You really need to be more specific.There are a wide range of "permissible" levels for example in contaminated land situations. You could look at Soil Guideline Values from the UK are equivalent values in Canada or Australia but you will find that limits for Mn and Mg are hard to come by. Soil magnesium values can be  massive, for example in dolomite area and do no harm to humans. However there may be toxicity to plants. Or is it harm to plants that you are worried about. In  terms of acceptable concentrations in plants this will depend on what the plants are used for. Human food plants and animal feed plants may have different acceptable concentrations but unless they are to be ingested the "limits" will be hard to find.

So let us know a bit more detail and we can give you what you need, if it is there, without sending loads of stuff that is not of use to you.

Sir Alan P Newman,

I am working on heavy metal accumulation in plants and I want to know the permissible limits of these metals for plants. I want to get knowledge of  acceptable concentrations for animal feed plants that are also being used as traditional medicines in rural areas..

The EU directive on animal feed specified limits for arsenic, cadmium , mercury and lead. Look at the first link.

For soils take a look at the second link.

For human food try the third link.

These may at least be a start

http://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:02002L0032-20131227&qid=1421850100043&from=EN

http://randd.defra.gov.uk/Document.aspx?Document=SP0547_7265_FRA.pdf

http://webarchive.nationalarchives.gov.uk/20120206100416/http://food.gov.uk/multimedia/pdfs/fsismetals0107.pdf

Hi, maybe you can search using the keyword "REFERENCE PLANT" that can tell you, more and less, the normal level of metals in an specific plant species with that you can compare later your heavy metal concentration levels in the plants that you are working with.

Source: Markert, B. (1991) Establishing of ‘Reference Plant’ for inorganic characterization of different plant species by chemical fingerprinting. Water, Air, and Soil Pollution 64: 533-538

If you are working on plant accumulation of heavy metals, you need to standardize the permissible limit by trial and error experimentation. You know, plant do vary in their capacity to accumulate different metals. Plants with hyper accumulation tendencies usually show the amount of a metal in its tissues more that what is available in soils. You may select such plants from contaminated sites and experiment with them to confirm their hyper accumulation tendencies for specific metals and finally you will be able to declare the permissible limit for your plant!

I would suggest you to consult an old book Diagnostic Criteria for Plants and Soils.by HD Chapman published by Univ. Calif., Div. Agric. Sci., U.S. A. 1966 pp. 793 pp.

http://link.springer.com/article/10.1007%2FBF00483363

http://onlinelibrary.wiley.com/doi/10.1002/jpln.19661130211/abstract

Permissible for what is the question. And the situation in soils and plants can be quite different.

Soils: first, some regions can have their soils naturally rich in some elements and there is not much you can do about it. If you have anthropogenic enrichment or contamination, then that is another story and remediation could be possible.

You also have to consider what is bioavailable for the plants and then for humans/animals if the plants are to be used for food /feed. That is very important and will dictate the analyses you will use to detect the different forms of an element, instead of just a global elemental analysis.

Permissible for plants to grow well and be cultivated: then, this is mostly a question of plant physiology and biochemistry. These are more agronomic considerations than regulations. Moreover, it can be highly species specific.

Permissible for human consumption (which is maybe what you are really after). Well, again, consider bioavaliability and also what are the plants used for. Are they eaten directly by humans or through several trophic levels, which may alter the original composition through their own metabolism? Which parts (levels in roots and leaves can be quite different)? Are they used as ornementals and hence not eaten? Are they used to extract a certain compound/fraction, which is not associated with the element(s) of concern. So, applications of what is done with the biomass may set different regulations and limits.

Another point, these things can be highly country specific. In Canada, I have to follow the regulations of the Canadian Food Inspection Agency. In the USA, it is the Food and Drug Administration. In the European Union, it is the EU directives. So, what is the situation in your country? What is the agency responsible for these regulations and their enforcement?

Also to consider: is the product for domestic consumption (the regulations of your country apply) or for exportation (what is, then, the agreement(s) with the destination country(ies)?).

Yes, it is not a simple answer... I hope to have indicated some of the sources of the complexity.

You can refer to the document below. There are some examples of national limits for toxic metals in herbal medicines and products according to WHO guidelines.

http://apps.who.int/medicinedocs/documents/s14878e/s14878e.pdf

I recommend Wafae's answer

Best Regards

I recommend Alan P Newman