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" Native soil' denotes experiments or studies where crops were harvested from a field soil or natural environment and the copper level determined from a soil sample to estimate copper fertility.

Information not available. When available in references, values have been expressed as an average concentration ± standard error.

a montmorillonite [(Al,Mg)2(OH)2Si4O10] soil at 50 mg kg1 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 maysL.) 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 kg1), followed by flowers (18.1mg kg"1), leaves (15.5mg kg"1), and stems (8.4mg 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 Km = 0.11 |imol L"1 and a mean Cmin = 0.045 |imol L"1 over a copper concentration range of 0.08 to 3.59 |imol 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, 144 |g Cu g"1) with an iron-deficient solution increased copper accumulation by roots and shoots in two wheat cultivars from 6 to 25 |g Cu g"1 (cv. Aroona) and 8 to 29 |g 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).

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 8mg kg"1 for legumes (range 1 to 28 mg kg1) and 4mg kg"1 for grasses (range 1 to 16mg 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 kg1 (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

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