Subcellular distribution and chemical speciation of lead in Panicum repens under different cultivation methods

Torpedo grass (Panicum repens) has ecological restoration advantages in polluted water-level-fluctuation zones of rivers, lakes, and water reservoirs because of its strong ability to purify heavy metals and other pollutants and remarkable adaptability to extreme drought and flooding conditions. Through three pot experiments with different lead treatments (T₁: hydroponic culture + 400 mg·L⁻¹ Pb; T₂: hydroponic culture + 800 mg·L⁻¹ Pb; T₃: soil culture + 400 mg·L⁻¹ Pb), the subcellular and chemical distribution characteristics of the roots, stems, and leaves of torpedo grass were investigated after 5, 15, and 25 days of treatment. This study aimed to uncover the heavy metal tolerance mechanisms and dynamic patterns of migration and transport in torpedo grass under different concentrations of Pb stress and cultivation methods. The results showed that the migration rate of the roots of torpedo grass to the upper part of the plant significantly decreased (P < 0.05) with the increase in Pb treatment concentration (T₁→T₂) and the transition from of soil cultivation to hydroponics (T₁→T₃), while it significantly increased with treatment time (P < 0.05). The average Pb contents of the roots under T₁, T₂, and T₃ treatments were 5148.56, 6721.51, and 257.73 mg·kg⁻¹, respectively, whereas the corresponding stem/root ratios were 1.37%, 1.44%, and 26.42%, respectively. Torpedo grass mainly detoxified Pb damage through the cell wall, with average maximum percentages of 69.10%, 73.13%, and 66.55% for T₁, T₂, and T₃, respectively. The second most important component is the cellular state, followed by soluble components, with significant differences in the proportion of each subcellular fraction component (P < 0.05). The average Pb content in the cell wall of the same organ showed the trend: T₂ > T₁ > T₃. The chemical state of Pb in the root and stem was mainly distributed in the less active HAc and HCl states, respectively. The distribution ratio of Pb in the leaves of torpedo grass was not apparent, and the distribution of Pb in various chemical states tended to be balanced, ranging from 11.49% to 23.81%. The accumulation of Pb (T₁ and T₂) in the roots of torpedo grass showed an inflection point at 15 d, with a significant increase from 5th day to 15th day (P < 0.05), followed by a slow decrease from 15th day to 25th day (P > 0.05). This indicated that the accumulation of Pb in the roots of torpedo grass reached the absorption limit at 15th day, which may be attributed to the activation of a specific escape mechanism in the roots of torpedo grass. Overall, the root Pb accumulation ability of torpedo grass under pure hydroponic conditions was significantly greater than that under soil cultivation, making it an apparent advantage for purifying polluted riverbank deep-water areas.