I've just returned from another visit to Australia.
I spent a week inspecting various distant places where C. gracilis trees grow.
Information and photos starting from http://www.homecitrusgrowers.co.uk/aust ... s2019.html
Citrange/Mike
No-one really knows because no-one has yet found viable seeds.
Known groups of plants are too far apart to understand how they could cross-pollinate, but perhaps there are many unknown trees in such a vast area. So perhaps cross-pollination does sometimes occur.
It needs someone in Australia to take a real interest in the species and manually pollinate between tree groups.
It's difficult to arrange this from here!
You are right, Ilya, there are no examples in botanical gardens.
There are no seeds and it is very difficult to graft.
Last trip in 2016 I gave excellent budwood to an expert Australian citrus grafter who said there would be no problem - but in fact none succeeded.
I know only two people who have managed to graft it. One is a nursery owner in Darwin who then sold his two plants but doesn't know to whom. The other is an Australian citrus researcher in Bundaberg who has two potted specimens but they have never flowered.
Mike
I have seen on the website of George Brown Darwin Botanical Garden that they have "the exceptional NT native plant collections in the woodland area".
It is very frustrating that they do not care for this unique species growing not far.
It is an excellent botanic garden - even without C. gracilis!
It does have two trees of Citrus garrawayi from neighbouring Queensland.
From the shape of the fallen fruit I found, I think one of them may be a hybrid with another citrus.
No seeds in any of these fruit!
Mike