LAUSR

Abstract Growing medicinal herbs on chromium-rich soils can pose severe threats to humans, animals, plants, and ecosystems. Leaf senescence is a typical sign of chromium contamination and may be regulated by foliar application (melatonin) or soil amendment (zeolite) treatments. The favorable roles of melatonin and/or zeolite in the alleviation of chromium-induced senescence processes related to growth, some biochemical and ultrastructural attributes, and essential oil yield in marjoram (Origanum marjorana L.) plants were studied. Marjoram biomass, photosynthetic pigment concentration, nutrient percentage, water status, ascorbic acid level, essential oil yield, and catalase activity declined significantly under chromium-pollution. On the other hand, organic and antioxidant solute levels, superoxide dismutase and guiacol peroxidase activity, oxidized biomolecule levels, as well as chromium concentrations were increased in the herb under chromium contamination. Melatonin and/or zeolite application under normal or chromium-polluted conditions may compensate to some extent growth retardation, essential oil yield, and metabolic disturbance processes. The greatest herbal essential oil yield was obtained in the case of the application of melatonin plus zeolite under normal conditions. Under chromium pollution, cell organelles exhibited a noticeable malformation and lysis, but, melatonin plus zeolite application counteracts the malformation observed with chromium-pollution. The current investigation postulated that either soil amendment with 5 g zeolite kg−1 soil and/or foliar application with 100 μM melatonin have found a useful role to mitigate chromium-induced senescence processes in marjoram herb. Their ameliorating effects may be connected with the regulation of reactive oxygen species scavenging systems, stimulation of osmotic adjustment and preservation of plant cell ultrastructure. Melatonin and/or zeolite can make chromium-rich soils appropriate for the cultivation and production of high-quality herbs by minimizing chromium bioavailability and toxicity to plants.