After reading this article you will learn about:- 1. Origin of Carrot 2. Production of Carrot 3. Floral Biology and Pollination 4. Qualitative Genes 5. Breeding Goals 6. Varietal Groups 7. Genetic Resources 8. Selection Criteria 9. Breeding Methods 10. Applications of Biotechnology 11. Research and Trends in Carrot Seed Production 12. Varieties.
- Origin of Carrot
- Production of Carrot
- Floral Biology and Pollination of Carrot
- Qualitative Genes in Carrot
- Breeding Goals of Carrot
- Varietal Groups of Carrot
- Genetic Resources of Carrot
- Selection Criteria of Carrot
- Breeding Methods of Carrot
- Applications of Biotechnology of Carrot
- Research and Trends in Carrot Seed Production
- Varieties of Carrot
1. Origin of Carrot:
The genus Daucus has many wild forms that grow mostly in the Mediterranean region and south-west Asia. Afghanistan is believed to be the primary center of genetic diversity. There are evidences that purple carrot together with a yellow variant spread from Afghanistan to the Mediterranean region as early as the tenth or eleventh century.
The white and orange carrots are probably mutations of the yellow form. The domestic carrot readily crosses with widely adapted wild carrot known as Queen Anne’s Lace.
2. Production of Carrot:
Carrot (Daucus carota L., 2n = 2x = 18) is widely grown both for fresh consumption and processing. It is among the top-ten most economically important vegetable crops in the world both in terms of acreage and market value. In 2005, the world production of carrot stood at 24 million tones on 1.1 million hectares. The total traded carrot seed crop at global level is estimated as 100 million USD.
Carrot is cultivated on large acreage in Europe including the former USSR, America, Asia and Africa. In India estimated acreage of carrot is about 2,00,000 ha. This speaks about the economic importance of carrot. Carrot is a rich source of high carotene content (pro-vitamin A) and reportedly has anti-carcinogenic effect due to carotene. It is the most widely grown member of the Apiaceae or Umbelliferae.
3. Floral Biology and Pollination of Carrot:
The inflorescence of carrot is a compound umbel. A primary umbel can have over 1000 flowers at maturity, whereas secondary, tertiary and quaternary umbels bear fewer flowers. Floral development is centripetal i.e. the flowers to dehisce first are on the outer edges of the outer umbellets. Carrot is protandrous.
After straightening of filament, the pollen is shed and stamens quickly fall. After this, the petals open fully and the style elongates. The style is divided into two parts. The petals of petaloid plants are persistent unlike those of brown-anther, male sterile plants.
Flowers are epigynous. There are five small sepals, five petals, five stamens and two carpels. Emasculation is laborious and time consuming. As soon as the first bud in an umbel opens, the whole umbel of the female parent is bagged in a muslin/cloth bag.
The flowers are removed daily until peak flowering has reached. Anthers are removed from the early opening outer flowers in the outer whorl of umbellets until sufficient flowers are emasculated.
Unopened central florets in the emasculated umbellets and all late flowering umbellets are removed. Thus, only the emasculated flowers are left on the female inflorescence inside the bag. A pollen bearing umbel from previously protected male plant is inserted into the bag of the female parent along with some house-flies to ensure pollination. Daily for a few days in the morning, the male umbel is gently rubbed against the emasculated umbel to enhance artificial cross-pollination.
Sometimes, 1-2 flowering umbels of both the parents are enclosed in the same cloth cage along with some house-flies. Seed from each parent is sown in adjacent rows. The hybrids and the parents could be identified (not always) and necessary roguing done to remove the selfed plants.
Simon has described another alternative commonly used in Europe for intercrossing male fertile plants. A single isolated umbel will not develop seeds even though pollen is present in the flowers as this is protanadrous. This umbel can serve as the seed parent in a cross if, one week after anthesis, the flowers of such an isolated umbel are sprinkled with water to flush out pollen.
After it dries, pollen from the intended pollen parent can be introduced with a brush and the seed parent umbel again placed in isolation. Seeds thus produced are nearly always hybrids. Sometimes two parents to be crossed are covered by a plastic or cloth screen pollination cage.
Flies are released in the pollination cage to move pollen or pollen is moved by hand or brush. In this system, selfed and crossed seeds are harvested together. The selfed and crossed progenies need to be identified by phenotypic or molecular markers or by hybrid vigour when inbreds are crossed.
4. Qualitative Genes in Carrot:
As per compilation by Simon (1984) and Peterson and Simon (1986) only 20 genes (Table 22.1) have been described.
5. Breeding Goals of Carrot:
i. High root yield
ii. Good eating quality
iii. Scarlet/orange colour roots
iv. High carotene content in roots
v. Uniformity in root shape and size
vi Thick flesh roots
vii. Thin and self coloured core in roots
viii. Broad shouldered, cylindrical, uniformly tapering or stump rooted carrot with non- branching habit
ix. Early rooting
x. Cracking free roots (major gene Ck known for root cracking)
xi. Smooth, shiny root
xii. High sugar and dry matter in roots
xiii. Slow bolting habit
xiv. Heat tolerance
xv. Crown or upper surface (shoulder) free from green colour, flat or slightly up-lifted rather than concave or shrunken
xvi. Resistance to: Alternaria blight (Alternaria dauci), Cercospora leaf blight (Cercospora carotae)
6. Varietal Groups of Carrot:
Simon (2008) have described several varietal groups based on root colour, root length, colour intensity, core colour and size, root tip shape and days to edible root maturity. Those same distinctions are still used to help categorize carrot varietal groups even today.
The major root types used today include varietal groups as:
European – Nantes, Chantenay, Danvers, Paris Market, Flakkee, Berlicum, Amsterdam Forcing
Asian – Kuroda
North American – Imperator
South American – Brasilia
Numerous cultivars have been named for all of these root types. The typical carrot root shapes are illustrated in Fig. 22.1.
European Nantes types (cylindrical roots, orange in colour) and Japanese Kuroda type (conical roots, deep orange in colour) are common in India and Asia at large. A dark orange selection from Gosun by Mr. Kuroda in 1950s resulted in the Kuroda-Gosun cultivar that has become grown widely, often simply referred to as Kuroda in India. Most of private sector seed companies are marketing Kuroda carrot on a large scale.
7. Genetic Resources of Carrot:
As for most vegetables, carrot genetic resources are in the form of open-pollinated cultivars. The U.S. and European databases are on internet.
Centres on carrot germplasm accessions are as follow:
1. USDA – ARS (http://www(dot)ars – grin(dot)gov/ npgs = 723 accessions of Daucus carota.
2. European cooperative programme for Crop Genetic Resources – ECP/GR (http:// www(dot)warwick(dot)ac(dot)uk/fac/sci/hri2/about staff/dastley/gbrhrigru/ecpumbel/ = 5037 accessions of D. carota.
3. National Centre for Vegetable Crops research- Carrot Breeding Collection (CNPH), Eurpresa Brasileira de Pesquisa Agropecularia-Brazil = 200 accessions.
4. BAZ – Inst, of Horticultural Crops, Germany = 5 species, 25 subspecies, 30 wild relatives.
5. National Gene Bank – Rural Development Administration, Korea (http:// genebank(dot)rda(dot)go(dot)kr = 695 accessions.
8. Selection Criteria of Carrot:
Colour and Quality:
Visual examination of roots, cross section of roots and longitudinal section of roots is effective. Same colour should extend from crown to down tap root. The colour of xylem, phloem and vascular cambium should match as far as possible.
Sugar and Flavour:
A thin cross-sectional slice could be cut and tested. The roots with harsh flavour are eliminated. Total sugars which contribute to sweetness and are an important component of general preference can be estimated by a refractometer. Selection for high soluble solids and dry matter is also possible by specific gravity. High dry matter is useful for processors. Percentage dry weight is easily determined by weighing a fresh sample, drying and reweighing.
Bolting may cause serious losses in yield and quality, hence it is important to apply selection pressure for non-bolting.
Susceptible cultivars are generally planted between rows of breeding lines and the spreader row plants are inoculated to ensure the spread of disease. This practice is applicable for both alternaria leaf blight and cercospora leaf spot.
9. Breeding Methods of Carrot:
Open-pollinated Varieties and Synthetics:
In the recent past, mass selection and pedigree selection within the different populations have been the most important breeding methods. Open-pollinated varieties are adapted to different situations. Nevertheless, all efforts to breed varieties with high level of uniformity have been limited by genetic factors originating from breeding methods used for open-pollinated varieties.
One reason for this is the heterozygosity of the open- pollinated varieties, while another is the inbreeding depression resulting from Radom selfing of plants within the populations. Inbreeding depression is considerable as quoted by Stein and Nothnagel (1995) in a review (Table 22.2).
All open-pollinated varieties suffer from inbreeding depression and a limited degree of uniformity, and hybrid breeding of carrots has now been started by the carrot breeders intensively to improve uniformity.
In view of severe inbreeding depression in carrot, selection for resistance to inbreeding depression is a major consideration in carrot where self-pollination is essential to breeding progress. In this situation, it is better to develop new inbreds from intercrosses between two good inbreds.
In case where extreme uniformity seems to be unnecessary, e.g. for juice or pulp production or for regions with weakly developed agriculture, breeding of synthetics would also be worthwhile.
As an outcrossing diploid with significant inbreeding depression, adequate population size is vital to maintain population vigor in development of open-pollinated cultivars.
The development of most open-pollinated carrot cultivars begins with a pre-existing open-pollinated cultivar and selection is exercised for one or more traits. Population improvement for carrot typically starts with inter crosses among plants of open-pollinated varieties followed by phenotypic mass selection for root shape, smoothness, length and desirable root colour in a particular local production area.
Where facilities are available, selection should also be done for disease resistance (Alternaria leaf blight), root quality, cavity spot, nematodes, flavour, carotene content and processing quality. Plants with premature bolting, excessive root cracking and poor plant vigour are always removed.
Several of the more successful open-pollinated carrot cultivars of North America began with an intercross between two cultivars, like the breeding of ‘Imperator’. Other examples include ‘Waltham Hicolor’ from ‘Hutchinson’ x ‘Turkey Red Carrot’ and ‘Gold Pak’ from ‘Long Imperator’ x ‘Nantes’.
Thompson (1961) and Hanschke and Gabelman (1963) were the first scientists to detect and analyze male sterility. The first carrot hybrids were sold in early 1960s in the USA. Today more than 100 hybrid varieties exist worldwide. The percentage of hybrids is 60-90% in Europe for early and late varieties and in USA, the value has reached to almost 100%.
Hybrid breeding in carrot is generally based on two systems of cytoplasmic male sterility (CMS) with different genetic backgrounds and origin. These are brown anther type and the petaloid type. The brown anther type (ba) is present in all cultivated orange coloured open pollinated varieties.
The phenotype is characterized by deformed, brown coloured anthers without functional pollen caused by a genetic block in meiosis. According to O. Banga and his colleagues (1964) the type ba results from an interaction of the Sa cytoplasm with two independent nuclear genes (homozygous aa or dominant B).
The two complementary genes E and D operate with their dominant alleles as restorer genes. Due to this complex inheritance, many test crosses are necessary for the development of a suitable maintainer.
The cytoplasm of the petaloid type (pt) is derived from a wild form of Daucus carota L. and has been introduced into many open-pollinated varieties of cultivated carrot. The ‘pt’ type is characterized by a transformation of the anthers into petals or petal like structures which are unable to produce functional pollen.
For the inheritance of this type, an interaction between Sp-cytoplasm and two independent dominant genes (M1; M2) was postulated by T.S. Morelock in 1974. A maintainer for this type cannot be detected in the F) because of the dominant state of the M genes. Backcrosses should be performed by use of the male-fertile genotype (Sp) m1 m1, m2m2 as a tester in order to find dominant alleles.
As a matter of fact, petaloid male sterility was discovered in North American wild carrot by Munger in 1953. It has also been found in other North American wild carrots. Petaloidy is a homeostatic mutation where a second whorl of petals exists in place of anthers.
Petaloid CMS is the most widely used form of male sterility for production of commercial carrot hybrids in North America today. It is stable over a wide range of environments throughout flowering and seed production, although in some genetic backgrounds petaloidy breaks down and late season umbels can be fertile.
The incorporation of CMS into carrot inbreds for production of hybrid cultivars is similar to that for other crops. Generally this process begins at the F2 or F3 generation with the intercrossing of individual selected fertile plants to a male sterile plant. In the next generation progeny from the male sterile are examined for male fertility in the process of backcrossing for sterile line development.
Presence of male fertile plants indicates the presence of restorer alleles from the original fertile parent. It has been easy to identify and select male sterility maintaining carrot inbreds from a wide range of germplasm backgrounds but the incidence of restorers varies widely.
A third CMS system has been detected in an alloplasmic form of orange-coloured carrot originating from a cross between the wild carrot D. carota gummifer Hook. fil. and the cultivated carrot D C. sativus Foffm by T. Nothangel in 1992.
This type of male sterility called ‘gum’ type, is characterized by a total reduction of anthers and petals. Recent results on the genetic mechanisms suggest that an interaction of the ‘gummifer’ cytoplasm with a recessive allele (gugu) in the nucleus is responsible for the expression of this type of male sterility.
The two CMS systems ‘ba’ and ‘pt’ generally suffer from instability of male sterility under specific conditions. The instability is mainly influenced by high temperature. Nevertheless, observations over many years have revealed that other factors such as dry conditions, growing time or long day conditions operate provocatively.
A strict selection scheme is therefore necessary because carrot is partially andromonoecial, i.e. in umbels of higher order, male flowers can be produced. Umbels of the 5—7th order must be examined carefully.
The development of the ms and maintainer lines is very laborious process due to the dominant state of male sterility. Crossing, backcrossing, selfing and testing of the progenies in the following two generations, including isolation of the positive progenies, are characteristic steps in breeding processes.
The breeder is forced to eliminate male fertile plants or phenotypes with partial male fertility within the ms lines developed, and to develop new lines with low inbreeding depression. Such lines can be found but they rarely have good combining ability.
An excellent uniformity of the hybrid varieties demands a high degree of homozygosity in the ms and maintainer lines but selection of lines with low inbreeding depression reduces the probability of finding good parents for hybrid varieties. To overcome this problem, the development of three-way hybrids has been recommended. In this way, the production of hybrid seed is realized on ms F1 lines (Fig. 22.1)
For this method, an original maintainer and a second line (exchange maintainer) are necessary. The universal maintainers which can maintain all ms lines due to complete homozygous state of the maintainer genes is desirable for such a system.
A universal maintainer must also possess the specific characters of the ms line and a good combining ability. Exchange maintainers have therefore been propagated which were selected following a diallel with all lines of one type.
These lines must have the same genetic state as the specific ms line, a state which can be found relatively frequently. Thus, the best line for performance and uniformity can be selected.
Recently, exchange maintainer lines for the most important ms lines have become available and can be used for seed production of new lines. Pollinator lines which are necessary for hybrid seed production are derived from op-varieties or special breeding lines by using a top-cross system for the testing of general combining ability.
10. Applications of Biotechnology of Carrot:
Genome size is relatively small. Variation in molecular genetic markers is quite extensive, and both genetic transformation and regeneration of carrot are readily achieved. Therefore, carrot is a good candidate for biotechnological applications.
The carrot genetic linkage map includes around 1000 isozymes, RFLPs, AFLPs, RAPDs, and other molecular markers. From 10-25% of genomic clones tested are estimated to be in high copy numbers, while 20-30% of the RFLP probes and RAPD primers, and 40% of AFLP bands were polymorphic. The incidence of molecular marker polymorphism and map size to date makes marker-assisted selection a viable option.
Reports of success have not yet been published but markers linked to nematode resistance, CMS restorers, carotene content, and sugar content are being sought. One significant limitation to the application of molecular markers to carrot improvement is the need for radioactive labels, which cannot to be used in many laboratories. Because of this, efforts are underway to convert AFLPs to PCR-based co-dominant markers and to develop microsatellite markers.
One of the most intensively studied traits of carrots today continues to be CMS with a particular emphasis on molecular evaluation. Mitochondrial restriction fragment pattern and protein product differences between fertile, petaloid, and brown anther cytoplasm’s have been compared and analysis of variation for a few specific genes, such as atpA have been reported.
Seven monogenic traits have been mapped for carrot. These are yel, cola, Rs, Mj-1, Y, Y2 and P1. QTLs have been mapped for carrot total carotenoids and five component carotenoids namely, phytoene, alpha-carotene, beta-carotene, zeta-carotene and lycopene and majority of the structural genes of the carotenoid pathway is placed into this map (Just et. al., 2007). MAS has been reported for two genes : MJ-1 and Rs.
With the pioneering work of F. C. Steward and co-workers in 1964, totipotency of plant cells was first demonstrated using carrot. Carrot has, in fact, proven to be a model organism for plant tissue culture, transformation, and regeneration.
Taking advantage of carrot’s facile manipulation in cell culture, carrot- Daucus capillifolius and carrot-bishop’s weed (Aegopodium podagraria) protoplast fusions were among the first examples of successfully regenerated somatic hybrids.
Maize transposable elements have been successfully introduced into carrot cell cultures and found to be mobile. Two interesting experiments with carrot cell cultures demonstrate evidence for easy regeneration of carrot callus after one year of being air-dried and stored at room temperature.
Transgenics with genes for disease resistance and enhanced root color have been field-tested. With more genes of potential agricultural application cloned and improved, public perception of transgenics, generation, and testing of carrot transgenics will increase.
Now there are reports that carrot transformation is quite efficient. Carrot transformants have been developed for hepatitis B vaccine and other unique plant products but like most other vegetables, commercial release of the carrot transgenics has not materialized. However, it is to be recognized that herbicide and fungal resistance could have lot of advantages in crop production.
11. Research and Trends in Carrot Seed Production:
The individual carrot flowers, in common with most other species in Umbelliferae, are borne on terminal branches in compound umbels. There is a distinct order of flowering which relates to umbel position. The first umbel to flower is the primary (sometimes referred to as the ‘king’ umbel) which is terminal to the main stalk.
Branches from the main stalk form secondary umbels, and subsequent branches from these form tertiary umbels. Quaternary branches and umbels may also be formed.
The modern methods of carrot root crop production for the fresh market and processing require high quality carrots with the minimum of variation between individual roots derived from the same seed lot. The relationships between seed weight and endosperm characteristics and effect of seed position on seed quality have been reviewed by George (1999).
The variation in weight of individual seeds and weight of their embryo can contribute to subsequent variability in seedling size. Seeds from primary umbels are of better quality in terms of uniformity and producing heavy seedlings. Therefore, it is better to have high plant densities as there will be less branching and therefore a high proportion of primary to secondary umbels than at lower plant densities.
Further overall seed quality is improved if manual harvesting is restricted to primary umbels only so that seeds of shorter maturity span are harvested. This is usually needed to propagate basic seeds.
For quality seed production in early stages, root-to-seed production is followed where fully evaluated and selected roots are planted for seed production. Root to seed production is used for nucleus/breeder/foundation seed production. It is also quite common for commercial carrot seed production as this method provides an opportunity to the breeder and the seed producer for selection of roots with desired quality and phenotype.
The production location should be free of wild carrots. Limited size plot can be covered by pollinating honey-bee hives. There could be geographical isolation by at least a few kilometers and pollination be accomplished by honeybees or naturally occurring insect pollinators. Seed to seed production though possible should be avoided.
Cultivar Description of Carrot:
There are open-pollinated and hybrid cultivars. The following outline is based on UPOV (1990) and as described by George (1999).
Length (including petiole): very short, short, medium, long or very long
Division: very fine, fine, medium, coarse or very coarse
Intensity of green colour: light, medium or dark
Anthocyanin coloration of petiole: absent or present
2. Carrot roots:
Length: very short, short, medium, long or very long
Width: narrow, medium or broad
Ratio width/length: very small, small, medium, large or very large
Shape of longitudinal section: circular, obovate, obtriangular or narrowly oblong
Shape of shoulder: flat, flat to rounded, rounded, rounded to conical or conical
Tip: blunt, slightly pointed or pointed
External colour: white, yellow, orange or red
Intensity of external colour: light, medium or dark
Extent of green colour of skin of shoulder: absent or very small, small, medium, large or very large
Diameter of core relative to total diameter: very small, small, medium, large or very large
Colour of core: white, yellow, orange or red
Intensity of colour of core: light, medium or dark
Colour of cortex: white, yellow, orange or red
Intensity of colour of cortex: light, medium or dark
Colour of core compared with colour of cortex: lighter, same or darker
Green coloration of interior of top (longitudinal section): absent or very weak, weak, medium, strong or very strong
Time of maturity: very early, early, medium, late or very late
Breeder/foundation seed – 1600 m
Certified seed – 1000 m
Seed Production Methods:
1. Seed – to – seed
2. Root – to – seed
1. 600-1000 kg/ha
2. 1000 seed weight – Approximately 0.8 g
3. SMR – 50
12. Varieties of Carrot:
This has been bred at Indian Agricultural Research Institute, New Delhi and is an old release of 1963. It was evolved from a cross of Local Red and Nantes. The roots are scarlet in colour. It is rich in carotene (38 mg/100 g edible portion). Roots stay longer in field without bolting. Seed production in north Indian plains is successful.
This is a tropical or Asiatic type cultivar. It has been developed at Indian Agricultural Research Institute, New Delhi from a cross between Pusa Kesar and Nantes and was released by the Institute in 1985.
It has short top, smooth roots with orange flesh and self-coloured core. It is richer in carotene content (11571 IU/100 g) than Pusa Kesar (7753 IU/100 g) and produces seeds in plains. It is suitable for early sowing and takes 110-120 days to attain harvest maturity and yields 260-280 q/ha.
This variety belongs to European or temperate types. Its seed production is possible only in the hilly areas. It is an introduction by IARI Regional Station, Katrain. The roots are half long (12-15 cm), slim, well-shaped, cylindrical with stumped end forming a small thin tail.
The cortex and core are deep orange. It ranks good in quality, but the top is brittle making pulling difficult. Keeping quality is poor. It is suitable for cultivation in cooler months. It takes 110-120 days for root formation.
Pusa Yamdagni (Selection 5):
This has been developed at IARI, Regional Station, Katrain from an inter-varietal cross between EC 9981 and Nantes to combine earliness and size of root of the former and self-coloured core character of the latter. It has performed well both in the hills and plains. It is early in maturity. It has been released by the central variety release committee.
It is an old variety developed in Japan by Mr. Kuroda but is very popular in Indian seed companies for large scale sales. It has long, sweet, tender orange colour roots with wide shoulders. Roots are tapering to a blunt point. Roots are smooth, uniform, and conical in shape. Roots have better storability.