Environmental Engineering Reference
In-Depth Information
hyperaccumulator. species. two-grooved. milkvetch. (
Astragalus bisulcatus
).
and.prince's.plume.(
Stanleya pinnata
).
Plants. known. to. accumulate. high. levels. of. S. compounds,. such. as. many.
Brassica
.and.
Allium
.genera.(mustards.and.cabbages,.onion.and.garlic),.also.
are. good. Se. accumulators.. These. S-loving. species. accumulate. Se. to. fairly.
high.levels.(0.1%.of.dry.wt..or.1000.mg.Se/kg.dry.wt.).when.supplied.with.
adequate. external. Se. levels,. and. have. been. called. accumulator. species.. Se.
accumulator.species.likely.do.not.have.Se-speciic.pathways.but.take.up.and.
metabolize.Se.and.S.indiscriminately.at.elevated.rates.compared.to.nonac-
cumulators.. Se. hyperaccumulators. are. found. in. the. families. Brassicaceae,.
Fabaceae,. and. Asteraceae. and. typically. accumulate. Se. to. levels. 100-fold.
higher.than.surrounding.vegetation.in.the.ield.(Beath.et.al..1939a,b)..Since.
hyperaccumulators.preferentially.take.up.Se.over.S.and.show.different.pat-
terns. for. these. two. elements. in. terms. of. seasonal. luctuations. and. tissue.
distribution,. they. likely. are. able. to. distinguish. between. S. and. Se. (Galeas.
et. al.. 2007,. White. et. al.. 2007).. Hyperaccumulators. accumulate. Se. up. to. 1%.
of. their. dry. weight. (10,000.mg. Se/kg. dry. wt.). from. soil. typically. contain-
ing.2-10.mg.Se/kg.without.suffering.any.toxicity.(Neuhierl.and.Böck.1996,.
Neuhierl.et.al..1999,.Persans.and.Salt.2000,.Ellis.et.al..2004,.LeDuc.et.al..2004)..
Se.hyperaccumulators.are.endemic.to.seleniferous.soils.and.thus.appear.to.
physiologically.or.ecologically.require.Se..Perhaps.hyperaccumulators.need.
Se. as. a. defense. compound. against. herbivores. or. pathogens,. as. discussed.
in.more.detail.later..It.has.also.been.suggested.that.Se.may.be.essential.for.
hyperaccumulator. physiology,. since. hyperaccumulators. grow. much. better.
in. the. presence. of. Se. than. without. it. (more. than. twofold. higher. biomass.
production. in. some. experiments).. However,. to. date. there. is. no. proof. that.
hyperaccumulators. or. any. higher. plants. require. Se. to. complete. their. life.
cycle.. The. positive. growth. response. of. hyperaccumulators. to. Se. may. also.
be.due.to.alleviation.of.phosphorus.toxicity,.which.is.much.less.pronounced.
when.plants.are.grown.at.lower.phosphorus.levels.(Broyer.et.al..1972)..In.the.
following.text,.we.give.an.overview.of.Se.metabolism.in.plants.in.nonhyper-
accumulators.and.hyperaccumulators.
SeMetabolisminPlants
The. predominant. forms. of. Se. in. the. environment. are. inorganic. selenate.
and.selenite..Selenate,.or.Se(VI),.is.the.most.oxidized.form.of.Se.and.the.pre-
dominant.bioavailable.form.in.oxic.soils,.while.selenite,.or.Se(IV),.is.more.
abundant.in. (more.anoxic).wetland.conditions..In.addition,.inorganic.ele-
mental.Se,.Se(0),.can.become.dominant.under.reducing,.anoxic.conditions..
Both.selenate.and.selenite.are.bioavailable.and.readily.taken.up.by.plants..
Selenate. is. taken. up. and. mobilized. in. plants. by. means. of. sulfate-proton.
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