Antioxidant Responses in Mangrove Roots Confer Adaptation to Mature Laguncularia racemosa C.F. Gaertn Exposed to Crude Petroleum Oil in Acidic Soil

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Crude petroleum oil (CPO), acting as an abiotic soil factor, significantly influences the chemical properties of tropical riverine mangrove roots; however, there has been a lack of systematic explanation regarding the effects of varying concentrations of CPO on reactive oxygen species synthesis, antioxidant activity, lipid peroxidation, and non-structural carbohydrates in the roots of young and mature mangrove trees, particularly concerning their adaptation to contaminated acidic organic soils. Thus, levels of H2O2, enzymatic and non-enzymatic antioxidants, proline, glucose, fructose, sucrose and malondialdehyde (MDA) were studied in roots of young (≤five years) Laguncularia racemosa (Lr1) and Rhizophora mangle (Rm) trees and mature (≥40 years) Laguncularia racemosa (Lr2) trees. The roots of all trees were sampled in soil that was non-toxic (1.2 to 1.6 g/kg) and toxic (6.8 to 6.9 g/kg) with respect to total petroleum hydrocarbons (TPHs), respectively. The roots of Lr2 and Lr1 trees responded to toxic CPO with increased levels of phenolic acids and phenylalanine amino lyase (PAL) activity in addition to increased FRAP (ferric reducing antioxidant power) and total antioxidant levels. A significant increase in MDA levels, indicating oxidative stress conditions in Lr2 exposed to CPO, proved important, despite elevated antioxidant levels; this suggests that the activated antioxidant response fails to completely prevent membrane damage. Remarkably, most chemical indicators of antioxidant activity and carbon mobilization analyzed in this study were significantly depressed in roots of young Rm trees. Thus, Lr2 trees, followed by Lr1, appear to have adapted better than their Rm counterparts to exposure to CPO contaminants, a trait that may provide a competitive advantage in contaminated riverine–lagoon forests.