Wednesday, 21 February 2018

Beetroot: 3. Classification and Botany

Beetroot (2004)

Stephen Nottingham

© Copyright: Stephen Nottingham 2004

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 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 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.
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 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.

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© Copyright Stephen Nottingham, 2004

July 2004 SFN.


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