The Role Of Copper In Plants
Copper is associated with a number of metalo-proteins. Its essential metabolic role is evident from the presence in cytochrome oxidase. Copper participates in protein and carbohydrate metabolism and N2 fixation. As much as 70% of plants Cu is concentrated in chlorophyll. It is important for photosynthesis, respiration, detoxification of superoxide radicals, and lignification. In general deficiency of Cu reduces reproductive growth of plants (fruit and seed formation) more than vegetative growth. In addition, Cu is involved in the desaturation and hydrogenation of fatty acids.
Copper Status in Soils and Plants
The average total concentration of Cu in the lithosphere is 70mg/kg. Sulfides are predominant Cu-containing minerals in the earth crust. Chalcopyrite (CuFeS2) chalcocite (cu2S) and bornite (CuFeS) are important Cu-containing minerals. Soils generally contain 2-100mg Cu/mg the average being about 30mg/kg of soil. Copper is slightly less abundant in soils than Zn. Most soil Cu is very insoluble, but a significant pool of diffusible Cu occurs mainly as organic complexes in equilibrium with a very low cu level of the soil solution.
Copper availability is generally associated quite closely with total Cu content. Mineral and organic soils with less than 6 and 30mg total Cu/kg soil, respectively, are suspected to be Cu-deficient.
Both organic and inorganic extractions are used for soil analysis. The DTPA (diethylene triamine penta-acetic acid) method developed at Colorado State University is very effective for simultaneous extraction of Cu, Fe, Mn and Zn. Recently, the AB_DTPA (ammonium bivarbonate-DTPA) test has become more popular because in addition to Cu, Fe, Mn and Zn it also extracts NO3-N and K.
Soil Factors Contributing to Copper Deficiency
Soil Cu availability is affected by several soil properties, including total reserves, texture, pH and organic matter. Sandy soils have low Cu content and suffer more frequently from Cu deficiency. Availability of Cu decreases with increasing pH because of greatly diminished solubility and increased sorption on soil colloids. Its availability decreases rapidly above pH 7 and Cu deficiency is believed to be a frequent problem in alkaline soil of Pakistan. Copper is more strongly bound than Mn, Zn and Fe on organic matter and Cu deficiency is common in organic soils like peats and mucks. Poor drainage condition aggravates Cu deficiency.
In Pakistan where histols are not found, Cu deficiency is less common than that of Zn, fe and B. high levels of either N or P can contribute to Cu deficiency. This may result from dilution due to N or P fertilizer induced higher crop growth relative to increased Cu uptake by plant roots.
Symptoms of Copper Deficiency in Plants
Cereals exhibit characteristic Cu deficiency symptoms first in leaf tips at tilling stage. At the grain filling stage the ears are jagged and mostly empty. In severally cu-deficient wheat, new leaves are pale yellow, lack turgor, and become rolled and yellow. Older leaves become limp and bent at ligule.
The leaves die and dry to a bleached gray. In fruit trees, dieback of terminal growth is a prominent feature, especially in oranges. The characteristic features are often large, dark green leaves on long and soft angular shoots. The generative phase of flowering and fruit formation is also severally affected. Fruits may be bumpy, have a rind covered with reddish brown excrescences, and may split.
Crop Sensitivity to Copper Deficiency
Plant species vary in their sensitivity to Cu deficiency. Vertical differences in tolerance to low Cu supply are also important and sometimes are as great as interspecies and oat varieties to Cu deficiency.
Correction of Copper Deficiency
Several inorganic compounds are used for Cu fertilizer. The most commonly used source is CusO4-5H2O which also contains 25.5% S. copper chelates such as Cu-EDTA are also used; however they are expensive. Chelates of micronutrients are metal organic complexes which, though soluble themselves do not ionize to any degree.
