediation of heavy metals (HMs). However, the regulatory mechanism determining the performance of legumes at lead (Pb) exposure have not been elucidated. Here, we cultivated black locust ( Robinia pseudoacacia L.), a leguminous woody pioneer species at three S supply levels ( i.e., deficient, moderate, and high S) with rhizobia inoculation and investigated the interaction of these treatments upon Pb exposure. Our results revealed that the root system of Robinia has a strong Pb accumulation and anti-oxidative capacity that protect the leaves from Pb toxicity. Compared with moderate S supply, high S supply significantly increased Pb accumulation in roots by promoting the synthesis of reduced S compounds ( i.e., thiols, phytochelatin), and also strengthened the antioxidant system in leaves. Weakened defense at deficient S supply was indicated by enhanced oxidative damage. Rhizobia inoculation alleviated the oxidative damage of its Robinia host by immobilizing Pb to reduce its absorption by root cells. Together with enhanced Pb chelation in leaves, these mechanisms strengthen Pb detoxification in the Robinia-rhizobia symbiosis. Our results indicate that appropriate S supply can improve the defense of legume-rhizobia symbiosis against HM toxicity.

Boron (B) is an essential micronutrient for plant growth and development; however, the process of B toxicity in citrus production is still poorly understood. We proposed a hypothesis that B toxicity in citrus trees is related to the characteristics of B transport from soil to leaf or fruit. For this, a field experiment was conducted for two treatments, control (B free or without B) and B fertilizer treatment (100 g Na2B4O710H2O plant-1), to investigate the effects on plant growth, nutrient uptake, fruit yield and quality, and B transport in 10-year-old pomelo trees [Citrus grandis (L.) Osbeck cv. Guanximiyou]. Our results showed that excess B fertilization directly led to B toxicity in pomelo trees by dramatically increasing soil total B and water-soluble B contents. B toxicity induced interveinal chlorosis in leaves and decreased leaf biomass and function, resulting in a decreased 45.3% fruit yield by reducing 30.6% fruit load and 21.4% single fruit weight. Also, B toxicity induced changes in mineral elements between leaf positions and fruit parts, in which the concentrations of B, potassium, and magnesium were increased while those of nitrogen and iron were decreased. Under B toxicity conditions, fruit quality parameters of total soluble solids (TSS), TSS/titratable acidity (TA), total soluble sugar, sucrose, pH, vitamin C, and total phenol contents decreased, which were regulated by the lower carbohydrate production in new leaves and the lower transport capacity in old leaves. Moreover, B toxicity significantly increased the transfer factor and bio-concentration factor of B in pomelo plants, with higher levels in leaf organs than in fruit organs. Taken together, excess B fertilization-induced B toxicity in pomelo trees, with induced growth inhibition and nutrient disorder, results in reduced fruit yield and quality, which are related to B transport from soil to organs. The findings of this study highlight the understanding of B toxicity in citrus plants and strengthen B management in pomelo production for high yield and high quality.

The prolonged exposure of agricultural soils to heavy metals from wastewater, particularly in areas near industrial facilities, poses a significant threat to the well-being of living organisms. The World Health Organization (WHO) has established standard permissible limits for heavy metals in agricultural soils to mitigate potential health hazards. Nevertheless, some agricultural fields continue to be irrigated with wastewater containing industrial effluents. This study aimed to assess the concentration of lead in soil samples collected from agricultural fields near industrial areas. Subsequently, we determined the lethal concentration (LC50) of lead (Pb) and other heavy metals for two Collembola species, namely Folsomia candida , a standard organism for soil ecotoxicity tests, and comparing it with Proisotoma minuta . The research further examined the toxic effects of lead exposure on these two species, revealing depletion in the energy reservoirs and alterations in the tissue histology of both organisms. The study revealed that lead can induce genotoxic damage as it evidently has moderate binding affinity with the ct-DNA and hence can cause DNA fragmentation and the formation of micronuclei. Elevated lipid peroxidation (LPO) levels and protein carbonylation levels were observed, alongside a reduction in antioxidant enzymes (CAT, SOD & GPx). These findings suggest that lead disrupts the balance between oxidants and the antioxidant enzyme system, impairing defense mechanisms and consequential derogatory damage within microarthropods. The investigation elucidates a complex network of various signaling pathways compromised as a result of lead toxicity. Hence, it presents a novel perspective that underscores the pressing necessity for implementing an integrated risk assessment framework at the investigated site.