Toxicity and Bioavailability of Heavy Metals and Hydrocarbons in Mangrove Wetlands and Their Remediation - Environmental Contamination: Health Risks and Ecological Restoration - page 379

species. B. gymnorrhiza .had.the.least.accumulation.of.Fe.plaque.on.root.sur-
face..All.these.studies.suggested.the.importance.of.the.interactions.between.
ROL.and.Fe.plaque.formation.and.their.effects,.which.merits.further.study.
Biochemical Response
Oxidative. stress. on. plants. is. associated. with. an. excessive. generation. of.
reactive. oxygen. species. (ROS),. including. superoxide. ( O 2 − ),. hydrogen. per-
oxide. (H 2 O 2 ),. and. hydroxyl. free. radicals. (·OH),. which. can. be. induced. by.
various.environmental.stresses.such.as.extreme.temperatures,.salt,.hypoxia,.
UV. radiation,. and. toxic. pollutants. (Bartosz. 1997).. The. highly. reactive. and.
nonspeciic. nature. of. ROS. has. been. reported. to. contribute. to. cell. dam-
age,. including. lipid. peroxidation,. inactivation. of. enzymes. and. other. func-
tional.proteins,.base.modiications,.and.strand.breaks.of.DNA.(reviewed.by.
Bartosz.1997)..MDA.is.a.inal.decomposition.product.of.lipid.peroxidation..
The. positive. relationship. between. ROS. and. MDA. accumulation. (Smirnoff.
1995). suggests. that. MDA. can. be. used. as. an. indicator. of. the. status. of. lipid.
peroxidation..In.response.to.oxidative.stress,.plants.have.developed.a.vari-
ety.of.antioxidative.defense.systems.to.scavenge.excessive.ROS.and.dimin-
ish.their.harm.to.plants..The.antioxidative.defense.systems.can.be.classiied.
into.two.groups,.antioxidative.enzymatic.system.and.nonenzymatic.system..
Antioxidative. enzymes. include. SOD,. POD,. and. catalase. (CAT). (Scandalios.
1993,. Vangronsveld. and. Clijsters. 1994).. SOD. is. involved. in. the. irst. step. of.
ROS. elimination,. which. catalyzes. the. conversion. of. O 2 − . to. H 2 O 2, . and. O 2 ,.
and.H 2 O 2 .can.be.further.decomposed.by.CAT.and.POD.(Parida.et.al..2004)..
Nonenzymatic.constituents.include.glutathione,.carotenoids,.and.ascorbate.
phenolic. compounds. (Tukendorf. and. Rauser. 1990).. Phenolic. compounds,.
including.tannins.and.derived.polyphenols,.are.among.the.most.widely.dis-
tributed. plant. secondary. metabolites. and. can. be. induced. by. stress. (Dixon.
and.Paiva.1995)..Phenolic.compounds,.mainly.due.to.their.redox.properties,.
also.play.an.important.role.in.absorbing.and.neutralizing.free.radicals.and.
can. function. as. antioxidants. (Lavid. et. al.. 2001).. The. antioxidant. capacity.
of. phenolic. compounds. is. also. attributed. to. their. ability. to. chelate. metals.
(Hernes.et.al..2001,.Schützendübel.and.Polle.2002,.Kraus.et.al..2003).
The. activities. of. antioxidative. enzymes. and. contents. of. nonenzymatic.
constituents. are. associated. with. plant. tolerance. to. metal. stress;. however,.
both.positive.and.negative.responses.have.been.observed,.depending.on.the.
plant. species. and. tissue. analyzed,. as. well. as. the. species. and. intensity. of.
the.metal.stress.(Schützendübel.and.Polle.2002)..POD.activities.in.the.leaves.
of. A. marina . were. positively. correlated. with. the. amounts. of. heavy. metals.
(Cu,. Zn,. and. Pb). accumulated. in. leaf. tissues. (MacFarlane. 2002).. However,.
Liu.et.al..(2008).found.that.both.SOD.and.POD.activities.in.the.roots.of. Sedum
alfredii .were.negatively.correlated.with.Pb.concentrations.in.the.substrate.
Research.on.antioxidative.defense.systems.in.mangrove.plants,.in.response.
to. environmental. stresses,. including. waterlogging,. salt,. wastewater,. heavy.