Re: What are the symptoms of copper toxicity in plants?

Copper is an essential element in plants, as all plants contain copper.
Copper is an fundamental part of many enzymes involved in redox reactions.

Some of the enzymes in which copper plays a role are plastocyanin, involved
the transfer of electrons in the light reactions during photosynthesis,
ascorbic acid oxidase, which catalyzes the oxidation of ascorbic acid in the
presence of oxygen, monoamine oxidase, cytochrome oxidase, which is a
respiratory enzyme in mitochondria, laccase and tyrosinase which both
oxidize different phenolic compounds, and uricase.

Copper is absorbed in both the divalent cupric ion (Cu2+) or as the
monovalent cuprous ion (Cu3+) depending upon the moisture and oxygen content
of the soil.

A plant deficient in copper may exhibit dark green leaves with necrotic
spots. These necrotic spots may first be evident at the tips of young leaves
and then proceed down the leaf margins. This dark green coloration is
indicative of a high nitrogen concentration. The leaves may become twisted
or malformed, turn a yellow-green color and abcise in extreme deficiencies.

There is not much available on toxicity due to difficulties in identifying
toxicity symptoms in plants.  Typically necrotic spotting of the lower leaves
is found in copper toxicity afflicted plants.  The only sure way to determine
toxicity of micronutrient metals is to do thorough tissue ananlysis.  

Normal soluble fertilizers have approx. 0.05 to 0.1 ppm copper in them, sometimes 
in a chelated form to aid in nutrient availablity.