Unraveling the role of zinc for phytostabilization of chromium in petroleum hydrocarbon polluted soil by <i>Tagetes erecta</i>

Soil pollution due to heavy metals and petroleum hydrocarbons (PHCs), has become an alarming issue worldwide. Micronutrients, especially Zinc (Zn), have shown enormous capacity to improve growth and oxidative stress tolerance of plants under co-stressed conditions. The current study evaluated the capability of zinc to reduce Chromium (Cr) uptake in <i>Tagetes erecta L</i> exposed to PHCs toxicity. The pot experiment was carried out using soil amended with chromium (150 mg kg^-1) and PHCs (2 g crude oil/kg soil). Results demonstrated that Cr and PHCs stress significantly reduced growth metrics and photosynthetic pigments but substantially increased the contents of antioxidant enzymes, phenols, flavonoids, proline, malondialdehyde (MDA), and hydrogen peroxide (H₂O₂) contents in <i>T. erecta</i>. However, zinc application at various doses (3, 6, 12 mg kg^-1) ameliorated the toxicity of PHCs and Cr in <i>T erecta</i>. Specifically, Zn3 (12 mg kg^-1) treatment increased shoot weight, total chlorophyll, antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD), by 17.2, 18.3, 56, 49, and 67%, respectively in <i>T. erecta</i> under Cr + PHCs treatment. Additionally, the Zn3 application lowered MDA, H₂O₂, and Cr uptake in root and shoot by 23.5, 31.9, 36 and 42% respectively in <i>T. erect</i> subjected to Cr + PHCs stress.