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  Section: Plant Nutrition » Micronutrients » Copper
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Uptake and Metabolism

The Element Copper
  Copper Chemistry
Copper in Plants
  Uptake and Metabolism
Copper Deficiency in Plants
Copper Toxicity in Plants
Copper in the Soil
  Geological Distribution of Copper in Soils
  Copper Availability in Soils
Copper in Human and Animal Nutrition
  Dietary Sources of Copper
  Metabolism of Copper Forms
Copper and Human Health
  Copper Deficiency and Toxicity in Humans

The rate of copper uptake in plants is among the lowest of all the essential elements (9). Uptake of copper by plant roots is an active process, affected mainly by the copper species. Copper is most readily available at or below pH 6.0 (4). Most sources report copper availability in soils to decrease above 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 different copper 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-1 dry weight on both soils (10). Copper concentrations 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 (pH 8.7) (11). In contrast, however, if a mixture of Cd (II), Cu (II), Ni(II), and Zn(II) was applied to a montmorillonite [(Al,Mg)2(OH)2Si4O10] soil at 50 mg kg-1 each, there were no differences in growth of alfalfa (Medicago sativa L.) between soil pH treatments of 4.5, 5.8, and 7.1, and plants grown 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 tissues (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 without copper additions, with little copper translocation to shoot tissues occurring (14). On a Savannah fine sandy loam pasture soil in Mississippi containing 12.3 mg Cu kg-1, analysis of 16 different forage species revealed that root tissues accumulated the highest copper concentrations (28.8 mg kg-1), followed by flowers (18.1 mg kg-1), leaves (15.5 mg kg-1), and stems (8.4 mg kg-1) (16). Copper most likely 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 Km0.11mol L-1 and a mean Cmin0.045mol L-1 over a copper concentration range of 0.08 to 3.59mol L-1. Within roots, 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 high copper supply, significant percentages of copper can be bound to the cell wall fractions. Within green tissues, copper is bound in plastocyanin and protein fractions. As much as 50% or more of plant copper localized 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 activity, pH of the solution, and concentration of Ca2+ (20). Copper concentration of four Canadian wheat (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, but abundant phosphorus supply decreased copper accumulation (22). Fertilizing a calcareous soil (pH 8.7, 144g Cu g-1) with an iron-deficient solution increased copper accumulation by roots and shoots in two wheat cultivars from 6 to 25g Cu g-1 (cv. Aroona) and 8 to 29g Cu g-1 (cv. Songlen) (13). In this 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 corn leaf blades (23). Selenite (SeO3-2) and selenate (SeO4-2) depressed copper uptake, expressed as a percentage 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).

Descriptions of the Interaction of Copper in Plant Tissues with Various Elements

Iron and copper metabolism appear to be associated in plants and in yeast (25,26). Ferric-chelate reductase is expressed on the root surface of plants and the plasma membrane of yeast under conditions of iron deficiency (25). Lesuisse and Labbe (27) reported that ferric reductase reduces Cu2+ in yeast and may be involved in copper uptake. Increases in manganese, magnesium, and potassium accumulation 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 status of the membrane (25,26). Evidence of this process was also supported by findings in a copper-sensitive 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 be significant 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-1 for legumes (range 1 to 28 mg kg-1) and 4 mg kg-1 for grasses (range 1 to 16 mg kg-1) (30). The copper content of native pasture plants in central southern Norway ranged from 0.9 to 27.2 mg kg-1 (28). Copper concentrations of tomato leaves from 105 greenhouses in Turkey ranged from 2.4 to 1490 mg kg-1 (31). Vegetables classified as having a low response to copper applications are asparagus (Asparagus officinalis L.), bean (Phaseolus vulgaris L.), pea, and potato (Solanum tuberosum L.). Vegetables classified as having a high response to 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-1 for lentil (Lens culinaris Medik), 2.8 mg kg-1 for faba bean (Vicia faba L.), 2.6 mg kg-1 for chickpea (Cicer arietinum L.), and 1.5 mg kg-1 for wheat (Triticum aestivum L.) (33). Leaves of dwarf birch (Betula nana L.) had considerably lower copper levels than mountain birch (Betula pubescens 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 soybean (Glycine max Merr.), the copper content of branch seeds was 20 g g-1 whereas seeds from the main stems contained 14 g g-1 (35). Addition of 10 g CuCl2.2H2O g-1 to nutrient solution culture significantly suppressed leaf area in expanding cucumber (Cucumis sativus L.) leaves, whereas copper addition significantly limited photosynthesis in mature leaves (34). However, the suppression in photosynthesis was attributed to an altered source-sink relationship rather than the toxic effect of copper (34). Nitrogen and copper were the only elements that showed no gradation in concentration along the entire corn leaf blade (23).

The copper content of many edible plant parts is not correlated to the amount of soil copper (15,36,29,37,38). No correlations could be made between the level of applied copper and the amount 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, copper concentrations of edible tomato fruit and onion bulbs were similar (36). Although soil copper levels ranged from 26 to 199 mg kg-1, spring wheat (Triticum aestivum L.) grain accumulated only between 2.12 and 6.84 mg Cu kg-1 (15). Comparing a control soil containing 18 mg Cu kg-1 and a slag-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-1 dry 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-1 dry weight; for lettuce were 11 and 40 mg Cu kg-1 dry 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-1 dry 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 5 g Cu plant-1 when grown on an agricultural soil (pH 6.3, 6.9% organic matter) contaminated with 591 mg Cu kg-1 (18). On the other hand, increasing copper treatments from 0.3 M CuSO4 to 10-5, 10-4, and 10-3 M Cu2+ increased root copper levels in sunflower (Helianthus annuus L.) from 42, 108, 138, and 1070 g Cu g-1 dry weight, respectively, but not at the expense of growth (39). Contrary to results from many uptake 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) with CuSO4.5H2O being the most common copper source because of its low cost and high water solubility (41). Copper can be broadcasted, banded, or applied as a foliar spray. Foliar application of chelated copper materials can be used to correct deficiency during the growing season (41).

Copper Fertilizer Sources and Their Approximate Copper Content

Limitations may apply to the amount of copper to be applied to land during a growing season. For example, in Italy, additions of copper from fertilizers, including sewage sludge, cannot exceed 5 kg ha-1 year-1 (29). Cupric oxide was ineffective in correcting copper deficiency in the year of application but did show residual effects in subsequent years (42). Copper sulfate has been shown to increase the yield of plantlet regeneration from callus in tissue culture (43). In cereal crops, copper is required for anther and pollen development, and deficiencies can lead to pollen abortion and male sterility (44). When the concentration of copper sulfate was increased 100-fold over control treatments to 10M, the rate of responding anthers in barley (Hordeum vulgare L.) increased from 57 to 72% and the number of regenerated plantlets per responding anther increased from 2.4 to 11% (44).

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