Arsenic (As) contamination in soil presents significant challenges to plant growth and development, impacting agricultural productivity, food safety, ecosystem stability, and human health. This study investigates the effects of As toxicity on the medicinal plant Ocimum basilicum L. cultivar CIM-Saumya by assessing the impact of varying As concentrations (1, 5, 10, and 25 mg kg-1 of soil) on various physio-biochemical and microscopic parameters. Controlled experiments were conducted to assess the photosynthetic rate, gas exchange, and the activities of carbonic anhydrase (CA), Rubisco, and nitrate reductase (NR) enzymes. In addition, the concentrations of non-enzymatic antioxidants (proline, flavonoids, and phenolic compounds) and enzymatic antioxidants (superoxide dismutase (SOD), catalase (CAT), and peroxidase (POX) were analyzed. Alterations in glandular trichomes, essential oil (EO) content, and composition were also evaluated. Confocal laser scanning microscopy (CLSM) was utilized to examine root cell viability and detect reactive oxygen species (ROS). Our results revealed that As exposure significantly inhibited physio-biochemical activities in O. basilicum, with low As concentrations (1 mg kg-1) enhancing EO content by 18.75 %. However, higher As concentrations (25 mg kg-1) induced oxidative stress, evidenced by increased malondialdehyde (MDA), ROS accumulation, reduced trichome size and density, and smaller stomatal apertures. The highest As concentration resulted in a 53.12 % reduction in EO content. These findings demonstrate that O. basilicum exhibits differential responses to As stress, with low concentrations enhancing EO production, while high concentrations cause oxidative damage and reduced EO content, providing insights into the plant's adaptive strategies and potential alterations in its aroma and therapeutic properties under As stress.

Trichomes play a key role in both heavy metal tolerance and herbivory defense, and both stressors have been shown to induce increased trichome density. However, the combined effect of these stressors on trichome density in general, and specifically on metal-hyperaccumulating plants, has yet to be examined. The aim of this study was to test the effect of cadmium availability and herbivory on leaf trichome density and herbivore deterrence in the metal hyperaccumulator Helianthus annuus. To test this, H. Annuus plants were grown in control pots or pots inoculated with 10 mg/kg cadmium and were subjected to either no herbivory or simulated herbivory using mechanical damage and foliar jasmonic acid application. Herbivore deterrence was tested in a feeding assay using Spodoptera littoralis caterpillars. Interestingly, while the trichome density of H. annuus increased by 79% or 53.5% under high cadmium availability or simulated herbivory, respectively, it decreased by 26% when the stressors were combined. Furthermore, regardless of cadmium availability, simulated herbivory induced a 40% increase in deterrence of S. littoralis. These findings suggest that the combination of metal availability and herbivory might present excessive stress to hyperaccumulators. Moreover, they suggest that the risk of metal bioaccumulation in phytoremediation can be reduced by simulated herbivory.