Screening and mechanistic study of organic foliar agents for reducing mercury and methylmercury in pakchoi.

Foliar application has demonstrated potential in mitigating the bioaccumulation of heavy metals. Nevertheless, limited research has focused on the effects of foliar application in alleviating mercury (Hg) stress, especially for the more toxic methylmercury (MeHg). In the present study, several organic acids, including the rarely reported tartaric acid (TA) were initially screened for their efficacy through in-situ pot experiment in a mining area. Among them, tartaric acid and humic acid (HA) were more effective in reducing Hg accumulation in pakchoi. with reduction rates of 46 % and 31 %, respectively. To investigate the underlying mechanisms of this reduction, subsequent pot experiment was performed, incorporating additional soil application treatments of the organic acids for comparative analysis, thereby providing further insight into their modes of action. The results revealed that the foliar application of both TA and HA effectively reduced the total Hg (THg) concentrations in pakchoi, with TA showing a better effect (59 %). In contrast to foliar application, the application of TA and HA to the soil resulted in an overall increasing trend in THg content in pakchoi. Compared to the control, both foliar spraying and soil addition effectively reduced the MeHg concentrations in pakchoi (TA:45 %-56 % and HA:56 %-61 %). Compared to the control and soil application, foliar-applied TA effectively reduced Hg uptake by modulating mineral nutrient absorption, stimulating the synthesis of non-protein thiols (NPTs) for chelation and vacuolar sequestration of free Hg, and enhancing Hg fixation by the cell wall. No correlation was observed between plant antioxidant capacity and THg, indicating that enhanced antioxidative response is not a primary mechanism of foliar organic acid-induced THg reduction in plants. These findings indicate that foliar application of TA or HA offers a potential solution for the safe agricultural use of Hg-contaminated soils.