turnip
菁 centers on the turnip plant, specifically its edible root. The meaning is narrow and concrete, referring to this particular vegetable.
菁 combines the grass radical 艹 with 青, which likely contributes the sound and may relate to the green color of the turnip plant. The exact historical development is uncertain.
The grass radical 艹 sits above 青 (blue-green), like the leafy green tops of a turnip growing above the blue-green root. Together they make the turnip plant.
For セイ, imagine a turnip saying 'say' as it pops out of the ground: say -> セイ, and the turnip speaks its name.
—