Touriga Nacional is a red wine grape varietal indigenous to Portugal, and revered as one of the finest grapes used for producing quality Port wines. Despite the very low yields and small berries, the varietal plays a key part in Port wine blends, and is increasingly being used for table wine in the Douro and Dão wine region. 
The tiny berries have a high skin to pulp ratio which improves the amount of extract in the wine. The varietal is known to produce intense, very aromatic wines with high tannin content. In the Douro it is grown in searing heat in steep schist vineyards that are more rock than soil. The alternative name of 'Mortagua' pays tribute to these harsh conditions. Typically trained under the Guyot system, and needs severe pruning to keep the canopy under control.

 

In contrast, the vine produces just a few bunches of blue-black grapes which vary in size from small to tiny on the bunch. The vine is vigorous; though producing mostly foliage and only a few bunches of grapes, compared to other local red varietals. In the most recent census in 2015, there was around 3600 hectares of Touriga Nacional planted in Portugal.
Thus yields are among the lowest of any commercial grape varietal. It constitutes a small percentage in the blend of most Port wines and due to its low productivity accounts for only 2% of the Douro Valley's production.
For a several years now, research have been working on trying to clone Touriga Nacional - to produce vines that are able to better pollinate, with the aim of increasing yields by 15% and sugar content by 10%. The varietal is grown for quality rather than quantity. Touriga Nacional produces wine with finesse, structure, body and warmth - and yields dark, concentrated, highly tannic and aromatic wines, with an intense black fruit character.
The depth and richness of flavour in the best Port wines owes much to the flavours of Touriga Nacional, but Port wines nearly always contain a mixture of several varietals. In fact about 50+ red varietals are authorised for inclusion.
The most common grape varietals in red Port are: Touriga Nacional, Touriga Franca, Tinta Barroca, Tinta Roriz, Tinta Cão, Souzao, Tinta Amarela and Mourisco Tinto.