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Botanical classification of citrus fruits




Meyer lemon

Meyer lemon
© Jorma Koskinen
Classification of citrus
Moro blood orange

Moro blood orange
© Jorma Koskinen
Which botanical name is the correct one?
Why author names?
Classification examples
History
Today
New classification of edible citrus
Newest technology
Other citrus groups
Botanical names on Citrus Pages
Monoembryony, nucellar embryony and
polyembryony in citrus seeds
Monoembryony
Nucellar embryony
Polyembryony
Which citrus seeds will grow true to type?
References
Literature












Classification of citrus

Australian Sunrise Lime Any botanical classification of citrus fruit faces several difficulties. All citrus types hybridise easily. New hybrids are continuously developed to obtain desired qualities such as seedlessness, juiciness and fresh taste. New hybrids spontaneously arise by cross-pollination. The differences between a species, a new botanical variant, a different cultivated (horticultural) variety and a minor modification can be small indeed. In the case of older varieties and hybrids the most modern methods of molecular research are sometimes needed to distinguish different citrus types from each other.


Which botanical name is the correct one?
Australian Desert Lime



'Dobashi Beni' mandarin



Cocktail grapefruit



Sour orange



Bergamot



'Atwood' sweet orange



'Honey' pomelo



Washinton Navel orange



Bearss lime
 
 
This is a question many citrus enthusiasts have in their mind. Unfortunately there is no single answer that would cover all the known citrus plants. However, certain decisions made in international botanical conferences in the 1990's do simplify matters considerably.

Tokyo code 1994
Starting from 1994 the oldest correct classification of a plant is the one to be used in scientific studies accompanied by the name of its author (Tokyo Code 1994, Chapter II, Section III, Article 11.3. where it says: "For any taxon from family to genus inclusive, the correct name is the earliest legitimate one." What does legitimate mean? To put it short it means a correct written description in Latin of the plant using accepted botanical and taxonomic practices and terms and placing a dried specimen of the plant (called a holotype) with all the needed parts and specifications in a public botanical library (called Herbarium) for safekeeping so that this specimen may be referred to in case of later dispute or doubt. Most recently also photographs available on the Internet with correct documentation are valid. The author then presents his findings to his peers in international botanical conferences and publishes articles in important botanical journals to get documented approval .

Sometimes the needed documentation is lost, sometimes it was not made properly to begin with so that it is impossible by today's standards to determine with certainty which species, subspecies, variety or form the name given by an author exactly referred to, especially if the holotype is lost or was never preserved and there are no contemporary botanists who would have confirmed the classification in their own writings. In such a case this classification is ignored and the oldest correct classification is used.
Nowadays
Nowadays only the International Botanical Congress, which has been held every five years since 1900, can change the name of a plant according to rules set out in the International Code of Botanical Nomenclature. The currently valid code is the Vienna Code of 2006 available on the Internet. Records of all botanical conferences since 1950 are kept at the International Association for Plant Taxonomy in Vienna, headed by D.J. Mabberley, President since 2005.

Earlier practices
In earlier times authors wrote books often with fine drawings (examples further down). They would send the book or in case of a single plant the needed documents to prominent colleagues around the world to wait for their approval. The acknowledgement would eventually come when the plant in question was referred to by a prominent colleague attaching the new author's name to the new plant.

Why author names?

There are many reasons and again it is not easy to give a single short answer. The main reason is to be able to tell classifications from one another. In earlier times the post was slow and five years after giving a new name a botanist could find that 15 years earlier another botanist somewhere around the globe had already used the same name but of a different plant completely. Author names can be abbreviations, only Carolus Linnaeus (Carl von Linné after his ennoblement) has an initial (L.)  Author names are also important when a name given by one author covers only a part of the plants included in the classification of another author. In a case like that the two names cannot be used as synonyms (example D further down). Only names that refer to the same species or the same members of a family can be used as synonyms.


Classification examples

Example A: Trifoliate orange Citrus trifoliata L.
Citrus trifoliata L. is the accepted name as opposed to Poncirus trifoliata Raf. because Carolus Linnaeus (L.) made his classification of the Trifoliate orange in 1763 and Constantine Rafinesque (Raf.) in 1838. 

In case a later botanist has amended the description of a plant and assigned the name to another plant the name of the previous author is in brackets followed by the name of the new author. The same is true when a different genus name but the same species name was used earlier (Aurantium in the following example).

Example B: Pomelo Citrus maxima (Burm.) Merril 
Johannes Burman originally used the name (Aurantium maximum) in 1755 of several types of pomelo hybrids in the Far-East, but Elmer Merril specified the type and re-assigned the name in 1917 to the pomelo we know today. In the meanwhile Linnaeus had created a new genus Citrus. For the same reason Citrus maxima is the accepted botanical name of the pomelo instead of Citrus grandis Hassk. because Burman's classification predates the one given by Justus Hasskarl in 1842
 
Example C: Limequat Citrus × floridana (J. Ingram & H. Moore) Mabb.
The species name floridana of the previous classification X Citrofortunella floridana of J. Ingram and H. Moore was retained by Mabberley when he created a new name Citrus × floridana obviously to avoid the hybrid genus Citrofortunella (see example E microcarpa). Therefore the names of the earlier authors are in brackets and also to show that the plant in question is the same and not a new species.
Malayan kumquat



Fukushu kumquat



Calamondin



Calamondin



Satsuma 'Aoshima'



Australian Round Lime
Matters get more complicated when authors have used the same name of different plants or a name that only covers certain varieties of the same plant.

Example D: Round kumquat Citrus japonica Thunb. 'Marumi'
The genus name Fortunella was introduced by Swingle in 1915. Whereas the various kumquat types were earlier considered varieties of the same species Swingle created a new species for each type: Fortunella margarita for the 'Nagami' type, Fortunella japonica for the 'Marumi' type and so on. Since the original classification is valid the various kumquat types 'Nagami', 'Meiwa', 'Fukushu', 'Hong Kong', 'Malayan' and 'Marumi' are cultivated varieties of the one species Citrus japonica Thunb. created by Carl Peter Thunberg in 1784. Thus the name Fortunella japonica (Thunb.) Swingle for the Round kumquat is no longer valid and can not be used as a synonym either since the Thunberg name Citrus japonica denotes all kumquats.

A good example is the Calamondin Citrus × microcarpa Bunge. More has been written about the correct classification and origin of the Calamondin than most other citrus plants. And yet, the matter is quite simple.

Example E: Calamondin Citrus × microcarpa Bunge
The exact hybrid nature of the Calamondin remains to be established. It is commonly accepted to be a hybrid of a sour mandarin type and a kumquat. The most frequently mentioned candidates are the Sour mandarin Citrus sunki Tanaka of 1927 (which is the later Citrus reticulata var. austera of Swingle of 1942) and the Oval or 'Nagami' kumquat Citrus japonica Thunb. 'Nagami' (example D).  It is still unclear to which plant the classification Citrus madurensis by Loureiro (1717-1791) referred to because of lack of documentation. Blanco created Citrus mitis in 1837. J.Ingram & H.E.Moore used the name X Citrofortunella mitis in 1975 and the name Citrofortunella × microcarpa was given by Wijnands in 1984. The last two are obvious attempts to acknowledge the hybrid nature of the Calamondin Citrofortunella suggesting both the genus Citrus of the mandarin parent and the genus Fortunella of kumquats. The name Citrofortunella is problematic because of its hybrid nature, as are other hybrid genus names. The whole purpose of creating the International Code of Botanical Nomenclature is establishing unambiguous plant names with the smallest propability of misunderstanding or error. As the genus Fortunella of Swingle is no longer valid either (example D) the concept has no basis. The oldest valid description and classification of the Calamondin was given by Alexander Andrejewitsch von Bunge (1803-1890) as Citrus microcarpa Bunge in 1833. When a plant is obviously of a hybrid nature the sign × is today attached between the genus and species name. Since 1997 the valid botanical name of the Calamondin is and remains Citrus × microcarpa Bunge. The hybrid sign × is only added in writing, it is not pronounced. To type it press down and hold Alt-key, type 0215 on the numeric keypad with Num lock on, release Alt-key.

This is also the reason behind the changes in the names of the Native Australian citrus types explained below. The later genus names of Microcitrus (1915) and Eremocitrus (1914) created by Swingle have now been replaced by the original Citrus classifications of Ferdinand Mueller (F.Muell.) of 1858 and F.M. Bailey from the period 1890-1915.

The assumption that the most recent finding is always the valid truth does not apply to botany as long as the plant in question is the same. New species are always new species and continue to be given new names.



History

The history of the botany of citrus fruit is fairly long. One of the best-known early books is about the oranges of Wen-Chou, China called 記嘉桔錄 literally: Kee Jia Citrus Record (Citrus records of Ji Jia)  written in 1178 AD by Han Yanzhi (Han Yen-Chih Chü lü) the governour of the region. The book was translated in 1923 by Michael J. Hagerty as Monograph on the oranges of Wen-Chou, Chekiang. It describes the 26 citrus varieties grown in the region, their cultivation techniques and use.

In the Western world citrus fruits have been a source of interest to botanists for nearly 400 years; almost as long as the plants have been known in the Western hemisphere. Some of the first attempts at a comprehensive view were done by Volkamer, Linnaeus, Gallesio and Risso. They were based on the appearance of the plant; the size and shape of plant, trunk, leaves and flowers and the number of petals and stamens. These parameters continue to be used in botany today. At the beginning of the twentieth century prominent citrus botanists like Swingle and Tanaka had more modern methods of analysis at their disposal and were able to make their scientific conclusions with the help of accurate laboratory measurements. Since then we have progressed to today's possibilities of DNA research .
Nürnbergische Hesperides title page
Volkamer
Johann Volkamer's (1644 - 1720) Nürnbergische Hesperides published in 1708 is one of the most important early books on citrus botany. The book with its beautiful drawings is available online as a digitised book.  
Traité du citrus title page
Gallesio
Traité du citrus (1811) by Giorgio Gallesio (1772-1839) is also available as a digitised book
.
Engraving of an orange from Risso & Poiteau

Pierre Antoine Risso

Risso & Poiteau
One of the most famous historical citrus classifications is Histoire Naturelle des Orangers published in 1818 by Pierre Antoine Poiteau (1766-1854) and Joseph Antoine Risso (1777-1845).  It brought many new varieties into our attention. Whether all of them deserve the distinction of true species is still under discussion but many of their classifications are still valid today. One of the virtues of the book are the many colour drawings in the Redouté-Bessa style. In the 19th century these were of great help in distinguishing between different citrus types. The original book is now a historical antiquarian rarity. So far nobydy has digitized the book in colour but Google Books has a black and white version
Walter Tennyson Swingle
Swingle
Walter Tennyson Swingle (1871 1952) was an American botanist. He had a major influence on modern scientific approach to citrus research. He created Citrange, a hybrid of sweet orange and trifoliate orange.  He assigned the genera Fortunella to kumquats as well as Microcitrus and Eremocitrus to native Australian citrus types. He is the author of 95 botanical names6 of citrus plants, the Ichang papeda Citrus ichangensis and the Mexican lime (Key lime, Common lime) Citrus aurantiifolia perhaps the most familiar among them. In his major work The Botany of Citrus and Its Wild Relatives, published in 1943 (available on the Internet), Swingle combined all citrus fruit into 16 species and their subspecies with various cultivated varieties.
Tyozaburo Tanaka
Tanaka
Tyôzaburô Tanaka (1885 - 1976) is the author of 180 botanical names6 in the citrus family Rutaceae.  His most important publications include Citrus fruits of Japan 1922, Species problem in Citrus 1954 and Tanaka's Cyclopedia of Edible Plants of the World 1976. Tanaka divided his citrus fruit into 162 species.




Today

Of modern botanists D.J. Mabberley (b.1948), since 2005 president of IATP, deserves special mention. He has presented the most interesting new views of citrus and the relationships between "the true citrus types." In his paper on native Australian citrus types he introduces a new species and assigns the Swingle genera Microcitrus and Eremocitrus in the genus Citrus. In A classification for edible Citrus he states that there are only three citrus species, citron, pomelo and mandarin, which are then involved in several hybrids. In his more recent study CITRUS Linnaeus he broadens the scope to include all the most common citrus species.

D.J. Mabberley

Mabberley's classification of edible citrus
In the above mentined studies Mabberley states that in the edible citrus group there are only three citrus species:
Citrus medic
a L. citron,
Citrus maxima
(Burm.) Merril pomelo and
Citrus reticulata
Blanco mandarin.
Each of these is involved in several hybrids as follows:

1. Citrus medica, citron, which is involved in

  • Citrus × limon(citron × sour orange) lemon and similar hybrids like Palestine Sweet lime and Volkamer lemon.
  • Citrus × jambhiri(citron × mandarin) rough lemon and similar hybrids like Rangpur lime, Mandarin lime and types like 'Otaheite'.
  • Citrus × aurantiifolia(citron × lemon × Ichang papeda) lime (Mexican lime)
  • Citrus × bergamia, (citron × sour orange) bergamot, also considered a citron × sour orange cross.

2. Citrus maxima, pomelo, which is involved in

a) Citrus × aurantium (pomelo × mandarin) which includes three pomelo hybrids

  • Citrus × aurantium (pomelo × mandarin) sour orange. The sour orange has inherited more features of pomelo than mandarin.
  • Citrus × sinensis (pomelo × mandarin) sweet orange. The sweet orange has inherited more features of mandarin than pomelo. This group also includes all the crosses of orange, mandarin and grapefruit such as tangors, ortaniques, tangelos and their backcrosses like Page and Nova.
  • Citrus × paradisi (pomelo × orange) the grapefruit.

3. Citrus reticulata Blanco, mandarin. This includes mandarin, satsuma, clementine and tangerine.

According to Mabberley: "This scheme provides a workable system for botanists and fruit-growers alike."


Newest technology
Since 1997 the use of molecular markers and other most recent technology of DNA analysis have provided new information. In an Analysis of the origin of several citrus species a group of scientists in the University of California found lemon Citrus × limon to be a cross of sour orange Citrus × aurantium and citron Citrus medica. They also found that lime has inherited genes of lemon, citron and the Small-flowered papeda Citrus micrantha. The same group found the bergamot Citrus × bergamia to be a cross of sour orange and limetta Citrus limetta

Another group of scientists working at the University of Catania, Italy, stated in A study of the genetic origins of various citrus species that citron and sour orange are the parents of lemon, and Citrus micrantha and citron are the parents of lime. They further pointed out that besides lemon also the Palestine sweet lime, bergamot and Volkamer lemon are hybrids of citron and sour orange while citron × mandarin hybrids were assumed of the Rangpur lime and the Rough lemon.

It will be interesting to wait for the results of new studies once they have been reviewed by peers and published either in scientific journals or in international conferences.


Tangelo 'Orlando'


'Bajoura' citron © Petr Broža


Persian lime, Citrus latifolia



Ellendale tangor, Citrus reticulata 'Ellendale'


Owari satsuma, Citrus unshiu 'Owari'


Tarocco blood orange, Citrus sinensis 'Tarocco'



Classification of other citrus groups

Fortunella obovata, Fukushu kumquat



Trifoliate orange 'Rich', Poncirus trifoliata



Russel River lime, Microcitrus inodora



Australian desert lime, Eremocitrus glauca
    
Kumquats 

Citrus japonica
Thunb. (previously Fortunella Swingle)
Markedly resembling the other citrus fruits in general and obviously closely related to the Calamondin and some of the small-fruited mandarins, the kumquats were earlier included in the genus Citrus until W.T. Swingle in 1915 created the genus Fortunella. Mabberley re-assigns kumquats to the original Citrus japonica Thunb. (1784). This group also includes kumquat hybrids like lemonquats, mandarinquats, orangequats as well as
Limequats Citrus × floridana Mabb., a classification created by Mabberley in 1998 (Mexican lime × kumquat),
Faustrimedin 'Australian Sunrise Lime' Citrus × oliveri, Mabb. 2004 (Australian finger lime × Calamondin) and
Citrangequat Citrus × georgiana Mabb. 2004 (Citrange × kumquat).

Trifoliate orange and other rootstock
Citrus trifoliata L. (previously Poncirus trifoliata Raf.)
Linnaeus included the Trifoliate orange in the genus Citrus. The genus Poncirus was established by Rafinesque in 1815. It was not until a hundred years later that Swingle gained acceptance for its restoration to a separate standing. As an outdoor ornamental the trifoliate orange is commonly grown in the temperate regions of China, Japan, Western Europe and the eastern United States. It is sometimes used as a very effective hedge. The description also includes the common trifoliate hybrids: citranges, citrumelos, citremons and many other types used as rootstock. Mabberley re-establishes the original Citrus trifoliata of Linnaeus.

Native Australian citrus
(Previously Microcitrus Swingle and Eremocitrus Swingle)
This Citrus group contains seven species, five of which are native to Australia with the other two found in New Guinea. Four Australian species occur in rainforests and their margins from Cape York Peninsula south to the Northern Rivers of New South Wales. They produce small, round or finger-shaped fruit, with a pleasant but very acid juice. The Australian desert lime Citrus glauca occurs in southeastern Queensland is a small tree or a large shrub, sometimes only a few feet high. It is the only plant in the whole orange subfamily that is able to survive extreme drought. Its hybrids include eremolemons, eremoranges, eremoradias (a hybrid with the sour orange) and citrangeremos (a hybrid with citrange). Mabberley uses the original genus Citrus of  F.M. Mueller (1825 - 1896), Government Botanist of Victoria and F.M. Bailey (1827-1915), Colonial Botanist of Queensland. The re-application of the genus Citrus is not an attempt to raise the status of the native Australian Citrus types but an automatic consequence of the Tokyo Code of 1994.

 

Kabosu papeda, Citrus sphaerocarpa



Cochin China Atalantia, Atalantia citroides



Mock orange, Murraya paniculata

Papedas
Papedas form a subgenus of the genus Citrus. They are a group of acid wild citrus types growing in the monsoon region and parts of Japan and China. The best-known papeda is perhaps the Kaffir lime Citrus hystrix DC. Papedas have pulp vesicules with numerous droplets of acrid oil, which often makes them inedible. Papedas belong to the oldest and most primitive known citrus types and according to recent studies have contributed their genes to many well-known citrus fruits, most notably the various kinds of limes. Citrus micrantha, the Small-flowered papeda, is one of the parents of the common lime (Key lime), Citrus aurantiifolia Swingle.


Distant citrus relatives
Clymenia, Clausena (Wampee), Glycosmis (Orangeberry), Murraya (Curry leaf and Mock orange), Severinia (Boxthorn) and Triphasia (Limeberry) are some of the more common distant citrus fruit relatives in the citrus subfamily described on the Distant Citrus relatives page.


The citrus subfamily Aurantioideae
All of the above mentioned citrus trees belong the citrus subfamily Aurantioideae (sometimes called Citroideae) in the Rutaceae family. A table listing the tribes, subtribes, subtribal groups, genera and species of the citrus subfamily gives an overview of the botanical relationships of the citrus genera listed above. It concentrates on the citrus types most familiar to the average consumer and citrus enthusiast: the edible citrus fruits, the most common types grown for decorative purposes as well as the ones used as rootstock. The classifications used are pre-1994.


Botanical names on Citrus Pages

Why are some botanical names on Citrus Pages different from names used on other websites?

In accordance with the 1994 Tokyo code of the International Code of Botanical Nomenclature where it says: "For any taxon from family to genus inclusive, the correct name is the earliest legitimate one" the oldest valid botanical name as used by D.J. Mabberley in his studies 1, 2 and 3, is given first. It is followed by the most commonly known later synonyms so that each may make his or her own choice. The present author has made a special effort to include as many earlier, previously valid botanical names as possible. He has followed the practice in leading literature and taken note of the reference studies listed below.

The fruit are divided into fourteen groups. Sometimes the division into groups is determined solely by the food use of each fruit. Thus the fruits in the lime group are not all closely related but form a collection of several different kinds of citrus fruit that are used in the kitchen in much the same way as limes. The same is true of lemons. The only completely homogeneous groups are pomelos, grapefruit and sweet oranges, all of which contain cultivated varieties of one citrus type only.

Around 310 different types of citrus fruit are presented with a brief description and a photograph. Another 90 closely related cultivated varieties are mentioned bringing the number of varieties to approximately 400. The botanical index contains about 280 alternative Latin names for the 131 citrus types that at one time or another were deemed valid species and were thus given a botanical name by an author.



Orangequat 'Nippon'


Mandarin 'Nasnaran' © C. Jacquemond / INRA


Pear lemon, Citrus lumia





Monoembryony, nucellar embryony and polyembryony in citrus seeds.
 

Glossary:
  • Apomixis - development of an embryo without the fusion of male and female cells.
  • Monoembryony – development of only one (usually zygotic) embryo in a seed.
  • Nucellar embryony - embryos form directly from the nucellar tissue of the seed without the genes of the father plant.
  • Polyembryony - occurrence of more than one (usually nucellar) embryo in a seed.
  • Zygotic - sexually produced, contains genes of both parents.
 

Monoembryony
Monoembryony is the usual case in many living creatures. A seed is formed that consists of one embryo that is sexually produced (zygotic) and inherits the genes of both parents. In other plants (not Citrus) the seeds being true to type is often caused by the incompatibility of the pollen of other species.
 
Three important citrus types produce monoembryonic and therefore zygotic seeds only

'Clementine' mandarin
Pomelo, Citrus maxima
'Temple' tangelo

These three have been widely used in breeding because they are of high quality and all crosses of these as mother plants produce hybrid seeds and not nucellars. A more detailed list of monoembryonic citrus types can be found below.
 

Nucellar embryony 
Nucellar embryony (notated Nu+) is a form of seed reproduction that occurs in many citrus varieties. During the germination of seeds from plants that possess this genetic trait, the nucellar tissue which surrounds the embryo sac in the ovule can produce additional embryos (polyembryony) which are genetically identical to the parent plant. These nucellar seedlings are essentially clones of the parent. By contrast, zygotic seedlings are sexually produced and inherit genetic material from both parents. Nucellar embryos begin development as soon as pollination occurs while zygotic embryos take four weeks to develop; thus, nucellar embryos often crowd out the zygotic embryos. Pollination is usually needed to trigger also nucellar development.
 
Nucellar embryony is important to the citrus industry. Most commercial rootstock varieties produce mainly nucellar seedlings which do not inherit any of the traits of the "father" plant. This allows for the production of uniform rootstock which yields consistent results in fruit production. However, this trait causes problems for cross-breeding. Hybridising two plants that produce mostly nucellar embryos can be very difficult. Sweet oranges (89.3 –97.3% polyembryonic [*8]) cannot be successfully crossed. Practically all currently available sweet orange varieties are spontaneous mutations that first occurred either as seedlings or more often as limb sports or bud mutations.
 
There are some important implications of nucellar embryony.
Rootstocks such as 'Rough Lemon', 'Sour Orange', and Trifoliate orange’  can be produced true-to-type from seed. This is important for nurseries because virus is not transmitted through either nucellar or zygotic seed. Thus, nurseries can produce virus-free, clonal rootstocks from seed. Virus can be eliminated from infected clones but this is very expensive.

Because nucellar seedlings are juvenile it takes a long time to obtain productive clones. These can then be propagated by budding from the indexed nucellar mother tree.
 
There are various degrees of nucellar embryony. Some cultivars produce only nucellar embryos, and some vary in the percentage of nucellar embryos. The ‘Volkamer lemon’ can produce about 50% nucellar embryos while the rest are zygotic [7*.]
 
Nucellar seedlings can have slightly variable traits. Often nucellar bud lines are more vigorous and more productive. Sometimes nucellar seedlings lose a quality. The nucellar seedlings of Thompson grapefruit lose the pink pigmentation whereas the seedlings of Foster grapefruit have stronger pigmentation than the mother plant.

 
Polyembryony
 
Polyembryony means the occurrence of several embryos in one seed. The amount of polyembryony within a seed varies from type to type In lemons Citrus × limon 25 – 43% of seeds are polyembryonic the rest being zygotic [7.*]
 
Polyembryony seems not to be caused by a single gene or factor. In a study of seeds of the Valencia orange polyembryony seemed to coincide with a difference in the albumin and globulin structure and content of the nucellar tissue. [9*]  In a recent study in Japan several genes were found to contribute towards the forming of nucellar embryos and several citrus types seemed to have changes in different genomic regions that coincided with nucellar embryony. [10*]
 
The amount of polyembryony in the seeds of Volkamer lemon is affected by weather, location, year of harvest and type of pollinator but there seems to be no correlating factor between the fruit characteristics and polyembryony. It was not possible to determine which Volkamer lemon fruit may have more polyembryony just by comparing their characteristics. In contrast, a high correlation has been found in the characteristics of ‘Cleopatra’ mandarin. [11*]  

A list of the most common polyemryonic citrus types can be found below.


Which citrus fruits will come true to type from seed?

Tom McClendon writes in Hardy Citrus for the South East:

“Most common citrus such as oranges, grapefruit, lemons and most mandarins are polyembryonic and will come true to type. Because most citrus have this trait, hybridization can be very difficult to achieve. In the late 19th century, when the first attempts at controlled hybridization were attempted by the United States Department of Agriculture in Florida, Walter T. Swingle reported that more than 1,100 sweet orange seeds pollinated with trifoliate orange pollen were required to produce the first citranges, and seven of these came from a single fruit. The good news is that polyembryony helps stabilize varieties, which allows seeds to be passed around with little chance of spreading diseases such as viruses. This unique characteristic allows amateurs to grow citrus from seed, something you can’t do with, say, apples.” [12*]


Highly monoembryonic citrus types:
Will produce zygotic monoembryonic seeds that will not come true to type.

Many “true Citrus species” such as

Citrus cavaleriei
(C. ichangensis) Ichang papeda
Citrus glauca (Eremocitrus glauca) Australian desert lime
Citrus indica
Citrus japonica (Fortunella) most kumquats
Citrus maxima pomelo (pummelo, Schaddock)
Citrus medica citron

And the citron hybrids:

Citrus × latifolia Persian lime (Tahiti lime, Bearss lime)
Citrus × bergamia Bergamot.

One true citrus species, mandarin Citrus reticulata, is highly variable. The following are monoembryonic:

Clementines (Marisol, Oronules, Loretina, Beatriz, Clemenpons, Arrufatina, Esbal, Oroval, Clemenules, Orogrande,
                        Tomatera, Fina, Nour, Hernandina, Clementard.) 
Ellendale
Encore
Fortune
Fremont (50% monoembryonic)
Temple
Ugli
Umatilla
Wilking


Highly polyembryonic citrus types

Will mostly produce nucellar polyembryonic seeds that will grow true to type.

Citrus × aurantiifolia Mexican lime (Key lime, West Indian lime)
Citrus × insitorumCitroncirus webberii) Citranges, such as Rusk, Troyer etc.
Citrus × jambhiri ‘Rough lemon’, ‘Rangpur’ lime, ‘Otaheite’ lime
Citrus × limettioides Palestine lime (Indian sweet lime)
Citrus × microcarpa ‘Calamondin’
Citrus × paradisi  Grapefruit (Marsh, Star Ruby, Redblush, Chironja, Smooth Flat Seville)
Citrus × sinensis Sweet oranges  (Blonde, navel and blood oranges)
Citrus amblycarpa 'Nasnaran' mandarin
Citrus depressa ‘Shekwasha’ mandarin
Citrus karna ‘Karna’, ‘Khatta’
Citrus kinokuni ‘Kishu mandarin’
Citrus lycopersicaeformis ‘Kokni’ or ‘Monkey mandarin’
Citrus macrophylla ‘Alemow’
Citrus reshni ‘Cleopatra’ mandarin
Citrus sunki (Citrus reticulata var. austera) Sour mandarin
Citrus trifoliata (Poncirus trifoliata) Trifoliate orange

The following mandarin varieties are polyembryonic:

Dancy
Emperor
Empress
Fairchild
Kinnow
Mediterranean (Avana, Tardivo di Ciaculli)
Naartje
Nova (Clemenvilla)
Ortanique
Ponkan
Sampson
Satsumas (Hashimoto, Okitsu, Clausellina, Owari)
Willowleaf

 
The following lemon types are polyembryonic:
Fino (Primofiori)
Verna (Berna)
Eureka
Lisbon
for other lemon varieties see Intermediate embryony below:

And a few more citrus types:

Marrakech limetta
Fukushu kumquat (Fortunella obovata)
Nanshôdaidai


Intermediate embryony

Is shown by some lemon types that produce 25-40% polyembryonic seeds and Volkamer lemon that tends to produce one nucellar and one zygotic embryo in each seed depending on climate, year and type of pollinator. [7*], [11*]







References


1
 
Australian Citreae with notes on other Aurantioideae (Rutaceae)
   Mabberley, D.J. Rijksherbarium, University of Leiden, Netherlands and Royal Botanic Gardens,
   Mrs Macquaries Road, Sydney, NSW 2000, Australia (1998)  Telopea 7(4):333–344.
   http://www.rbgsyd.nsw.gov.au/__data/assets/pdf_file/0003/73236/Tel7Mab333.pdf


2 A classification for edible Citrus    
   D.J. Mabberley,  Rijksherbarium, University of Leiden, Netherlands and
   Royal BotanicGardens,
Mrs Macquaries Road, Sydney, NSW 2000, Australia (1997)  Telopea 7(2): 167–172.
   http://www.rbgsyd.nsw.gov.au/__data/assets/pdf_file/0019/73216/Tel7Mab167.pdf


3 CITRUS Linnaeus, Sp. Pl. 2: 782. 1753.
   Zhang Dianxiang, David J. Mabberley, Fl. China 11: 90–96. 2008.
   http://flora.huh.harvard.edu/china/PDF/PDF11/Citrus.pdf


4 RFLP analysis of the origin of Citrus bergamia, Citrus jambhiri, and Citrus limonia          
   Federici, C.T., Roose, M.L. and Scora, R.W. 2000. Acta Hort. (ISHS) 535:55-64
   http://www.actahort.org/books/535/535_6.htm
   http://plantbiology.ucr.edu/faculty/roose.html#2

5 Citrus phylogeny and genetic origin of important species as investigated by molecular markers
   E. Nicolosi, Z. N. Deng, A. Gentile, S. La Malfa, G. Continella and E. Tribulato
   Istituto di Coltivazioni arboree, University of Catania, Italy. Theoretical and Applied Genetics 100(8): 1155-1166.

   http://www.springerlink.com/content/tjggcl5wyyu6l69d/

6 The International Plant Names Index (IPNI) database of authors.

7 Assessment of polyembryony in lemon: rescue and in vitro culture of immature embryos
   O. Pérez-Tornero and I. Porras
   Instituto Murciano de Investigación y Desarrollo Agrario y Alimentario, C/ Mayor s/n, 30150 Murcia, Spain
   Plant Cell, Tissue and Organ Culture 2008, 93: 173-180
   http://resources.metapress.com/pdf-preview.axd?code=235001u8w4877334&size=largest
  
8 Studies on Polyembryony and Improvement of Breeding Efficiency of Oranges.
   Hwang A-shiang and Yeuh Ching-shi.
   http://www.tari.gov.tw/taric/uploads/publication_no27-10.pdf
 
9 Polyembryony in Citrus
   Accumulation of Seed Storage Proteins in Seeds and in Embryos Cultured in Vitro
   Anna M. Koltunow, Tetsushi Hidaka and Simon P. Robinson
   Plant Physiol. (1 996) 11 O: 599-609
   http://www.ncbi.nlm.nih.gov/pmc/articles/PMC157756/
 
10Marker enrichment and construction of haplotype-specific BAC contigs for the polyembryony genomic region in Citrus
   Michiharu Nakano, Tokurou Shimizu, Hiroshi Fujii, Takehiko Shimada,Tomoko Endo, Hirohisa Nesumi, Takeshi Kuniga and Mitsuo          Omura
   Breeding Science Vol. 58 (2008) , No. 4 375-383
   http://www.jstage.jst.go.jp/article/jsbbs/58/4/58_375/_article
 
11Polyembryony and identification of Volkamerian lemon zygotic and nucellar seedlings using RAPD
   María Andrade-Rodríguez, Angel Villegas-Monter, Guillermo Carrillo-Castañeda, and Armando García-Velázquez.
   Pesq. agropec. bras., Brasília, v.39, n.6, p.551-559, jun. 2004
   http://webnotes.sct.embrapa.br/pdf/pab2004/junho/39n06a06.pdf
  
12Hardy Citrus for the South East
   Tom McClendon
   Southeastern Palm Society SPS Publishing
   http://www.sepalms.org/Publications/Hardy%20Citrus%20sample.pdf



Australian Outback limes





Tangelo 'Nova' © C. Jacquemond / INRA





Meyer lemon, Citrus limon var. meyerii





Malayan kumquat





Trifoliate orange 'Rich', Poncirus trifoliata



Literature and other sources
Walter T. Swingle and Philip C. Reece: The Botany of Citrus and Its Wild Relatives. Chapter 3 of The Citrus Industry Vol 1 pp 190 - 430. Originally published in 1943 this is one of the best-known taxonomic descriptions of citrus fruit. Now also available in its entirety on the Internet.

Robert Willard Hodgson: Horticultural Varieties of Citrus  Chapter 4 of The Citrus Industry.
An extensive description of both common and rare cultivated varieties of citrus, also available on the Internet.

R. Cottin: Citrus of the World, A citrus directory, SRA - INRA - CIRAD © 2002   A catalogue of more than 5500 citrus names, classified by botanical, common and cultivar names. Includes a useful comparison of Swingle and Tanaka terminology listing equivalent names of both.

D.J. Mabberley: The Plant-Book, Second edition, Oxford University Press © 1997,  858 pp.

James Saunt: Citrus Varieties of the World,  Second edition, Sinclair UK © 2000, 160 pp.
           
University of California, Riverside Citrus Variety Collection, Citrus varieties
A presentation of the most important common varieties at the UCR Experiment Station.

University of California, Riverside CCPP Citrus Clonal Protection Program, Variety data
A presentation of the holdings of the CCPP citrus variety collection with the relevant data.

Répartition des variétés par espèces. Station de recherche agronomique SRA-INRA Corse.
Détails des variétés par espèces. Station de recherche agronomique SRA-INRA Corse.

These two large databases detail the holdings of the INRA Citrus Research Station in Corsica, France.


Molecular characterization and genetic diversity among Japanese acid citrus based on RAPD markers 
   A. Asadi Abkenar and S. Isshiki 2002. Laboratory of Biotechnology and Plant Breeding, Saga University, Japan.

   Journal of Horticultural Science & Biotechnology
(2003) 78 (1) 108-112
   http://www.citrusvariety.ucr.edu/citrus/documents/JapaneseAcidCitrus.pdf

Native Australian Citrus – wild species, cultivars and hybrids
   Primary Industries and Resources, Government of South Australia (PIRSA) FS No: 7/03
   http://www.ausbushfoods.com/new/upload/factsheets/ancitrus.pdf

Molecular Genetic Analysis of  Nucellar Embryony in Citrus
   Mikeal Roose:  Department of Botany and Plant Sciences, University of California at Riverside
   Citrus Research Board 2000 Annual Report.
   http://www.citrusresearch.com/documents/c7a4fcdf-c23a-4742-8992-d1303b4c636b.PDF
 
'Chandler' pomelo



'Lumia' citron




Trifoliate orange 'Argentine'



Yuzu, Citrus junos








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Page up-dated 25 February 2011

 
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