Anyone know of any more northerly grapefruits?

From the Home Citrus Growers site I recently discovered:

World's Northernmost Fruiting Grapefruit? (Home Citrus Growers)

Anybody know of anything further north than London (there's a claim for Porlock, but if you're going to make fruiting a criterion I think you have to actually ripen the fruit)? Really I think the UK is probably the only place with a shot at growing it this far north. I've seen some kind of citrus growing in a greenhouse in Iceland (I can't recall what it was) but I think to count it really has to be outdoors...where's the challenge in growing it in a greenhouse...even if it is an immensely cool greenhouse heated only by infernal depths of the earth.

The Grapefruit Family Tree

I was going to post this in repsonse to Gregor Mendel's comment to the Google queries post, but it was sufficiently cool I thought it rated its own post:

The History of Red Grapefruit, courtesy of Texas A&M.

I thought the coolest part was the family tree at the top...I also liked the red/yellow chimera down at the bottom (In fact, I'm going to swipe that photo and make it the Image of the Day.)

A few things I noticed which relate to previous conversation:

Apparently mutation breeding in Texas was performed by irradiating 'Ruby Red', which produced the experimental line, A&I 1-48. A budsport of A&I 1-48 with darker red fruit became the commercially successful 'Rio Red'. I've heard that a lot of what is marketed as 'Ruby Red' these days is in fact 'Rio Red', but they use the other because of the better name recognition (I don't remember where I heard this, so I have no idea if it's true). It seems A&I 1-48 produced a variety of other red sports as well, though I don't know whether any have been released as cultivars...a further sport of 'Rio Red' was named and released as 'Texas Red'.

'Star Ruby' appears to be the seedless result of irradiating 'Hudson' budwood, so there's an example of mutation breeding successfully producing seedlessness in grapefruit, though it's still not a mandarin.

Attack of The Clones

Citrus crops have been with us for a very long time. No one knows for sure how long ago citrus began to be collected and cultivated in it's native range, Southeast Asia, but by the fourth century B.C. it had made its way to ancient Greece, where it was mentioned as a wedding gift of the gods. As any visit to a good-sized grocery store will tell you, there are a stunning array of citrus fruits out there: orange, lemons, limes, grapefruit, pummelos, mandarins,tangerines, tangelos, etc. There are even different cultivars of each of these, making for what seems like an amazing range of cultivated diversity.

Things are not entirely how they seem, though. First of all, there is a tendency to think of each of the different fruits as a different species, and taxonomists have been nice enough to oblige us with species names for each. Taxonomists are in the unenviable position of trying to impose simple distinctions on a system which is neither simple nor distinct, and in this case they create the illusion of many different species, where, in fact, there are basically only four which account for everything you'll ever see in the grocery store: the pummelo, mandarin, citron, and lime. Virtually all the rest are hybrids between these four, some of ancient, probably natural, origin, and some more recent. Here are a few common hybrids:

Lemon = (Lime x Citron)
Sweet Orange = (Pummelo x Mandarin)
Grapefruit = (Sweet Orange x Pummelo)
Tangelo = (Grapefruit x Mandarin)
Temple Orange = (Mandarin x Sweet Orange)
Tahiti Lime = (either Lime x Citron or Lime x Lemon)
Chinese Lemon = (Mandarin x Lime)

You might think that all this intercrossing would quickly result in a wide spectrum of different citrus fruits, rather than these few categories, and you'd probably be right, except for one weird quirk of citrus: it virtually never forms hybrids. Most of these species produce seeds which are what is called "nucellar", which means that rather than hybrid plants, the seeds produce clones of the female (sometimes many, many clones per seed...they're polyembryonic, too). These hybrids are the very rare exception, not the rule.

If you can't make new hybrids, generating new cultivars becomes very, very difficult. All you get is copies of the original. So virtually all new cultivars are the product of mutations.

Take grapefruit, for example: All grapefruits, everywhere, it is thought, come from a single hybridization between sweet orange (Citrus sinensis) and pummelo (Citrus grandis) in Barbados, some time around 1750. Every cultivar, from 'Ruby Red' to 'Marsh' to 'Thompson', is essentially the same genotype, and in fact are basically indistinguishable at the genetic level. When hybrids are made, they suffer so badly from inbreeding depression (after all, a cross between grapefruit cultivars is essentially a selfing) that the plants rarely survive long.

So breeding citrus has become a crazy mix of induced mutations, protoplast fusion, and embryo rescue. Anything to introduce a little genetic variation. And as the usual process of hybridization and selections is basically broken, things like transformation take on added importance for moving genes of interest into cultivated types.

Much of this work is going on at the Core Citrus Transformation Facility at the University of Florida research center at Lake Alfred. Steve Wagar also has a decent page on the various permutations of citrus, and the associated nomenclature.