Uptake of copper by plants is affected by many factorsincluding the soil pH, the prevailing chemical species, and the concentration of copper present inthe soil.. Uptake ofcopper by plant
Trang 110 Copper
David E Kopsell
University of Wisconsin-Platteville, Platteville, Wisconsin
Dean A Kopsell
University of Tennessee, Knoxville, Tennessee
CONTENTS
10.1 The Element Copper 293
10.1.1 Introduction 293
10.1.2 Copper Chemistry 294
10.2 Copper in Plants 294
10.2.1 Introduction 294
10.2.2 Uptake and Metabolism 294
10.2.3 Phytoremediation 313
10.3 Copper Deficiency in Plants 314
10.4 Copper Toxicity in Plants 315
10.5 Copper in the Soil 316
10.5.1 Introduction 316
10.5.2 Geological Distribution of Copper in Soils 317
10.5.3 Copper Availability in Soils 317
10.6 Copper in Human and Animal Nutrition 321
10.6.1 Introduction 321
10.6.2 Dietary Sources of Copper 321
10.6.3 Metabolism of Copper Forms 321
10.7 Copper and Human Health 322
10.7.1 Introduction 322
10.7.2 Copper Deficiency and Toxicity in Humans 322
References 323
10.1 THE ELEMENT COPPER
10.1.1 INTRODUCTION
Copper is one of the oldest known metals and is the 25th most abundant element in the Earth’s crust The words ‘aes Cyprium’ appeared in Roman writings describing copper, to denote that much of the metal at the time came from Cyprus Refinement of copper metal dates back to 5000 BC The metal by itself is soft, but when mixed with zinc produces brass and when mixed with tin produces bronze Copper is malleable, ductile, and a good conductor of electricity In its natural state, it is a reddish solid with a bright metallic luster
293
Trang 2Copper has an atomic number 29 and atomic mass of 63.55 It belongs to Group I-B transition als The melting point of copper is 1084.6°C Copper occurs naturally in the cuprous (I, Cu⫹) andcupric (II, Cu2 ⫹) valence states There is a single electron in the outer 4s orbital The 3d10orbitaldoes not effectively shield this outer electron from the positive nuclear charge, and therefore the 4s1electron is difficult to remove from the Cu atom (1) The first ionization potential is 7.72 eV and thesecond is 20.29 eV Because the second ionization potential is much higher than the first, a variety
met-of stable Cu⫹species exist (2) The ionization state of copper depends on the physical environment,the solvent, and the concentration of ligands present In solution, copper is present as Cu2 ⫹or com-plexes of this ion The cuprous ion Cu1 ⫹is unstable in aqueous solutions at concentrations greaterthan 10⫺7M (3) However, in wet soils, Cu1 ⫹is moderately stable at typically expected conditions(10⫺6to 10⫺7M) Under such conditions, hydrated Cu1 ⫹would be the dominant copper species (1).Copper can exist as two natural isotopes,63Cu and 65Cu, with relative abundances of 69.09 and30.91%, respectively (4) In the Earth’s crust, copper is present as stable sulfides in minerals ratherthan silicates or oxides (3) The Cu1 ⫹ion is present more commonly in minerals formed at consid-erable depth, whereas Cu2 ⫹is present close to the Earth’s surface (3)
The transition metals are noted for the variety of complexes they form with bases In these plexes, Cu1 ⫹and Cu2 ⫹act as electron acceptors Chelating bases are so named because they havetwo or more electron donor sites (often on O, S, or N atoms) that form a ‘claw’ around the copperion (1) Such complexes are important in soil chemistry and in plant nutrition The Cu1 ⫹ion formsstrong complexes with bases containing S, but Cu2 ⫹does not In the presence of these bases, Cu2 ⫹
com-acts as a strong oxidant (2)
10.2 COPPER IN PLANTS
10.2.1 INTRODUCTION
Copper was identified as a plant nutrient in the 1930s (5,6) Prior to this realization, one of the firstuses of copper in agriculture was in chemical weed control (7) Despite its essentiality, copper istoxic to plants at high concentrations (8) Uptake of copper by plants is affected by many factorsincluding the soil pH, the prevailing chemical species, and the concentration of copper present inthe soil Once inside the plant, copper is sparingly immobile Accumulation and expression of toxicsymptoms are often observed with root tissues Extensive use of copper-containing fungicides inlocalized areas and contamination of soils adjacent to mining operations has created problems oftoxicity in some agricultural regions Because of this problem, remediation of copper andidentification of tolerant plant species are receiving increased attention Concentrations of copper
in some plant species under different cultural conditions are reported in Table 10.1
The rate of copper uptake in plants is among the lowest of all the essential elements (9) Uptake ofcopper by plant roots is an active process, affected mainly by the copper species Copper is mostreadily available at or below pH 6.0 (4) Most sources report copper availability in soils to decreaseabove pH 7.0 Increasing soil pH will cause copper to bind more strongly to soil components.Copper bioavailability is increased under slightly acidic conditions due to the increase of Cu2 ⫹ions
in the soil solution On two soils in Spain, with similar pH values (8.0 and 8.1) but with differentcopper levels (0.64 and 1.92 mg Cu kg⫺1, respectively), leaf content of willow leaf foxglove
(Digitalis obscura L.) was equal, i.e., 7 mg kg⫺1dry weight on both soils (10) Copper
concentra-tions of tomato (Lycopersicon esculentum Mill.) and oilseed rape (canola, Brassica napus L.)
roots and shoots were significantly higher in an acidic soil (pH 4.3) than in a calcareous soil (pH8.7) (11) In contrast, however, if a mixture of Cd (II), Cu (II), Ni(II), and Zn(II) was applied to
Trang 18grown at pH 7.1 accumulated the highest amount copper (12) However, if soil pH is above 7.5,plants should be monitored for copper deficiency.
Copper has limited transport in plants; therefore, the highest concentrations are often in root
tis-sues (11,13,14,15) When corn (Zea mays L.) was grown in solution cultures at 10⫺5, 10⫺4, and 10⫺3M
Cu2 ⫹, copper content of roots was 1.5, 8, and 10-fold greater respectively, than in treatments withoutcopper additions, with little copper translocation to shoot tissues occurring (14) On a Savannah finesandy loam pasture soil in Mississippi containing 12.3 mg Cu kg⫺1, analysis of 16 different foragespecies revealed that root tissues accumulated the highest copper concentrations (28.8 mg kg⫺1), fol-lowed by flowers (18.1 mg kg⫺1), leaves (15.5 mg kg⫺1), and stems (8.4 mg kg⫺1) (16) Copper mostlikely enters roots in dissociated forms but is present in root tissues as a complex Nielsen (17)
observed that copper uptake followed Michaelis–Menten kinetics, with a Km⫽ 0.11µmol L⫺1and a
mean Cmin⫽ 0.045µmol L⫺1over a copper concentration range of 0.08 to 3.59µmol L⫺1 Withinroots, copper is associated principally with cell walls due to its affinity for carbonylic, carboxylic,phenolic, and sulfydryl groups as well as by coordination bonds with N, O, and S atoms (18) At highcopper supply, significant percentages of copper can be bound to the cell wall fractions Within greentissues, copper is bound in plastocyanin and protein fractions As much as 50% or more of plant copperlocalized in chloroplasts is bound to plastocyanin (19) The highest concentrations of shoot copper usually occur during phases of intense growth and high copper supply (9)
Accumulation of copper can be influenced by many competing elements (Table 10.2) Copper
uptake in lettuce (Lactuca sativa L.) in nutrient solution culture was affected by free copper ion ity, pH of the solution, and concentration of Ca2 ⫹(20) Copper concentration of four Canadian wheat
activ-(Triticum aestivum L.) cultivars was affected by cultivar and applied nitrogen, but the variance due to
applied nitrogen was fourfold greater than that due to cultivar (21) In Chinese cabbage (Brassica
pekinensis Rupr.), iron and phosphorus deficiencies in nutrient solution stimulated copper uptake, butabundant phosphorus supply decreased copper accumulation (22) Fertilizing a calcareous soil (pH 8.7,
144µg Cu g⫺1) with an iron-deficient solution increased copper accumulation by roots and shoots intwo wheat cultivars from 6 to 25µg Cu g⫺1(cv Aroona) and 8 to 29µg Cu g⫺1(cv Songlen) (13) Inthis same study, zinc deficiency did not significantly stimulate copper accumulation (13) Iron
deficiency in nutrient solution culture increased copper and nitrogen leaf contents uniformly along cornleaf blades (23) Selenite (SeO3⫺2) and selenate (SeO4⫺2) depressed copper uptake, expressed as a per-
centage of total copper supplied, in pea (Pisum sativum L.), but not in wheat (Triticum aestivum L cv.
Sunny) However, copper uptake and tissue concentration were not affected by selenium (24)
Iron and copper metabolism appear to be associated in plants and in yeast (25,26) Ferric-chelatereductase is expressed on the root surface of plants and the plasma membrane of yeast under condi-tions of iron deficiency (25) Lesuisse and Labbe (27) reported that ferric reductase reduces Cu2 ⫹inyeast and may be involved in copper uptake Increases in manganese, magnesium, and potassiumaccumulation were associated with iron deficiency in pea, suggesting that plasma reductases may have
a regulatory function in root ion-uptake processes via their influence on the oxidation–reduction tus of the membrane (25,26) Evidence of this process was also supported by findings in a copper-sen-
sta-sitive mutant (cup1-1) of mouse-ear cress (Arabidopsis thaliana L Heynh var Columbia), suggesting
that defects in iron metabolism may influence copper accumulation in plants (25)
The copper requirements among different plant species can vary greatly, and there can also besignificant within-species variation of copper accumulation (28,29) The median copper concentration
of forage plants in the United States was reported to be 8 mg kg⫺1for legumes (range 1 to 28 mg kg⫺1)and 4 mg kg⫺1for grasses (range 1 to 16 mg kg⫺1) (30) The copper content of native pasture plants incentral southern Norway ranged from 0.9 to 27.2 mg kg⫺1(28) Copper concentrations of tomato leavesfrom 105 greenhouses in Turkey ranged from 2.4 to 1490 mg kg⫺1(31) Vegetables classified as hav-
ing a low response to copper applications are asparagus (Asparagus of ficinalis L.), bean (Phaseolus vulgaris L.), pea, and potato (Solanum tuberosum L.) Vegetables classified as having a high response
Trang 19to copper are beet (Beta vulgaris L Crassa group), lettuce, onion (Allium cepa L.), and spinach (Spinacia oleracea L.) (32) In Australia, the critical copper concentration in young shoot tissue was
4.6 mg kg⫺1for lentil (Lens culinaris Medik), 2.8 mg kg⫺1for faba bean (Vicia faba L.), 2.6 mg kg⫺1for chickpea (Cicer arietinum L.), and 1.5 mg kg⫺1for wheat (Triticum aestivum L.) (33) Leaves of dwarf birch (Betula nana L.) had considerably lower copper levels than mountain birch (Betula pubes-
cens Ehrh.) and willow (Salix spp.) in central southern Norway (28).
The response of many crops to copper addition depends on their growth stages (20,34) In
soy-bean (Glycine max Merr.), the copper content of branch seeds was 20µg g⫺1whereas seeds from themain stems contained 14µg g⫺1(35) Addition of 10µg CuCl2⭈2H2O g⫺1to nutrient solution culturesignificantly suppressed leaf area in expanding cucumber (Cucumis sativus L.) leaves, whereas cop-
per addition significantly limited photosynthesis in mature leaves (34) However, the suppression inphotosynthesis was attributed to an altered source–sink relationship rather than the toxic effect ofcopper (34) Nitrogen and copper were the only elements that showed no gradation in concentrationalong the entire corn leaf blade (23)
Descriptions of the Interaction of Copper in Plant Tissues with Various Elements
Element Interaction with Cu in Plant Tissues a
Nitrogen (N) Increasing levels of N fertilizers may increase requirement for Cu due to increased growth
N fertilization linearly increases the Cu content of shoots High N levels may also inhibit translocation of Cu Phosphorus (P) Heavy use of P fertilizers can induce Cu de ficiencies in citrus
Excess P in solution culture decreased Cu accumulation in Brassicab
Potassium (K) Foliar K sprays have reduced the copper content of pecan
Calcium (Ca) Ca was shown to reduce Cu uptake in nutrient solution culture in lettuce c
Increasing Ca in solution culture improved reduced growth due to Cu toxicity
in mung bean d
Iron (Fe) High levels of Fe have produced leaf chlorosis in citrus and lettuce
Fe deficiency has stimulated copper uptake in solution culture in Brassicay and corn e
Excess Fe in nutrient solution culture lessened the e ffects of Cu toxicity in spinach f
Zinc (Zn) Cu signi ficantly inhibits the uptake of Zn
Zn will inhibit the uptake of Cu
Zn is believed to interfere with the Cu absorption process Manganese (Mn) Cu has been shown to stimulate uptake of Mn in several plant species
Molybdenum (Mo) Cu interferes with the role of Mo in the enzymatic reduction of nitrate
A mutual antagonism has been found between Cu and Mo in several plant species
Aluminum (Al) Al has been shown to adversely a ffect the uptake of Cu
aReproduced from H.A Mills, J.B Jones, Jr., in Plant Analysis Handbook II, MicroMacro Publishing, Inc., Athens, GA,
1996, 422pp., unless otherwise noted With permission.
bAdapted from Z Xiong, Y Li, B Xu, Ecotoxic Environ Safety, 53:200–205, 2002.
cAdapted from T Cheng, H.E Allen, Environ Toxic Chem., 20:2544–2511, 2001.
dAdapted from Z Shen, F Zhang, F Zhang, J Plant Nutr., 21:1153–1162, 1998.
eAdapted from A Mozafar, J Plant Nutr., 20:999–1005, 1997.
fAdapted from G Ouzounidou, I Illias, H Tranopoulou, S Karataglis, J Plant Nutr., 21:2089–2101, 1998.
Trang 20amount of that metal in edible parts of corn grain, sugar beet (Beta vulgaris L.) roots, and alfalfa
leaves (29) Despite differences of mean soil copper levels ranging from 160 to 750 mg kg⫺1, per concentrations of edible tomato fruit and onion bulbs were similar (36) Although soil copperlevels ranged from 26 to 199 mg kg⫺1, spring wheat (Triticum aestivum L.) grain accumulated only
cop-between 2.12 and 6.84 mg Cu kg⫺1(15) Comparing a control soil containing 18 mg Cu kg⫺1and aslag-contaminated soil containing 430 mg Cu kg⫺1, the respective copper concentrations for bean
(Phaseolus vulgaris L.) were 6.6 and 6.7 mg Cu kg⫺1dry weight; for kohlrabi (Brassica oleracea var gongylodes L.) were 1.9 and 2.8 mg Cu kg⫺1 dry weight; for mangold (Beta vulgaris L cv.
macrorhiza) were 11 and 18 mg Cu kg⫺1dry weight; for lettuce were 11 and 40 mg Cu kg⫺1dry
weight; for carrot (Daucus carota L.) were 5.1 and 8.1 mg Cu kg⫺1 dry weight; and for celery
(Apium graveolens var dulce Pers.) were 7.5 and 12 mg Cu kg⫺1dry weight (38)
Proportionally less accumulation of cadmium, lead, and copper occurred in Artemisia species
in Manitoba, Canada, at high soil metal concentrations than in soils with low metal concentrations
(37) Radish (Raphanus sativus L.) accumulated only 5µg Cu plant⫺1when grown on an tural soil (pH 6.3, 6.9% organic matter) contaminated with 591 mg Cu kg⫺1(18) On the other hand,increasing copper treatments from 0.3µM CuSO4to 10⫺5, 10⫺4, and 10⫺3M Cu2 ⫹increased rootcopper levels in sunflower (Helianthus annuus L.) from 42, 108, 138, and 1070µg Cu g⫺1 dryweight, respectively, but not at the expense of growth (39) Contrary to results from many uptake
agricul-and accumulation studies, the above ground portions of H annuus in this study accumulated more
copper than the roots (39)
Fertilizer sources of copper include copper chelate (Na2CuEDTA [13% Cu]), copper sulfate(CuSO4⭈5H2O [25% Cu]), cupric oxide (CuO [75% Cu]), and cuprous oxide (Cu2O [89% Cu]) (Table 10.3) The copper in micronutrient fertilizers is mainly as CuSO4⭈5H2O and CuO (40) withCuSO4⭈5H2O being the most common copper source because of its low cost and high water solubil-ity (41) Copper can be broadcasted, banded, or applied as a foliar spray Foliar application ofchelated copper materials can be used to correct deficiency during the growing season (41)
TABLE 10.3 Copper Fertilizer Sources and Their Approximate Copper Content
Copper(II) oxalate CuC2O4⭈2H 2 O 40 Copper(II) sulfate monohydrate CuSO4⭈H 2 O 35 Copper(II) sulfate pentahydrate CuSO4⭈5H 2 O 25
Copper(II) ammonium phosphate Cu(NH4)PO4⭈H 2 O 32 Copper(II) acetate Cu(C2H3O2)2⭈H 2 O 32 Cupric nitrate Cu(NO3)⭈nH2 O 31 Copper chelates Na2CuEDTA 13
Organic forms Animal manures ⬍0.5