The term phytotoxicity has normally been associated with phenomenon whereby a potentially harmful substance has accumulated in the plant tissue to a level affecting optimal growth and the development of the plant . Such a definition is not adequate for developing the phytotoxicity standards because plants that experience varying degrees of phytotoxicity exhibit a variety of symptoms during the course of growth, and differing levels of injury can result. However, retardation of plantgrowth may not be limited to accumulation of toxic substances since environmental factors associated with growing plants such as nutrient deficiencies, water, salt stress, root diseases, other chemical exposure produce similar visual symptoms and also result in yield depression . Positive confirmation of an incidence of metal toxicity requires that

 Plants have sustained injuries; 

1. A potentially phytotoxic metal has accumulated in the plant tissue; 

 2. The observed abnormalities are not due to other disorders of plant growth; 

 3.The biochemical mechanisms that cause metal toxicity to be harmful to plants are observed during the course of  gouuwth.

During the evolution of the plants only a few heavy metals were incorporated in metabolic process. phytotoxicity of Plants to various heavy metals occurs by surpassing critical levels.Several principle are elaborated: at the cellular levels it is the importance of the plasma membrane integrity at the explosion to elevated concentration of Ag,Cu, & Hg ,the regualtion of the concentration of free metals in the cystosol, and the cytosolic the decontamination by  a enhanced transport across the tonoplast into the vacuole ,at the whole plant level it is the translation velocity from root to shoot and the ability of leaves to accumulate a high amount of metals.

regards,

Prem Baboo

Prem Baboo has already explained substantively. The phytotoxicity of each element varies and depends on various physico-chemical properties of the sample matrix such as pH, temperature, salinity etc. It also depends on the type of metal, its valency state, bio-availability etc. Moreover the plant species, age and habitat etc also plays significant role in the toxicity assessment. 

Dear Juan Manuel,

You should give more details about your question. Are you searching for level in the soil or level in the plant? Unfortunately, there is no absolute answer. You first have to define what phytotoxicity is for you. A reduction in root growth may be the first step, necrosis of aboveground organs maybe the other extreme with all possible intermediate symptoms. Once you have define what phytotoxicity is for your goal, then you have to examine the corresponding concentration  of the heavy metal in the organs. This will give you the phytotoxicity concentration regarding your plant species (and possibly cultivar as it is likely depending on it).

Have a look at this paper :

Kopittke, P.M., Blamey, F.P.C., Asher, C.J., Menzies, N.W., 2010. Trace metal phytotoxicity in solution culture: a review. Journal of Experimental Botany 61, 945–954. doi:10.1093/jxb/erp385

The level of heavy metals in soils that give the corresponding phytotoxic concentration in the plant is depending on the bioavailability of the metals in the soil, the total concentration being of poor interest. The bioavailability is generally the availability with time of the free ion depending on the initial concentration in the soil solution, and on its buffering by sorption/desorption, by complex dissociation, transport by mass flow and diffusion (and possibly dissolution, changes in redox etc...).