This is the best answer ....

An interesting free recent publication

The Bioaccumulation and Physiological Effects of Heavy Metals in Carrots, Onions, and Potatoes and Dietary Implications for Cr and Ni: A Review

http://onlinelibrary.wiley.com/doi/10.1111/1750-3841.12433/abstract;jsessionid=C0AF2431696FED8AEE3E8C9E07B04F6C.f01t02

http://www.sciencedirect.com/science/article/pii/S0048969704007880

http://www.sciencedirect.com/science/article/pii/004313549400178A

http://www.tandfonline.com/doi/abs/10.1080/01904168109362846

http://www.ijer.ir/?_action=articleInfo&article=54

http://www.tandfonline.com/doi/abs/10.1080/00103629009368265

Dear Azra

Please have a look these new papers

http://www.tandfonline.com/doi/abs/10.1080/02757540.2017.1316491

https://www.ijcmas.com/6-2-2017/V.S.%20Reddy%20Kiran%20Kalyan,%20et%20al.pdf

http://jast-old.modares.ac.ir/article_18562_0.html

Best wishes

Please take a look at the following links.

Article Intake of heavy metals in selected varieties of onion (Alliu...

Article Assessment of heavy metals in onion and potato in imported a...

https://www.longdom.org/open-access/assessment-of-selected-heavy-metals-in-onion-bulb-and-onion-leaf-allium-cepal-in-selected-areas-of-central-rift-valley-of-oromia-r-2376-0354-1000217.pdf

https://www.degruyter.com/downloadpdf/j/oszn.2015.26.issue-3/oszn-2015-0017/oszn-2015-0017.pdf