Hi, Angger, it is not so easy to tell exactly. What is the aim of folira copper applications?

Do you want o study copper toxicity? I think you should try experimentally at least 4 different Cu concentrations, like 1, 3, 10 and 20 µM. Good luck!

many thanks Taras,

what method we use to know optimum nutrient absorbtion by foliar application?

Hi, Angger, this is dependent from the aim, of your project. You can feed leaf with copper and measure chlorophyll contents as a simplest marker after 12 hours on the light conditions. Good luck!.