Botanical Accuracy: The species names of citrus - a sweet, sour, and sticky mess

Wednesday, June 25, 2014

The species names of citrus - a sweet, sour, and sticky mess

Everybody knows the great and tasty citrus fruits - lemon, lime, orange, grapefruit, tangerine, and clementine, and maybe even the more rare bergamot, pomelo, tangelo, kumquat, yuzu, and tangelo.

Various slices of citrus fruits. (Image from USDA, public domain.)

Since ancient times these plants have been cultivated, domesticated, and used by humans, and hybridization (on purpose or accidental) have created new hybrids. Citrus plants really do like to hybridize and since many of these hybridization and domestication events happened in the past it has been really hard to figure out these questions:
1) What is a species? (biological boundaries and evolution)
2) What is a hybrid? (= cross between two species, biology, evolution, and genetics)
3) What are the parent species of a hybrid? (genetics)
4) What is the correct scientific name for a species or a hybrid? (scientific nomenclature)

With the help of Dr. David Mabberley, an expert on the genus Citrus, I tried to sort this out since so many scientific names for citrus used on beauty and body products are inconsistent and sometimes flat out wrong and need to be updated. Please note that this is a work in progress for science, and that more research is needed to clarify relationships further. Citrus relationships are a very messy business, indeed. So, here is the most up-to-date information, as far as I can discern.

This is a summary picture of the current hypotheses of the relationships of the most common species and hybrids of Citrus:

Origins and scientific names for the most commonly cultivated citrus fruits.
Based on data from D.J. Mabberley's publications.

In botanical taxonomy a species (*) can only have one current and unique scientific name. Other names are synonyms and their use should be phased out, and only be listed as synonyms in publications. This is to ease the communication and understanding in botany, horticulture, ethnobotany, and agriculture. We should all know what we talk about and we should talk about the same thing when we use a species name, right?
(* except for some fungi, but the mycologists are sorting that out now too, slowly. )

Hybrids either get their parents' names with an 'x' in between parent names (mother listed first), or a brand new species epithet preceded by a 'x'. So, in the figure above, the name for orange can be listed as Citrus maxima x Citrus reticulata, or Citrus x aurantium. You often see the name Citrus sinensis or Citrus x sinensis for oranges, but those are synonym names that should not be used anymore.

The group Citrus x aurantium includes a large array of different cultivars and varieties, such as orange, tangor, grapefruit, bitter orange, clementine and tangelo. This is because the initial hybrids where then back-crossed to different parents, so Citrus x aurantium is really a large hybrid complex.

So, what are then the most up to date names for cultivated and commercial citrus fruits? Here they are:

Common name	Current scientific name	Common inaccurate botanical and ingredient names
bergamot	Citrus × limon	Citrus aurantium var. bergamia, Citrus bergamia
bitter orange	Citrus × aurantium	Citrus amara, Citrus aurantium ssp. amara, Citrus iyo
cedrat	Citrus reticulata	Citrus x reticulata
citron	Citrus medica	Citrus medica var. acida
clementine	Citrus × aurantium	Citrus clementina
grapefruit	Citrus × aurantium	Citrus paradisi, Citrus × paradisi
kaffir lime (now called makrut lime)	Citrus hystrix
kumquat	Citrus japonica
lemon	Citrus × limon	Citrus limon, Citrus medica var. limon
lime	Citrus × aurantiifolia	Citrus aurantifolia
makrut lime	Citrus hystrix
mandarin	Citrus reticulata	Citrus x reticulata
orange	Citrus × aurantium	Citrus aurantium, Citrus aurantium var. sinensis, Citrus sinensis, Citrus × sinensis
pomelo	Citrus maxima	Citrus grandis
satsuma	Citrus reticulata	Citrus unshiu
Tahiti lime (seedless)	Citrus x latifolia
tangelo	Citrus × aurantium	Citrus tangelo
tangerine	Citrus reticulata	Citrus x reticulata, Citrus × tangerina
tangor	Citrus × aurantium	Citrus nobilis, Citrus x nobilis
yuzu	Citrus × junos	Citrus junos

Many of the commercial, non-commercial, government or non-profit databases and publications do not use these updated names. They are often lagging behind and changing names take time.

Want to read more about the delicious history and taxonomy of citrus plants? Try these scientific papers if you want something more substantial than Wikipedia.

Mabberley, D. J. 1997. A classification for edible Citrus (Rutaceae). Telopea 7: 167-172.

Mabberley, D. J. 2004. Citrus(Rutaceae): A review of recent advances in etymology, systematics, and medical applications. Blumea 49: 481-498

Zhang, D. & D. J. Mabberley. Citrus in Flora of China. [Lots of interesting information here!]

Li, X., R. Xie, Z. Lu, & Z. Zhou. 2010. The origin of cultivated Citrus as inferred from internal transcribed spacer and chloroplast DNA sequence and Amplified Fragment Length Polymorphism fingerprints. Journal of the American Society for Horticultural Science 135: 341-350. [Note - some of their results do not agree with the parent hypotheses presented above.]

(Many thanks to Dr. David J. Mabberley for expert help and information for this post.)

What is Botanical Accuracy?

Would you care if someone called a cat 'a mouse' in the description of a medieval painting?

Would you care if someone served you horse meat, but said it was beef? I bet you would.

Would you care if the wrong chemical was listed in the ingredient list of your shampoo or cereal?

Would you care if you bought one plant, but got another?

Would you care if there were species or ingredient mistakes in advertising, menus, herbal pills, and such things?

Would you care if books on plants are illustrated with the wrong plants?

If so, here is the place for you to read about such problems in the world of plants and plant products, where unfortunately such mistakes, inaccuracies, and problems are not uncommon. This is usually due to lack of botanical knowledge or expertise, or sometimes because of plain ignorance.

Inaccuracies are common when it comes to plants, because it seems like we humans care to learn less about green things like trees, flowers, and herbs than we care to learn about animals, even when we eat plants, paint them, plant them, extract their chemicals, or use them in numerous other ways.

Without plants in the world you and I certainly would be dead. Some plants can also kill you with their toxins, so it is best to know which plant is what species. Time to learn some botany!

About mistakes, inaccuracies, and errors

Science is a process of gaining knowledge and understanding of the world around us. It is a never-ending process, and what we think are true facts today might change tomorrow. In science we are aiming for having the best understanding possible today based on what we and our predecessors have learned until now.

This means that what is botanically accurate from a scientific viewpoint might (and will) change. Other experts in the field of botany know a lot more about their particular research plants than I do. New scientific findings and conclusions are being published every day. This is just normal and part of the scientific process; we improve on our knowledge all the time.

The important thing is our willingness to continuously aim for botanical accuracy and the highest scientific standards in our use of names and facts. When things are wrong, let's correct them. Let us not perpetuate wrong botanical knowledge by accepting its incorrect use on commercial products, in everyday language, or in other parts of our contemporary cultures. Through scientific education and specific corrections we will improve botany and science for everybody, in supermarkets, restaurants, and garden centers.

It is the perpetuation of incorrect facts that are the problem, not the need for correction. Everybody makes mistakes, and everybody learns, throughout their lives. We need words to be able to communicate and talk about things, so let's use the right words and the right species names.

Undoubtedly there will be mistakes and errors on this blog, or things that need to be updated. If you want to get in touch, please e-mail me at botanicalaccuracy at gmail.com. If you see a mistake on this blog, e-mail me with the link to the post and an explanation what is incorrect and should be updated. If you represent a company and want to get in touch, please use the same e-mail address.