Role of the molecular structure of humified organic matter in rice plant response to environmental lead pollution.

Humified organic matter has been shown to decrease Pb toxicity in plants. However, there are still gaps in our understanding of the mechanism by which this phenomenon occurs. In this study, we aimed to assess the ability of humic substances (HSs), humic acids (HAs), and fulvic acids (FAs) to enhance defense mechanisms in rice plants under lead (Pb)-stressed conditions. HS fractions were isolated from vermicompost using the chemical fractionation methodology established by the International Humic Substances Society. These fractions were characterized by solid-state NMR and FTIR. Chemometric analysis was used to compare humic structures and correlate them with bioactivity. Three treatments were tested to evaluate the protective effect of humic fractions on rice plants. The first experiment involved the application of humic fractions along with Pb. The second comprised pretreatment with humic fractions followed by subsequent exposure to Pb stress. The third experiment involved Pb stress and subsequent treatment with humic fractions. The root morphology and components of the antioxidative defense system were evaluated and quantified. The results showed that HS + Pb, HA + Pb, and FA + Pb treatment preserved root growth and reduced the levels of O and malondialdehyde (MDA) in the roots by up to 5% and 2%, respectively. Pretreatment of the plants with humic fractions promoted the maintenance of root growth and reduced the contents of O, HO, and MDA by up to 48%, 22%, and 20%, respectively. Combined application of humic fractions and Pb reduced the Pb content in plant tissues by up to 60%, while pretreatment reduced it by up to 80%. The protective capacity of humic fractions is related to the presence of peptides, lignin, and carbohydrate fragments in their molecular structures. These results suggest that products could be developed that can mitigate the adverse effects of heavy metals on agricultural crops.