Tag Archives: citrus breeding

Disentangling citrus

Citrus fruit have been much on my mind in this week of winter rain. That is because I have been setting myself a target of gathering a bucket of fallen fruit a day to squeeze for juice. I freeze the juice and mostly use it to make fresh jelly. I never left my childhood love of jelly behind, although these days I only make it from scratch with fresh juice and a bare minimum of sugar.

Too many windfalls on the heavily cropping Jaffa orange

We have about 25 citrus trees, many of which date back to the 1940s and 50s when Mark’s parents decided to try them in the garden as plants that are ornamental as well productive. Not for the first time, I wondered to myself whether the standard medical advice to avoid grapefruit if you are taking certain medications applied in NZ where the plants we grow as grapefruit are not actually grapefruit at all. That sent me down a very complicated path, trying to understand the citrus family.

Mandarins at the top, sweet oranges, tangerine or tangelo, lemons and limes in the middle, Wheeny grapefruit and Poorman’s orange or NZ grapefruit at the bottom

When I say a very complicated path, there doesn’t even seem to be common agreement on how many citrus species there are, let alone some very loose usage of the word species. Add in groups and related plants along with new terms to describe crosses between two species which are then given their own name and shown in species format – with or without an x in front showing it is a hybrid – and it is very confusing.

And the cut version (I will squeeze these for juice now)

Then there is the long – very long – history of distribution around the world and the evolution of unique citrus species even in New Guinea and Australia (yes, Australia has native citrus). While they originated in central and south eastern Asia, it seems that archaeological discoveries of seeds on Cyprus date its arrival to the Middle East and southern Europe back as far as 1200BC – or 1200BCE as now seems to be accepted usage – but it took another couple of thousand years before the more desirable forms reached that part of the world. The scale of time is as difficult to comprehend as the taxonomy is complicated.

Mandarins but named cultivars. Easy peel to the left, not so easy peel but possibly more delicious to the right

Pared down to bare essentials, it seems that most of our modern citrus in commercial production come from just three species in the wild: the pomelo (syn pummelo or shaddock, botanically Citrus maxima), the mandarin (Citrus reticulata) and the citron (Citrus medica).

Citron, courtesy of Wiki Commons. Mostly pith with little flesh.

The outliers in relatively common usage are the kumquat which is C. japonica, the kaffir or makrut lime which is C. hystrix and the fashionable Yuzu lemon which is not a lemon but has very obscure species in its parentage, possibly with sour mandarin.

Pomelo photo from Wiki Commons

When it comes to the mainstream citrus, we don’t have a citron here although if we were growing the curious ‘Buddha’s hand’, I could have picked that because it is one example. Nor do we have a pomelo. They are very large, by citrus standards, and paler in colour. I looked in town this week to see if I could find a pomelo to add to my board but there were none for sale at this time of year – a niche item, I suspect. Mandarins we have a-plenty although these are selected and named cultivars and the ones in the wild may well have looked different.

Sweet oranges, Jaffa at the top and a navel orange below

Oranges we also have in abundance here and no matter whether they are sweet table oranges or bitter marmalade oranges (Seville), they are a hybrid between pomelo and mandarin.

I remembered tangerines from my childhood and wondered what happened to them because it is many years since I have seen them. They are easy-peel and sweeter than an orange, originating from Tangiers in Morocco. It seems they are largely mandarin (C. reticulata) with a touch of pomelo.

Tangelo, photo from Wiki Commons, showing the characteristic nipple at the top

I see the newer tangelo originated in Barbados which is why it is often called the Jamaican tangelo, originally marketed as Ugli fruit. I vaguely remember when the tangelo became available and I think it was regarded as an improvement. In fact, it is a tangerine (so mostly mandarin) crossed with a pomelo which would then result in a majority of pomelo genes – hence the ‘tangelo’ name. This will be why it is not as sweet.

What we have as a tangelo but I am now wondering if our trees are tangerines instead

It was also a revelation for me to read the botanic descriptions and see the photos because I am now pretty damn sure that the two trees we have as tangelos are in fact tangerines. Mark thinks they were almost certainly sold under the tangelo name which makes me wonder whether New Zealanders are largely growing tangerines, not tangelos.

We have forgotten the proper name but it is a proper lemon, probably Lisbon or Yen Ben

Lemons get complicated. They are thought to have originated from a citron crossing with a bitter orange (so originally mandarin but already splitting into sub groups). In NZ, the Lisbon lemon (first found in Australia from seed imported from Portugal – hence Lisbon) and Yen Ben, which is a Lisbon lemon selection, are the main commercial varieties of true lemon that are grown.

And then there is the Meyer lemon which is probably the main home garden variety and is not a true lemon. It is less acidic and hardier, making it more successful in our cooler climate which is extremely marginal for the more tropical citrus. Again, it is a citron crossed with a mandarin/pomelo hybrid but not the bitter orange version, rather one with more mandarin genes than pomelo. Meyer is known for the fact that it doesn’t give the same pectin content for setting jams but for general culinary use, it functions as a lemon.

Tahitian lime

Limes get more complicated because they have a genetic line to the lesser known species that gave the key lime along with the lemon genes of citron and mandarin. We tree ripen ours and pick them when yellow rather than at the hard green stage they are usually sold as in the shops.

Wheeny grapefruit

The true grapefruit is the newest addition to the commercial range of citrus and is thought to have originated from a pomelo crossed with sweet orange (making pomelo the dominant genes with the addition of mandarin) in Barbados back in the early 1800s. It is one of the citrus most sensitive to colder conditions so what we call grapefruit in this country and what is generally commercially grown here are the Wheeny grapefruit and Poorman’s Orange. The Wheeny is thin-skinned and very juicy and is a chance seedling from Wheeny Creek in NSW in Australia. It is a pomelo hybrid, maybe crossed with bitter orange.

NZ grapefruit or Poorman’s Orange

The Poorman’s Orange  is often referred to internationally as the NZ grapefruit because it is grown widely here but not generally elsewhere. However, it originated in Asia, was taken to Australia and brought to NZ by Governor Grey to grow in his Kawau Island garden. It is thought to be pomelo x tangelo (so more pomelo than mandarin) and has much brighter flesh than a true grapefruit. If you have ever eaten grapefruit overseas and they were not the same as here, that is why. We don’t grow true grapefruit here.

It seems that citrus in the wild cross-pollinate readily. While many named cultivars will be the result of deliberate hybridising efforts down the centuries, it is likely they were starting with plants which had already done some mixing and mingling of their own accord. The history indicates that a fair number of types are simply selections of found plants, including hybrids and mutations.  

Back to my starting point of grapefruit interfering with some medications. I am no scientist but my understanding is that the problematic compound is furanocoumarin which is usually concentrated more in the peel than in the flesh and juice. Pomelos, citrons and the lesser-used papeda group have very high levels of this whereas the mandarins have very low levels. It seems that the rule of thumb might be that the sweeter citrus are less problematic.

The only reason I can see for singling out the grapefruit on medical advisories is that it is that it is the only one of that more bitter range that is consumed undiluted in quantity as a drink.

With the dominance of pomelo genes in the two varieties we grow as grapefruit in NZ, yes that advisory would apply equally to our fruit. Maybe keep to orange juice if you have been advised against consuming grapefruit.

Finally, three of my favourite photos. Fortunately we have plenty of fruit to share.

Kaka are a native parrot. They are not generally resident here but from time to time, one will stop by for a few weeks. This one ate quite a few mandarins but we were so thrilled to see it here that we were glad to share.
The tui are always in residence here in large numbers. Being nectar eaters, they will tuck into mandarins in winter.