Physiological and anatomical studies on coleus amboinicus lour Subjected to heavy metals stress

Abstract

The present study investigated the heavy metal-induced stress and tolerancepotential of Coleus amboinicus Lour., a widely used medicinal herb belongs toLamiaceae family. It is a large fleshy succulent perennial herb with aromaticpubescence and inherent medicinal properties due to the presence of manyphytochemicals and is known by a bunch of vernacular names ranging from Indianborage to “Panikoorka” in Malayalam.The plant's structural and functionalalterations were assessed through analyses of roots, stems, and leaves exposed toheavy metal stress. Rooted propagules were treated with Al (500 μM), Cr (150 μM),Cu (80 μM), and Hg (10 μM) in Hoagland nutrient medium. The immediateresponse to metal exposure was observed in morphology and anatomy. The effectsof heavy metals on development, growth, and metabolism were evaluated using arange of parameters, including stem, root, and leaf morphometry. Tolerance andstomatal indices were calculated to assess heavy metal impacts. Additionally,biochemical analyses were performed to quantify and qualify changes in protein,phenolic, proline, chlorophyll, and carotenoid pigment content. To investigate theheavy metal-induced oxidative stress, the activities of antioxidant enzymes,including catalase and superoxide dismutase, and malondialdehyde (MDA)production were analyzed in plant parts. Additionally, scanning electron microscopy(SEM) was employed to confirm anatomical changes in plant organs. Furthermore,SEM-energy dispersive X-ray spectroscopy (SEM-EDX) was used to determine thedistribution and localization of heavy metal ions in the plant tissues. Thebioaccumulation pattern of C. amboinicus for heavy metals was assessed usingInductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES). Given theplant's medicinal importance, Gas Chromatography-Mass Spectrometry (GC-MS)was employed to analyze the occurrence and distribution of bioactive secondarymetabolites.Morphological analysis revealed slight growth retardation in root lengthacross all metal treatments compared to the control, with no significant differences between metals. The tolerance index percentage showed significant reduction in allmetal treatments compared to the control, with negligible differences betweenmetals. The reduction in photosynthetic pigments like chlorophyll content alongwith increase in carotenoids provides the plant with protection from photo oxidativedamage.In C. amboinicus treated with Hg, there is an increased number of stomataand a wider stomatal opening; and also, the presence of lenticels. These traits aredirectly linked to the removal of volatile forms of Hg, which is an indicator oftoxicity sequestration. An interesting observation in the Cu treatment is the removalof oil globules from stem and leaf. SEM images give the clear evidences for theremoval of oil globules. The possible reason for the removal of oil globules may bedue to the reduction of a secondary metabolite named alpha-bergamotene after Cutreatment. It is a major component of essential oils in various plants and itcontributes to the aromatic profile and therapeutic properties of essential oils. Theenhanced cell wall thickness plays vital role in efficient sequestration of toxic levelsof Cu2+ in the cell wall, thereby preventing the entry of toxic ions to the cytoplasmand enhancing Cu stress tolerance in C. amboinicus. Anatomical examinationrevealed that plants treated with chromium, copper, and mercury exhibitedsignificant structural changes, including a broken epidermal layer and alterations invessel size and shape. In contrast, aluminium-treated plants showed minimalanatomical changes, likely due to aluminium's essential role as a micronutrient witha negligible stimulatory effect on plant growth. According to GCMS results, effectof heavy metals resulting in the absence of many secondary metabolites which isvital for the antibacterial property of C. amboinicus whereas occurrence of somenew bioactive components especially Tetradecanoic acid,12-methyl-methyl estershows same properties.