The two main components that determine the taste of Matcha are Catechins and L-Theanine. Catechins gives the bitterness and L-Theanine gives the umami flavour (tea sweetness).
As we mentioned in some of our articles, tea leaves used for Matcha processing are grown in the dark. 20 days before harvest, tea leaves are covered from direct sunlight. This shade growing proces will prevent sunlight breaking down L-Theanine and become Catechins. Therefore the highest grade Matcha must be harvested at the early stage as more L-Theanine will break down to Catechins at later stage. This is also the main reason why first flush Matcha is the highest grade Matcha.
Other than Tencha (Matcha), Gyokuro and Kabusecha are also grown under shade (but Kabusecha only for 10 days before harvest). Gyokuro and Kabusecha are also contain higher L-Theanine and lower Catechins which make them the highest grade green tea in Japan.
Sencha or other green teas are grown under direct sunlight, and they have high Catechins. This is the reason why Sencha, Bancha etc has refreshing astrigent flavour.