Copper

Copper is one of the oxygen carriers in the plant, acting similarly to iron in oxidoreductive processes.
It is a structural part of plastocyanin, the chloroplast protein responsible for electron transport between photosystem II and photosystem I in photosynthesis.
It is part of the cytochrome c oxidase enzyme, the respiratory enzyme responsible for catalyzing the transfer of electrons to oxygen in the mitochondrial cristae.
Copper is a component of the phenolase enzyme complex, which is involved in the oxidation of phenols.
Copper is also related to lignin biosynthesis, since it forms some of its precursors.

ABSORPTION
Roots absorb copper in the form of Cu+2 ion in aerated soils and as cuprous ion Cu+ in oxygen-poor or flooded soils.
Copper can also be absorbed by the plant via foliar application.
The divalent form can form chelates in both soil and nutrient solutions, so it must be monitored to avoid possible toxicity.

DEFICIENCIES

Copper is usually always available in the soil, so its deficiency is not common.

The first symptoms of copper deficiency appear in new leaves due to the immobility of this micronutrient. In the early stages of this deficiency, leaves begin to curl and show mild chlorosis with necrotic spots in some cases. Another early symptom of copper deficiency is a reduction in lignification, due to its role in lignin biosynthesis and the accumulation of phenols, as a cofactor of the enzyme phenolase.

If the deficiency progresses, new leaves will become smaller, duller, and may even wilt. Due to the necrotic growth of the apical meristems, lateral branches may be stunted.

An excess of phosphorus, potassium and other micronutrients can have an antagonistic effect on the absorption of copper by the plant.