Compounds
Ooika (covering scent)
The particular aroma that shade-grown tea develops, usually described as seaweed-like. It is strongest in gyokuro and good matcha, and its presence is a reliable tell that the tea was shaded.
In more detail
Shading raises the level of sulphur-containing aroma compounds in the leaf, and those are what the seaweed comparison is reaching for. It is pronounced in gyokuro and good matcha, and once you know it you can tell a shade-grown tea by smell alone.