Divergent Application Pathways of Silicon and Phytohormone in Mitigating Cadmium Accumulation in Rice

Heavy metal(loid) contaminated soil is widespread in the word. Exogenous addition of phytohormones have been proved to mitigate Cd toxicity, but the co-application of different phytohormones with Si through plant root or spraying are still limited. It is necessary to investigate an efficient strategy using phytohormones to ensure grain safety in Cd-polluted paddy fields. Three phytohormones (including melatonin [MT], jasmonic acid [JA] and salicylic acid [SA]) were applied alone or in combination with silicon (Si) fertilizer in a hydroponic experiment, screening MT as the best phytohormone on decreasing Cd accumulation and enhancing plant growth. Then a pot experiment was conduced to evaluate the MT application method (root or spraying) and the associated mechanisms. Hydroponic experiment revealed that foliar phytohormone application significantly increased total root length by 97.72–127.40% and improved chlorophyll synthesis. However, root-applied MT exhibited the most notable reduction on Cd accumulation (by 56.96%) and translocation factor (by 64.13%). Consequently, MT was selected for further validation in a soil-pot experiment through foliar and root application. The pot results confirmed that foliar Si and MT co-application was the most effective strategy, reducing Cd concentrations in grains, husks, leaves, shoots, and roots by 37.8%, 74.3%, 72.2%, 45.1%, and 30.3%, respectively. This maybe due to the subcellular sequestration (cell wall fraction increased by 17.3%), and the increased antioxidant protection. This study demonstrates that the co-application of Si and MT is a cost-effective, highly efficient strategy for ensuring grain safety in Cd-polluted paddy fields. Graphical