Beetroot (2004)
Stephen Nottingham
© Copyright: Stephen Nottingham 2004
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3. Classification and Botany
Beetroot
(beets) are classified, like all living organisms, in terms of class, order,
family, genus and species. Beets are flowering plants and therefore within
the class Dicotyledonae. Within this class, they are part of the order
Caryophyllales. Within this order, beets are part of the Chenopodiaceae
family. The Chenopodiaceae or goosefoot family of plants also includes other
edible species, including spinach (Spinacia oleracea), quinoa (Chenopodium
quinoa), orache or orach (Atriplex hortensis) and Good King Henry
(Chenopodium bonus-henricus). Beets are in the genus Beta and
the species Beta vulgaris. The cultivated forms of Beta vulgaris
are leaf beets (spinach beet and chard), beetroot (table beet or garden
beet), fodder beet and sugar beet. This chapter explains how beets are
classified within the genus Beta. The chapter concludes with a closer
look at the different forms of cultivated beet.
The Swedish botanist Linnaeus (Carl Linné,
1707-1778) described the species Beta vulgaris L. in 1753. He erected
the genus Beta L. specifically to incorporate this species. Linnaeus'
name is therefore given as the authority for the genus and species; it is
usually abbreviated (L. or sometimes Linn.).
The genus Beta probably first originated
in Mediterranean Europe, diversifying and spreading northward and eastward in
prehistoric times. A secondary region of Beta biodiversity developed
in the Near East. Annual, biennial and perennial plants occur in the genus.
A section is a taxonomic subdivision of a genus.
Sections were first erected within the genus Beta in 1927. Different
authors have modified these sections over the years. The original three
sections described were Vulgares, Corollinae and Patellares.
Today, four sections are used to classify all the plants within the genus Beta.
These four sections are Procumbentes (formerly Patellares), Nanae,
Corollinae and Beta (formerly Vulgares). At present,
within the genus Beta, these four sections encompass ten species and
three subspecies.
The oldest of the four sections in evolutionary
terms is Procumbentes. Botanically, Procumbentes is
distinguished by having hard round black fruits (seed clusters) lacking the
perianth segments that result in plants in other Beta sections having
'corky' fruit with irregular shapes. The section contains three species: Beta
patellaris Moq., Beta procumbens Chr. Sm., and Beta webbiana
Moq.. These species are all perennial (under favourable conditions), with
very short vegetative phases. They are distinguished from each other by leaf
shape: Beta patellaris having the broadest stem leaves and Beta
webbiana the narrowest. The centre of diversity for Procumbentes
is the Canary Islands.
Beta patellaris
(referred to as Beta campanulata by Vilmorin in 1923) is found in
patches (up to 1.5 m diameter) in coastal and low-lying dry rock areas
throughout the Canary Islands, and also in south-east Spain, near Almeria,
and some coastal areas of Morocco. It has small white twisted roots, and a
chromosome number of 36.
Beta procumbens has a
very limited distribution, being virtually restricted to the Canary Islands,
particularly Tenerife, Gran Canaria, Gomera, La Palma and Lanzarote. Isolated
populations might still exist in southern Portugal and north Africa. The
plant has small white twisted roots and a chromosome number of 18.
Beta webbiana is
virtually restricted to Tenerife, Gran Canaria and Fuerteventura in the
Canaries. Isolated populations might still exist in southern Portugal and
north Africa. It has very long fibrous roots and a chromosome number of 18.
The only species in the section Nanae is Beta
nana Boiss & Held. It is endemic to Greece and has a distribution
that is restricted to snowy patches on the Greek mountains Olympus,
Parnassos, Giona and Taiygetos in Greece. This plant is endangered and is
protected within nature reserves. Nanae is distinguished from other
sections due to a short inflorescence (only up to 10 cm) and its solitary
flowers. Beta nana is a small perennial plant, with a stout
cylindrical root and a chromosome number of 18.
Plants in the section Corollinae are hardy
perennials with strongly sclerified roots. Their distribution is centred on
Iran and Asia Minor. There are currently three species recognized in this
section: Beta lomatogona Fischbeck & Mey, Beta macrorhiza
Stev. and Beta corolliflora Zoss.
Beta lomatogona is
distinguished within the section by having flowers that are usually solitary.
It is found at relatively high altitudes, primarily in Anatolia, Armenia,
Azerbaijan, Iran and south-east Turkey, in steppes, arid-stony mountain
sides, dry wastelands and in agricultural land. Beta lomatogona has a
chromosome number of 18. The former species Beta intermedia Bunge is
now thought to be an apomictic type of Beta lomatogona having a
chromosome number of 45.
Beta macrorhiza is a
perennial that develops a bushy habit with maturity. It is distributed in
mountainous regions of Armenia, northern Azerbaijan, Dagestan and south-east
Turkey, typically in dry river beds and disturbed ground. It has a chromosome
number of 18.
Beta corolliflora is a
perennial that has a pyramidic growth habit. It is found in central Anatolia,
Armenia, Georgia and western Azerbaijan, at high altitude (1300-2450 metres),
on stream banks, in meadows and in moist disturbed land, including farmland.
It has a chromosome number of 36. Two types of Beta corolliflora
exist, the other being the former species Beta trigyna Wald. &
Kit., which is now considered an apomictic hexaploid (54 chromosomes) close
to Beta corolliflora.
The Beta section of the genus Beta
contains all the cultivated forms of Beta vulgaris and the wild sea
beet from which they are descended. The section contains three species: Beta
macrocarpa Gussone, Beta patula Aiton and Beta vulgaris L..
There are currently three subspecies (subsp.) recognized in this section: Beta
vulgaris subsp. adanensis (Pamukuoglu) Ford-Lloyd and Williams, Beta
vulgaris subsp. maritima (L.) Arcangeli and Beta vulgaris
subsp. vulgaris L.. The distribution of plants within the section Beta
is centred on the eastern Mediterranean region.
Beta macrocarpa is an
annual with a short vegetative phase, with long, ovate, glabrous (smooth)
green leaves. The glomerules (flowers) cluster on the inflorescence in groups
of around three. It is found in southern Portugal, southern Spain, the
Balearic Islands, the Canary Islands, Morocco, Algeria, southern France,
Greece, Cyprus, Israel and Turkey. Beta macrocarpa is a fairly common
species, for instance, in field margins and along roadsides. It is also
halophytic (salt tolerant) and found in particular saline habitats, such as
salt marshes in Portugal and salt mine workings in Portugal and the Canary
Islands. The chromosome number is usually 18, although natural polyploidy has
resulted in Canary Island populations having a chromosome number of 36.
Beta patula is a
perennial, under favourable frost-free conditions. It has small glabrous
leaves and is distinguished botanically from the other two species in the
section Beta by having flower clusters of around seven. Beta patula
has a very limited distribution, being found only on Illheu dos Embarcaderos
- a small island near Madiera. It has a chromosome number of 18. Beta
vulgaris subsp. adanensis is an annual, and also a perennial under
favourable (frost-free) conditions. It has long wedge-shaped leaves and
glomerules in groups of around three that are spaced out along the
inflorescence. It is found in Greece, Cyprus, Turkey and Syria. The
chromosome number is 18.
Beta vulgaris subsp.
maritima or sea beet is the ancestor of cultivated beet. It can grow
as a biennial or a perennial, under favourable (frost-free) conditions,
although many Mediterranean populations are annuals. It has long wedge-shaped
leaves in a rosette arrangement and is without a swollen root. The glomerules
occur in groups of around three and are crowded together on the
inflorescence. It is further differentiated from subsp. adanensis
through its smaller glomerules and flowers that are less flat. It has a
chromosome number of 18.
Sea beet occurs throughout the Mediterranean
region, along Atlantic coasts up into Scandinavia, throughout the Near and
Middle East, and into India. It thrives on stony and sandy beaches, rocky
cliffs, coastal grasslands, and salt marshes. Sea beet is also found in some
inland sites in the Mediterranean region, and in Iran and Azerbaijan.
Beta vulgaris subsp.
vulgaris is cultivated beet, in all its forms. Cultivated beets are
considered in terms of Cultivar Groups later in this chapter.
All plants within the section Beta can
readily interbreed. Wild and cultivated plants in the genus Beta are
outbreeding. This means that they are more likely to fertilize other
compatible plants (cross-fertilization) than to fertilize themselves
(self-fertilization). They share a mechanism, called the S-allele
incompatibility system, which acts to prevent self-fertilization. A
continuous variation in morphology within the section Beta would therefore
exist, if genetic lines were not preserved through artificial selection and
cultivation.
Interbreeding is rare between plants in different
sections of the genus Beta. One reason for this is the differences in
chromosome number in different sections. Chromosome number varies through
polyploidy, the presence of extra sets of chromosomes in the nucleus. For the
genus Beta, the chromosome number possibilities are 9 (haploid), 18
(diploid), 27 (tetraploid), 36 (triploid), 45 (pentaploid) or 54 (hexaploid).
Beta vulgaris in the Beta section has a chromosome number of
18, but chromosome number varies in other sections. Plant breeders have found
it easier to cross Beta vulgaris with plants in its own section. Wild
sea beet has contributed genes to many modern sugar beet cultivars, for
example, such as resistance to the fungal disease Cercospora. However,
important sources of pest and disease resistance have been identified in
plants in the Procumbentes and Corollinae sections. After much
experimentation, some of these genes have been bred into cultivated Beta
vulgaris. Beet cyst nematode resistance, for example, has been
transferred from Beta patellaris to sugar beet.
The incompatibility system than maintains
outbreeding has been known to break down naturally, however, in isolated
annual populations of wild beet in the genus Beta. Plants within these
populations can therefore be self-fertilized. Small populations of
predominantly inbreeding wild beets can be found in isolated locations
throughout the Mediterranean. New species of wild beet have been described
from such populations.
There are two approaches to classifying
cultivated plants such as Beta vulgaris. The first is the taxonomic
system called binomial nomenclature and the second is the more flexible
horticultural classification system.
The system of classification, in which organisms
are assigned to class, order, family, genus and species, is called binomial
nomenclature. This system was devised by the Swedish botanist Linnaeus (Carl
Linné 1707-1778). Binomial nomenclature can be extended below the species
level (infraspecific classification), by including the ranks of subspecies
(subsp.) and variety (var.).
Strict rules are applied for binomial
nomenclature. For plants, these rules have most recently been laid out in the
International Code of Botanical Nomenclature (ICBN), which was adopted by the
International Biological Congress in 1981 and published in 1983.
Binomial nomenclature has been very successful in
cataloguing the world's biodiversity, but its use below the species level may
not always be appropriate for cultivated plants. By definition, a species is
a group of organisms that interbreed. Populations of a species can become
separated due to geographic isolation or other factors. Such populations can
develop different characteristics, under different selection pressures, and
become distinct subspecies. Subspecies, by definition, must resemble each
other and, as a group, have characteristics that distinguish them from other
subspecies. Organisms within a subspecies breed more freely among themselves
than with other members of the species. Eventually a new species may arise
through speciation. This has occurred for wild beets in the genus Beta,
particularly in the Canary Islands. However, it has been artificial
selection, rather than natural selection, that has driven the evolution of
cultivated beets. This can be problematic for classification when cultivated
forms, assigned to different categories, can freely hybridize in the field.
When Linnaeus first described Beta vulgaris
in 1753, he erected three varieties within the species: wild ancestral beet Beta
vulgaris var. perennis, leaf beet Beta vulgaris var. cicla,
and garden beet Beta vulgaris var. rubra.
In 1763, Linnaeus decided that wild maritime or
sea beet should be a separate species, Beta maritima L., and the
varietal name perennis was discarded. Taxonomists have since brought
sea beet back into the Beta vulgaris species. Sea beet is now
considered a subspecies: Beta vulgaris subsp. maritima (L.)
Arcangeli. Its classification as a subspecies is appropriate because of the
ease with which it cross-breeds with cultivated Beta vulgaris, and the
insufficiently discontinuous morphology between it and cultivated forms
within the genus Beta to warrant it being a separate species.
Linnaeus' division of Beta vulgaris into
leaf beets (cicla) and root beets (rubra) has persisted;
although since Linnaeus' day, fodder and sugar beets have caused the root
beets to be further sub-classified.
For many years, the distinction between leaf and
root beets was raised to the subspecies level. Artificial selection has
exacerbated the morphological differences within Beta vulgaris, and
this was thought to represent real morphological, genetic and geographic
discontinuity within the species. An emphasis on the leaves gave rise to Beta
vulgaris subsp. cicla. The name 'cicla' derives from Sicula, the
name Theophrastus originally gave to leafy beets from Sicily. Selection for a
single swollen taproot gave rise to Beta vulgaris subsp. vulgaris.
However, the current taxonomic situation is that all cultivated forms of Beta
vulgaris belong to just one subspecies: Beta vulgaris subsp. vulgaris.
In taxonomic schemes, variety (var.) is the level
below subspecies. A variety is a group of individuals that differ distinctly
from but can interbreed with other varieties of the same species. The
characteristics used to classify a variety must be genetically inherited. In
Linnaeus' day, fodder beet and sugar beet were considered to be beetroot.
Today, all the different cultivated forms of Beta vulgaris subsp. vulgaris
are distinguished at the variety level. Chard is Beta vulgaris var. cicla,
beetroot or table beet is Beta vulgaris var. conditiva, fodder
beet is Beta vulgaris var. alba, and sugar beet is Beta
vulgaris var. altissima (formerly esculenta).
In a recent taxonomic key of the genus Beta,
the authors (Frese et al.) quote Paul Aellen, writing in 1938: "Studies
on Beta are getting more difficult the more you plunge in the
matter". The taxonomy of Beta has been modified numerous times
since Aellen wrote these words and the situation is still fluid. However,
recent genetics studies are helping to clarify the situation. The
classification of wild species in the genus Beta has recently been
rationalized and the overall number of species in the genus has been reduced.
The current taxonomic consensus is that the use
of subspecies and botanical variety within the ICBN scheme is confusing and
not strictly necessary for cultivated Beta vulgaris. Single genes
alone can determine swollen root shape and pigmentation, the different
cultivated forms of Beta vulgaris have not been genetically or
geographically isolated during their history, and there are no known barriers
to gene exchange with the section Beta. The subspecies division for
cultivated Beta vulgaris has been removed, although the different
cultivated forms are still assigned to different varieties. Beta
taxonomists, however, have suggested that it would be better, and less
confusing, if all infraspecific classification for cultivated Beta
vulgaris were done using the non-hierarchical horticultural
classification system.
Infraspecific classification is frequently
adopted for cultivated plants, because artificial selection results in clear
morphological differences within a species. However, binomial nomenclature
has a number of shortcomings when it comes to classifying cultivated plants
below the species level. The system of horticultural classification, which
uses the concepts of cultivated variety (cv. or var.) and cultivar race or group
(cv. Group), has emerged as the most useful method of infraspecific
classification for cultivated plants. The International Code of Nomenclature
for Cultivated Plants (ICNCP), published in 1980, gives the accepted rules
for applying this horticultural classification scheme.
The horticultural scheme has been erected for
different reasons to those underlying the Linnaean system of binomial
nomenclature. Rather than being a framework in which to catalogue all living
creatures, the horticultural system of classification has more practical
aims, for example, to achieve uniformity for seed registration and to form a
basis for Plant Breeders' Rights legislation. Hierarchical taxonomic
classification is often too rigid for people who work with cultivated plants.
The horticultural system is flexible system and aids communication, enabling
the transfer of clear and easily understood information between people
working with cultivated plants.
The concept of cultivar is central to the ICNCP
system. A cultivar is defined within the ICNCP as an assemblage of cultivated
plants that is clearly distinguished by any characteristics (e.g.
morphological or chemical), which when reproduced by sexual or asexual means
retains these distinguishing characteristics. A cultivar can be a clone, line
or assemblage of cross-fertilizing plants. Landraces or crop lines maintained
by traditional agricultural methods, often in areas of crop origin and
centres of genetic diversity, can be considered cultivars. Therefore, the
ICNCP is a much more open and flexible system than the ICBN system, which is
a closed classification system with rigid rankings. The ICNCP is considered
subordinate to the ICBN, although both codes can be effectively co-ordinated
in most cases.
The cultivar name of Beta vulgaris is
usually given after the species or subspecies name, which for all cultivated
beets is now Beta vulgaris subsp. vulgaris. Beetroot cultivars
include, for example, Beta vulgaris subsp. vulgaris cv.
Boltardy and Beta vulgaris subsp. vulgaris Red Ace F1.
Sometimes, cultivar is given after the variety name, for example, Beta
vulgaris var. conditiva Cheltenham Green Top. However, cultivar
names do not overlap for the different cultivated forms of Beta vulgaris.
Variety is often used synonymously with cultivar,
but the terms have distinct meanings. A cultivar is a plant that has been
developed and maintained by cultivation as a result of agricultural or
horticultural practices. The term cultivar is derived from cultivated
variety. In binomial nomenclature, botanical variety is a fixed rank below
subspecies. Cultivar is a category without rank, as long as it comes below
the taxonomic rank to which it is assigned. Therefore, it may appear, for
instance, below the rank of genus, species, subspecies or variety. In
addition, cultivars are man-made, while there is no indication of whether a
botanical variety exists naturally or needs to be maintained by artificial
selection.
Within the ICNCP system, assemblages of similar
cultivars can be grouped. The Leaf Beet cultivar grouping, Beta vulgaris
subsp. vulgaris Leaf Beet Group, sometimes known as the Cicla
grouping, contains Spinach Beet (perpetual spinach) and the chards.
Beetroot, forage beet and sugar beet are part of
the Crassa cultivar grouping. Within the Crassa grouping, beetroot, forage
beet and sugar beet are Beta vulgaris subsp. vulgaris Garden
Beet Group, Beta vulgaris subsp. vulgaris Fodder Beet Group,
and Beta vulgaris subsp. vulgaris Sugar Beet Group,
respectively.
In addition to this system, beetroot cultivars
are often further grouped with respect to root shape. Four categories of root
shape are usually recognized: (i) globe or spherical, (ii) long, (iii)
cylindrical or intermediate (half-long), and (iv) flat. The long and globe
categories have sometimes been subdivided into classes. In one scheme,
devised by Holland (1957) globe was subdivided into two classes, while long
root shape was subdivided into five classes. However, the advantage of
erecting subdivisions below the four basic root shapes is unclear. Today,
most of the beetroot grown is globe-shaped.
Cultivated beets
Leaf Beets: Beta vulgaris subsp. vulgaris Leaf Beet
Group
(Beta vulgaris var. cicla)
Leaf beets are of two types, depending on whether
or not a thick leaf midrib and petiole are present.
Leaf beets are known as salk in Arabic; tian cai
in Chinese; bette or blette in French; mangold in German; bieta a foglia in
Italian; acelga in Portuguese; svekla listovaja in Russian; and bleda or
acelga in Spanish.
Spinach beet or perpetual spinach is grown for
its leaves, which are used as greens or a potherb. It is distinct from
spinach (Spinacia oleracea) and New Zealand spinach (Tetragonia
expansa). Spinach beet does not have a thickened leaf midrib or a
thickened petiole (leaf stem). It also lacks a swollen taproot. Spinach beet
has never been bred intensively and there is no tradition of distinct
cultivars. Three European varieties were proposed by Helm in 1957, based on
foliage colour, but this classification has not persisted. Perpetual spinach
is usually sold generically as 'spinach beet' or 'leaf beet'. However, the
exception to the rule is the Italian cultivar of perpetual spinach called
Erbette, which is listed in seed catalogues. Leaf beet has been consumed
since ancient times throughout Europe. In parts of Ireland, where it is known
as 'wild spinach', it has also been revered as a cure for sick sheep. Outside
of Europe, spinach beet is an important crop in Northern India and parts of
Central and South America.
Chards are grown for their foliage and in
particular their thickened leaf midribs and petioles. As with spinach beet,
there is no swollen taproot. Although some leaf beets have fairly thick
roots, they are never fleshy. The white root of chard has in times past been
consumed medicinally, in the form of infusions, and very occasionally as
food, for example, in times of hardship. Chard is often used synonymously
with Swiss chard, but older chard varieties are sometimes considered to be
distinct from Swiss chard. Several types of Swiss chard can be distinguished,
based on petiole or leaf midrib colour and other characteristics. Swiss Chard
is sometimes also called seakale beet or silver beet. Lucullus is one of the
oldest chard varieties and it has green leaf blades and white petioles.
Chards occur in many colourful forms, including Bright Yellow Chard and Ruby
or Rhubarb Chard. A popular recent introduction is Bright Lights Swiss Chard,
an improved chard that has leaf midribs and petioles that occur in a mix of
colours.
Spinach beet and Chard are highly perishable and
do not transport well. Leaf beets are therefore rarely found in supermarkets
in the USA and Western Europe. However, leaf beets are a popular vegetable
grown for local consumption. They are particularly valued in warmer temperate
regions, such as the south of France, where the summer weather can be too hot
to grow other green leafy vegetables.
Beetroot: Beta vulgaris subsp. vulgaris Garden Beet
Group
(Beta vulgaris var. conditiva)
Beetroot has been primarily selected as a root
vegetable, although its leaves are edible. Larger cultivars are grown as a
staple root crop for winter sustenance, while smaller globe-shaped cultivars
are grown as a summer salad crop.
Beetroot is called beet in North America. It is
also referred to as garden beet, table beet or red beet in English; remolacha
(or betarraga) hortelena, remolacha mesa or remolacha roja in Spanish; and
beterraba hortela, beterraba de mesa or beterraba vermelha in Portuguese.
Beetroot is called betterave, betterave rouge or betterave potage in French;
rote Rote Beete or Runkelr German; Barbabietola in Italian; and rode biet or
kroot in Dutch. It is silig in Arabic; gen tian cai in Chinese; and svekla
stolovaja in Russian.
There are numerous cultivated varieties or
cultivars of beetroot. These are listed and described further in Chapter
Eight.
Fodder beet: Beta vulgaris subsp. vulgaris Fodder Beet
Group
(Beta vulgaris var. alba)
The leaves and roots of fodder or forage beet are
fed to livestock and other animals, either fresh or as silage. They have
large swollen roots that are classified in terms of their shape. Four basic
root shapes are recognized: (i) flat globe, (ii) cylinder, (iii) globe and
(iv) spindle. The taproots of flat globe and cylinder sit mainly above the
soil level, while globe and spindle have storage roots sitting roughly half
in and half out of the soil. In comparison, most beetroot cultivars have
taproots that lie mainly below the soil level. In the past, different
botanical varieties of fodder beet have been described based on their root
colour: scarlet, pink, orange, yellow and white. The scarlet and pinks forms,
however, are indistinguishable from large long-rooted beetroot. Modern forage
beet cultivars can be distinguished from beetroot because of their white and
zoned roots, their greater size, and their coarser root quality. Overall, the
Fodder Beet Group comprises a large number of cultivars, displaying a wide
range of variation in root shape and colour.
Mangel is an old variety that can be classified
as distinct from modern fodder beets. Mangel is a heavier plants that stick
out of the ground to a greater extent than improved fodder beet. Mangel
originated from a cross between a beetroot and a leaf beet, while modern
fodder beets result from a cross between mangel and sugar beet. The
cylindrical-shaped variety Eckendorfer was developed around 1840 by plant
breeders. Many of today's fodder beet cultivars are descended from it. Modern
fodder beet roots contain around 3-5% sugar and 6-8% protein by dry weight.
The feeding of beet to animals has contributed to
beetroot being unpopular or unfashionable as a food at certain times in parts
of Europe. This may partly be due to etymology. The cultivated form of Beta
vulgaris called mangel wurzel was originally known as mangold wurzel
(root of the beet). Mangolt was the old German word for Beta vulgaris
in general. However, this was corrupted among German speakers to
mangel-wurzel (root of scarcity). Mangel wurzel became the name for fodder
beets. The English translation of the title of Abbe Commerell's book, for
instance, was Culture and Use of the Mangel Wurzel, a Root of Scarcity.
The notion that beets are only to be consumed in times of hardship or fed to
animals has persisted in some areas until relatively recently.
Sugar beet: Beta vulgaris subsp. vulgaris Sugar Beet
Group
(Beta vulgaris var. altissima)
Sugar beet is the most recent of the cultivated
beet crops and the most important in commercial terms. The conical swollen
roots lack pigmentation and have a characteristically high sugar content.
Sugar beet cultivars have been classified according to their root size and
sugar content. There is much less variation in the Sugar Beet Group than in
other beet cultivar groups.
Up until the 1870s, the focus was on breeding
sugar beet to yield as much sugar as possible. However, after this time the
aims of breeders diverged. This resulted in three groupings of sugar beet
cultivars. Z-type (zucker-type) cultivars have small roots and high sucrose
levels, E-type (ertrag-type) cultivars have large roots but lower sucrose
levels, and N-type (normal-type) cultivars have medium-sized roots and
intermediate sucrose content. The high sugar Z-types were a continuation of
the continuous selection programmes for increasing the sugar levels in roots
(as a percentage of root fresh weight). However, it was found that these types
had a limited root yield potential. E-types were therefore selected for
larger root size and higher root yield. The so-called normal types were the
result of balancing both objectives. The classification of all sugar beet
cultivars into Z, E and N-types has only recently been discontinued.
Sugar beet has whitish conical roots, up to half
a metre in length. Modern sugar beet breeding programmes have produced a
range of high-yielding and disease-resistant cultivars. The sweet beet in
Marggraf's time contained up to 6% sucrose, whereas modern sugar beet
cultivars contain around 18% sucrose. Modern beetroot cultivars, in
comparison, typically contain between 6% and 10% sucrose.
Cultivated beet seed is botanically a fruit. Each
knobbly 'seed' is a cluster of dried fruit. One to six fruit stick together
to form a compressed corky structure called a glomerule or seedball.
Seedballs are around 3-7 mm in diameter, depending on cultivar. Each fruit
contains one seed or embryo, enclosed within its swollen cork-like base. The
cork layer contains phenolic compounds that act to inhibit germination.
Beetroot cultivars typically have glomerules containing around three true
seeds. The true seeds are kidney-shaped, brown to black, and around 1.4 mm in
diameter and 1.5 mm thick.
The glomerules or seedballs are said to be
multigerm when they contain more than one seed. More than one seed can
therefore germinate from each multigerm seedball. However, a number of modern
beetroot cultivars and all modern sugar beet cultivars have been bred to be
monogerm, with only one viable seed per seedball. This is beneficial for
cultivation because less thinning is needed at the seedling stage. For
mechanical planting of multigerm beetroot, the seedball is sometimes crushed
into uniform pieces to separate out the seeds. Beet seed maintains its
viability for around five years.
The search for monogerm beet seed started in
around 1900. However, success was not achieved until the 1930s, when V.F.
Savitsky (1902-1965) and co-workers identified monogerm sugar beet plants
growing at the Sugar Beet Institute in Kiev. After the Second World War,
Savitsky emigrated to the USA, where he identified five monogerm plants in a
sugar beet seed field in Michigan in 1948. His work led to the development of
commercial monogerm cultivars. By the 1960s, practically all sugar beet
farmers in the USA and Western Europe were growing monogerm cultivars.
Monogerm sugar beet seed is coated into a pellet,
which may contain fungicide or other pesticides, into a uniform shape that
aids sowing using precision drills. The improved germination rates of modern
monogerm seed enable it to be sown individually, into a final stand, without
the need for thinning.
The tops, foliage or leaves of Beta vulgaris
subsp. vulgaris grow in a basal rosette pattern. They emerge from the
crown of the hypocotyl in an alternate formation. New leaves grow interior to
older leaves. The leaves typically have a roughly triangular shape. They have
strong long petioles or leaf stalks that broaden towards the base. The leaves
can be dark or light green, or a dark red, depending on cultivated form and
cultivar, and have a shiny surface.
The leaf surface is covered with an amorphous wax
film. An analysis of compounds obtained by steam-distillation of Beta
vulgaris leaves revealed a group of chemicals called n-alkanes. An
alkane, in the terminology of organic chemistry, is a saturated hydrocarbon
that has its carbon atoms attached in a continuous or branched chain; ethane,
pentane, propane and isobutane are among the large family of alkane
molecules. Other plants in the family Chenopodiaceae have similar
combinations of n-alkanes to those identified from Beta vulgaris.
In a study conducted by Röttger it was found that
alkanes identical to those found in the wax layer of Beta vulgaris
leaves caused the beet fly (Pegomya betae) to lay eggs (oviposit) on
an artificial surface that was normally ignored. The beet fly has co-evolved
with Beta vulgaris to the extent it is now a specialist on this
species, and uses the distinct chemical profile of its leaves as an
oviposition stimulant. If a pregnant female fly senses their presence after
landing on a plant, she is likely to lay eggs; but if she does not, she will
fly to another plant.
The storage organ of beetroot, fodder beet and
sugar beet is usually called a root. This is common usage and, for instance,
has been used throughout this book. However, the swollen root of cultivated Beta
vulgaris is technically made up from both the root and the hypocotyl,
which is an enlarged region at the base of the stem. The hypocotyl is an
intermediate region between stem and root and accounts for the top or crown
part of the root. In sugar beet, the hypocotyl typically accounts for 10% of
the storage organ, while 90% is root derived. However, this proportion varies
and the amount derived from hypocotyl can be higher in fodder beet and
beetroot, particularly in varieties that have crowns that extend above the soil
surface.
Wild sea beet has little stem or root swelling
and a long, stout, tapering main or taproot with a dense network of small
side-roots. Leaf beets retain this basic tapering root form. In beetroot, the
swollen storage root can be globular, cylindrical, flat or tapered, depending
on the cultivated variety. The true taproot occurs below the hypocotyl.
Lateral, side or adventitious roots, in two opposite rows on its lower part,
grow from the true root. In loose soils, the rooting depth of beets can be up
to 300 cm.
The roots of cultivated beet are either white or
coloured. Chard and sugar beet have narrow and swollen white roots,
respectively. Mangels and beetroot have roots that are typically yellow and
red, respectively. Root colour is determined by the presence of pigments
called betalains. These will be considered further in Chapter Five.
The hypocotyl and true root consist of
alternating layers of conductive tissue and storage tissue. These are visible
as distinct circles or cambium rings when the roots are cut transversely.
Each ring is a vascular bundle comprising xylem, to the inside, and phloem,
to the outside. The conductive tissue or xylem is typically broad and dark,
and is involved in transporting water, sugar (sucrose) and nutrients around
the plant. The storage tissue or phloem is typically narrow and light, and is
where carbohydrates are transported and laid down during the first year's
growth. In some beetroot cultivars, the colour difference between the
darker-coloured bands of connective tissue and lighter-coloured bands of
storage tissue can be subtle. In others, it can be highly pronounced. In
Chioggia, for example, dark red bands alternate with almost white ones.
The girth of the root increases as additional
cambium layers are added. In sugar beet at harvest, there are usually 12 to
15 rings. In harvested beetroot the number is usually less, because they are
typically picked younger. Beetroot has been selected to have little lignin or
hard fibrous tissue in its cambium layer. This makes the root's texture
better for eating. Fodder beet and sugar beet, on the other hand, have been
bred primarily for larger size and/or higher sugar content. In sugar beet,
the concentration of sucrose is greatest in the very centre of the root.
Scientists are beginning to understand how
different plant hormones interact during the formation of cambium layers in
the roots of Beta vulgaris. These hormones produce different outcomes
in the different cultivated forms of beet. It may be possible to increase
levels of sugar storage in roots by manipulating plant hormones using
biotechnological approaches.
Cultivated beet is a biennial plant. In the case
of beetroot, fodder beet and sugar beet, foliage and an enlarged swollen
storage root are produced during the first year's growth. In the second year
of growth, resources stored in the root are diverted into producing a flower
spike and seeds. When cultivated, these crops are biennials grown as annuals.
However, to obtain seed, a second year's growth is usually required.
Beets have an inflorescence: literally a massing
together of flowers. Flowering occurs at the end of the second year's growth,
when a long single flower spike is produced. The flower spike occurs on the
top of an elongated stem, which is usually around 50-150 cm high, although
much higher flower spikes are possible. Flower spikes of 120 cm (4 feet) are
typical of common beetroot cultivars. The spike grows upward and is branched
(paniculate) at the top.
The inflorescence on the upper part of the flower
spike has flowers irregularly arranged up its length. The flowers are
greenish and sessile, being directly attached to the inflorescence with no
individual stalks, either singly or more usually in groups or clusters of
between two to five. The flowers are small and green or red in colour. As the
inflorescence grows, adjoining flowers within a cluster cohere, eventually
hardening to produce the uneven and wrinkled glomerules or seedballs.
Beta vulgaris
flowers are bisexual or hermaphrodite, having both male and female
reproductive organs. The stamen is the male part of the flower. Each flower
has five stamens. An anther, from which pollen is released, tops each of the
five stamens. The five stamens are fused at their bases to five petals and
they surround the female part of the flower. This consists of a short pistil,
topped by two to three stigmas, and a one-celled ovary embedded in a
structure called a receptacle. The pollen released by the anthers is
transported to the pistil. In the case of beetroot, the pollen is wind borne.
When pollen lands on a stigma, a pollen tube starts to grow down through the
pistil to the ovary, where fertilization takes place and seed is formed.
The seedballs turn brown on the flower spike,
maturing from the base upwards. The seedballs do not readily drop and so the
entire flower spike can be harvested when all the seeds are mature. Seedballs
can be removed by pinching the stalks where they join the spike. The seed
clusters should not be broken at this stage because this could injure the
seeds.
Flowering is initiated in the second growing
season after a prolonged period of cold weather (vernalisation), with
temperatures below 10°F (50°F) for 30 to 60 days. Such conditions are typical
in Northern Europe.
Cultivated Beta vulgaris that prematurely
goes to seed at the end of its first growing season it is said to have
"bolted". One of the most important factors in beet cultivation is
to avoid sowing seed too early in the year, which can cause plants to bolt. Bolting
can also be brought on by a sudden check on plant growth, such as that caused
by cold weather, drought or insect pest attack. Bolting is very rare in
perpetual spinach, which can therefore be harvested almost all year round,
but it has in the past been a major problem in commercial sugar beet and
beetroot cultivation.
One of the main aims of plant breeders has been
to produce plants that are resistant to bolting, for early season planting.
Workers at the Plant Breeding Institute in Cambridge, in England, first
obtained bolting resistance in sugar beet in around 1940. Because temperature
and day length are critical in determining bolting resistance, breeding work
is carried out under the climatic conditions in which the beet is to be
grown. The Cambridge team therefore also did breeding work in Scotland, and
obtained lines that did not bolt under the demanding early season conditions
there. These lines were used to produce commercial bolting-resistant
cultivars. All modern sugar beet cultivars incorporate bolting resistance.
Boltardy was one of the first bolting-resistant beetroot cultivars; it was
introduced in the early 1960s. Many recent beetroot cultivars incorporate
resistant to bolting. Bolting is therefore much less of a problem today than
in the past.
Beets are outbreeding and cross-fertilize rather
than self-fertilize. This makes it harder to obtain consistent lines for
breeding. The incorporation of beneficial traits into beets was helped by the
discovery of sugar beet plants with 'cytoplasmic male sterility', by F.V.
Owen in the 1940s. F1 hybrids could subsequently be bred through controlled
pollination and crossing of effectively inbred lines having male sterility.
Most modern sugar beet and an increasing number of beetroot varieties are obtained
in this way. F1 hybrids do not breed true and farmers need to obtain new seed
each year, derived from further crossing of inbreeding lines. Hybrids have
increased seedling vigour and enhanced resistance to pests and diseases.
Later in this book we will see if beetroot F1 hybrids also taste as good as
traditional varieties.
Barocka, K.H. (1959). Die einzelfrüchtigen Arten
der Gattung Beta L. im Hinblick auf ihre mögliche Verwendung zur
Einkreuzung in Beta vulgaris L. subsp. vulgaris (Zucker- und
Futterrübe) Der Züchter, 29 (5): 193-203.
Bosemark, N.O. (1993) Genetics and breeding, in:
D.A. Cooke & A.K. Scott (eds.) The Sugar Beet Crop. London:
Chapman and Hall, pp. 67-119.
Brandenburg, W.A. (1986) The cultivar concept,
in: B.T. Styles (ed.) Infraspecific Classification of Wild and Cultivated
Plants. London: Clarendon Press, pp. 87-98.
Buttler, K.P. (1977). Revision von Beta Sektion Corollinae
(Chenopodiaceae). I. Selbststerile Basisarten, Mitt. Bot. München, 13:
255-336.
CAB International (2001) CABI Crop Protection
Compendium, Global Module, 3rd Edition. Wallingford, UK: CAB
International.
Elliot, M.C. and G.D. Weston (1993) Biology and
physiology of the sugar-beet plant, in: D.A. Cooke & A.K. Scott (eds.) The
Sugar Beet Crop. London: Chapman and Hall, pp. 37-66.
Ford-Lloyd, B.V. and J.T. Williams (1975) A
revision of Beta section vulgares (Chenopodiaceae), with new
light on the origin of cultivated beets, Botanical Journal of the Linnean
Society, 71(2): 89-102.
Ford-Lloyd, B.V. (1986) Infraspecific variation
in wild and cultivated beets and its effects upon infraspecific
classification, in: B.T. Styles (ed.) Infraspecific Classification of Wild
and Cultivated Plants. London: Clarendon Press, pp. 331-334.
Frese, L., D. Ziegler and R. Krauss (2004) A
Taxonomic Guide for Wild and Cultivated Beets (Beta L.),
Hawkes, J.G. (1986) Infraspecific classification
- the problems, in: B.T. Styles (ed.) Infraspecific Classification of Wild
and Cultivated Plants. London: Clarendon Press, pp. 1-7.
Helm, J. (1957) Versuch einer
morphologisch-systematischen Gliederung der Art Beta vulgaris, Zuchter,
27: 203-222.
Holland, H. (1957) Classification and performance
of varieties of red beet, Report of the National Vegetable Research
Station, Wellsborne, 1956: 1-26.
Hosford, D.J., J.R. Lenton, G.F.J. Milford, T.O.
Pocock and M.C. Elliot (1984) Phytohormone changes during root growth in Beta
species, Plant Growth Regulation, 2: 371-380.
ICBN (1983) International Code of Botanical
Nomenclature, Regnum Vegetabile no. 111.
ICNCP (1980) 'International Code of Nomenclature
for Cultivated Plants', Regnum Vegetabile no. 104.
Kays, S.J. and J.C. Silva Dias (1996) Cultivated
Vegetables of the World: Latin Binomial, Common names in 15 Languages, Edible
Part, and Method of Preparation. Athens, Georgia, USA: Exon Press, pp.
30-31.
Lange, W., W.A. Brandenburg and Th.S.M. de Bock
(1999)Taxonomy and cultonomy of beet (Beta vulgaris L.), Botanical
Journal of the Linnean Society, 130: 81-96.
Letschert, J.P.W. (1993). 'Beta section Beta:
biogeographical patterns of variation and taxonomy'. PhD thesis published as
93-1 of the Wageningen Agricultural University Papers.
Linnaeus, C. (1753) Species Plantarum.
Stockholm.
Munerati, O. (1932) Sull incrocio della
barba-bietola coltivata con la beta selvaggia della costa adriatica, Industria
saccar ital., 25: 303-304.
Owen, F.V. (1942) Inheritance of cross- and
self-sterility and self-fertility in Beta vulgaris, Journal of
Agricultural Research, 69: 679-698.
Röttger, U. (1978) Blattwachskomponenten als
Schl?reize der Eiablage von Pegomya betae Curt., Mitteilungen der
Deutschen Gesellschaft für allgemeine und angewandte Entomologie, 1:
22-28.
Savitsky, V.F. (1950) Monogerm sugar beets in the
United States, Proceedings of the American Society of Sugar Beet
Technologists, 6: 156-159.
Speckmann, G.J. and Th. S. M. de Bock (1982) The
production of alien monosomic additions in Beta vulgaris as a source
for the introduction of resistance to beet root nematode (Heterodera
schachtii) from Beta species of the section Patellares, Euphytica,
31: 313.
Snyder, R. (2003) Beet (Beta vulgaris),
Williams, J.T., A.J. Scott and B.V. Ford-Lloyd
(1976). Patellaria: a new genus in the Chenopodiaceae, Feddes
Report, 97: 289-292.
Zossimovich, V.P. (1939) Evolution of cultivated
beet, Beta vulgaris, C.R. (Doklady) Acad. Sci. URSS, 24(1):
73-76.
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© Copyright Stephen Nottingham, 2004
sf.nottingham@btinternet.com
July 2004 SFN.
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