particularly for polyphenols

[GoldfishLord]

Phytochemicals​

Considerable differences exist between onion varieties in phytochemical content, particularly for polyphenols, with shallots having the highest level, six times the amount found in Vidalia onions.[38] Yellow onions have the highest total flavonoid content, an amount 11 times higher than in white onions.[38] Red onions have considerable content of anthocyanin pigments, with at least 25 different compounds identified representing 10% of total flavonoid content.[38]

Source: Wikipedia https://en.m.wikipedia.org/wiki/Onion

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What's the reason "for" was used there?

 

[jutfrank]

I presume you're asking about the meaning. It means 'in the case of'.

 

[GoldfishLord]

Does the "with shallots having the highest level, six times the amount found in Vidalia onions" part mean roughly "of all its varieties, the variety "shallot" has the highest level"?

 

[jutfrank]

Yes, that's right.