Lemon tree from Taiwan

The Taiwan lemon tree is a natural hybrid (pomelo x mandarin), originally from the island of Formosa, but now mainly cultivated in Japan under the name Natsudaidai.

It produces large fruits (250gr, 9cm) rather flattened, yellow then orange at maturity.
The abundant juice is highly acidic and also bitter, with a pronounced floral fragrance.
Harvested in winter through to spring.

The tree is vigorous, very productive and thorny, with an erect habit and dense foliage.
Extremely cold resistant, including the fruit.

References:
https://idtools.org/tools/1076/index.cfm?packageID=1179&entityID=8696