Dose-dependent effect of He-Ne laser on Pb tolerance and phytoremediation potential of milk Thistle (Silybum Marianum L.) Gaertn

In recent decades, limited studies have explored the beneficial effects of seed irradiation with Helium-Neon (He-Ne) laser on secondary metabolism and stress tolerance in medicinal plants. However, the impact of laser on heavy metal tolerance and accumulation in these herbs is still largely unknown. To address this, a factorial experiment was conducted to examine the effects of various durations (0, 20, 40 min) of seed irradiation with He-Ne laser (10 mW m⁻²) on Pb tolerance and phytoremediation potential of milk thistle (Silybum marianum L.) in soils spiked with varying Pb concentrations (0, 250, 500 ppm). Exposure to both concentrations of Pb decreased emergence, growth parameters, and chlorophyll (Chl.) contents, and seed irradiation with the laser significantly affected these attributes in a dose-dependent manner. The 20 min laser irradiation increased emergence percentage (29.48%, 54.48%), dry mass of roots (52.31%, 38.07%) and shoots (23.36%, 31.71%), height (18.60%, 17.06%), tolerance index (30.29%, 29.35%), Chl. a (25.76%, 21.37%) and Chl. b (52.32%, 60.33%) contents, total phenol content (43.55%, 27.40%), and DPPH scavenging activity (30%, 18.37%) in leaves of milk thistle at 250, and 500 ppm of Pb in soil. This treatment also reduced electrolyte leakage in roots (38.52%, 36.01%) and in leaves (19.22%, 30.05%) by enhancing the activity of peroxidase and ascorbate peroxidase enzymes, particularly at 500 ppm of Pb. Contrarily, 40 min irradiation had adverse effects on growth and tolerance index, increased EL levels, and lowered antioxidant enzyme activity at all Pb levels, highlighting the critical role of fine-tuned dose regulation. Interestingly, 20 min irradiation to the laser reduced Pb concentration in roots (13.26%) without altering its translocation to shoots while increasing the total Pb uptake (21.14%) under 500 ppm of Pb. These findings highlight the potential of seed laser irradiation as an innovative and eco-friendly technique to enhance the phytoremediation efficiency of milk thistle while preserving the quality and safety of plants cultivated in Pb-polluted soils for medicinal purposes.