Environmental Engineering Reference
In-Depth Information
epidermis. and. hypodermis. was. increased,. leading. to. a. reduced. oxygen.
supply. to. the. root. tip.. Compared. to. B. gymnorrhiza . and. E. agallocha ,. the.
epidermis.and.hypodermis.layers.in. Acanthus ilicifolius .were.the.thinnest,.
in. which. the. cells. without. suberized. walls. had. the. least. protection. from.
the. harmful. effect. of. wastewater. exposure.. These. results. suggested. that.
B. gymnorrhiza . was. the. least. affected. by. wastewater. discharge,. followed.
by. E. agallocha ,.and. Acanthus ilicifolius .was.the.most.susceptible.species.and.
was.not.suitable.for.treating.strong.wastewater.
The.ROL.spatial.patterns.of.mangrove.plants.were.found.to.have.a.direct.
link.to.their.metal.tolerance..Liu.et.al..(2009).investigated.the.toxicity.of.mixed.
heavy. metals. of. Pb,. Zn,. and. Cu. at. different. concentrations. (low,. medium,.
and. high:. 50-100-50,. 100-200-100,. and. 200-400-200.mg. Pb-Zn-Cu/kg. dry.
sand,.respectively).to.6.months.old.mangrove.seedlings.of. Aegiceras cornicu-
latum ,. A. marina ,.and. B. gymnorrhiza ..Heavy.metals.inhibited.plant.growth,.
reduced. the. amount. of. ROL,. and. changed. the. ROL. spatial. pattern,. which.
were.positively.correlated.with.metal.tolerance.. B. gymnorrhiza .had.the.high-
est. ROL. amount. and. the. tightest. barrier. to. ROL,. and. it. was. also. the. most.
tolerant. species. to. heavy. metals.. In. another. study. by. Cheng. et. al.. (2010),.
plant.uptake.and.tolerance.of.1.year.old.seedlings.of. Aegiceras corniculatum ,.
B. gymnorrhiza ,.and. Rhizophora stylosa .Griff.to.Zn.toxicity.were.compared..Zn.
exposure.led.to.a.signiicant.decrease.in.ROL.in.roots.of.all.three.species,.but.
the.degree.of.reduction.varied.among.different.species,.with.the.reduction.
percentages.following.the.descending.order.of. R. stylosa .>. B. gymnorrhiza .>.
Aegiceras corniculatum .(Figure.17.2)..The.ROL.reduction.was.suggested.to.be.
due.to.the.inhibition.of.root.permeability,.including.an.obvious.thickening.of.
outer.cortex.and.a.signiicant.increase.in.ligniication.of.cell.walls.. B. gymnor-
rhiza ,.which.possessed.the.“tightest.barrier”.to.ROL,.accumulated.the.least.
Zn.and.had.the.highest.Zn.tolerance..This.study.provided.new.evidence.of.
the.structural.adaptations.of.mangrove.plants.to.metal.tolerance.
Fe Plaque Formation
Another.important.consequence.of.ROL.is.the.induction.of.Fe.precipitation.
in.the.rhizosphere,.creating.iron-rich.root.coatings.or.Fe.plaque.on.root.sur-
faces.(Otte.et.al..1989,.Guo.et.al..2007,.Hu.et.al..2007)..Fe.plaque.is.a.mixture.
of. crystalline. and. amorphous. ferric. hydroxides. goethite. and. lepidocrocite.
(St-Cyr. et. al.. 1993,. Hu. et. al.. 2007,. Chen. et. al.. 2008a).. Its. formation. is. con-
trolled. by. a. number. of. physiochemical. factors,. such. as. the. availability. of.
Fe 2+ ,. organic. matter,. soil. texture,. and. redox. potential. (Mendelssohn. et. al..
1995)..Wastewater-borne.pollutants.also.affect.the.formation.of.Fe.plaque..Pi.
et.al..(2010a).reported.that.Fe.plaque.formation.on.the.root.surface.of.1.year.
old.seedlings.of. B. gymnorrhiza .and. E. agallocha .increased.signiicantly.after.
105.days.of.exposure.to.strong.synthetic.wastewater.(10.NW)..However,.the.
degree.of.increase.in.different.parts.of.the.root.differed.from.species.to.spe-
cies,. with. more. signiicant. increases. toward. the. basal. and. mature. zones.
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