Author: Arvind Nagar
Root vegetables are the mainstay of a winter diet. The most commonly grown root crops in India are carrot (Daucus carota L.), radish (Raphanus sativus L.), turnip (Brassica rapa L.) and beet root (Beta vulgaris L.). Due to their short duration and high productivity, root crops fit well in sequential, inter and relay cropping which enable maximum use of enable land. Root crops are generally storage organs enlarge to store energy in the form of carbohydrates. Root vegetable like carrots are highly efficient in producing the highest amount of nutrients and health promoting substances per unit area. Carrot is a rich source of high carotene content which is reported to be anti carcinogenic in nature
Improvement of root Crops
A clear cut objective of breeding is a useful prerequisite for greater chances of success in the improvement of a vegetable crop. Knowledge of inheritance of characters on the principles of genetics is quite essential and helpful in crop breeding. The breeding method depends upon breeding system, life cycle, mode of reproduction, sex forms and genetic architecture. The commonly used breeding methods for crop improvement are selection hybridization, backcrossing and heterosis breeding through utilization of hybrid vigour.
The development of hybrid and hybrid seed production in root crops can be made by the following methods:
- Emasculation (by pulling out male plants) and use of wind pollination-Garden beet).
- Free insect pollination. a. Using self- incompatibility: Radish and turnip b. Using male sterility: Carrot, radish and beet root The following steps are necessary for the production of hybrid seeds in these crops:
i) Inbreeding and production of inbred lines.
ii) Testing of combining ability.
iii) Improvement of inbred lines/varieties.
iv) Production of hybrid seeds.
CARROT (Daucus carota L.)
Breeding objectives: Major breeding goals have historically been focused on improving root productivity and appearance as well as seed productivity. Main breeding goals in carrot are:
- Early and high root yield
- Combining desirable characters of both European and Asiatic groups, especially high carotene content and ability to set seeds is plains.
- Developing F1 hybrids using cytoplasmic male sterility.
- Desirable uniform root size, shape (cylindrical), dark orange external and internal color, i.e. uniform in xylem and phloem, top-root ratio, i.e., small tops and smooth heavy tender root.
- Broad shouldered, cylindrical, uniformly tapering or stump rooted thin and self coloured slow bolting carrots.
- Resistance/ tolerance to defects such as excessive secondary root development, splitting, secondary growth and cavity spot.
- Resistance to Alterneria blight (A. dauci) and cercospora leaf blight (Cercospora carotae) and root knot nematode (Meloidogyne hapla).
- Tolerance to environmental stresses and wider adaptability. The production of F1 hybrid seed by hand emasculation and pollination is not possible commercially as the flowers are very small and single pollination gives only one or two seeds. In carrot, the inflorescence is compound umbel. The flowers are arranged in umbellets, and umbellets into an umbel. Within umbellets and umbels, floral development is centripetal and arranged in spiral. Flowers are protandrous. It produces the first order umbel called King umbel, second order (secondary) and third order (tertiary) and even umbels of fourth and fifth orders also. Pollen dehiscence may last one or two days per floret and 6 to 7 days per umbel. Stigma becomes receptive on the third or fourth day after anthesis and may remain receptive for about a week. Thus a plant may take about 4-6 weeks for complete blooming. The different order umbels flower at intervals of 8-12 days from each other. Anthesis in a single umbel is completed in 7-9 days. From King to II, III or more number of orders, there is a certain amount of embryo less seed which cause the sterility. Stigma fertility starts when dehiscence is completed in whole umbel. King umbel comes first in flower and gets pollens from secondary umbels and secondary umbels get pollens from tertiary umbels, and so on. Sufficient buds at peak stage of flowering are emasculated and the remaining young ones are removed. The pollen parent umbel is enclosed in the same bag along with the emasculated umbel of the female parent. Daily for a few days in the morning, the male umbel is gently rubbed over the female to ensure cross pollination. Some times when the pollen parent possesses some dominant marker gene with the help of which the hybrids can be distinguished in the seedling stage, it is not necessary to emasculate the flowers.
Welch and Grimball (1947) reported the first male sterile plant in 1945-46. Now morphologically there are 3 types of male sterility available:
- Wax coated and Swollen Anthers: These anthers do not produce any fertile pollen.
- Brown coloured Anthers: These anthers are shriveled and brown without any viable pollen.
- Anthers replaced by petals: These anthers are petaloid. This type of sterility is more stable.
Characteristics of Male Sterile Lines
- The male sterile plants are less vigorous than the male fertile plants.
- The cell differentiation of male sterile lines also differs. It may be either earlier or later than normal lines resulting in petaloid or brown anther type sterilities respectively.
- Male sterile lines produce off white to green coloured flowers, while fertile ones produce white flowers.
- Male sterile lines start anthesis much later than fertile lines.
- Nectar produced in the male sterile lines is often poor in quality as well as in quantity.
- Male fertile lines generally produce perfume like aroma while most of male sterile lines produce indistinct carrot like or no aroma in the flowers.
- Male sterile flowers are less visited by the honey bees.
A. Genic Male Sterility
Hausche and Gabelman (1963) reported that male sterility is conditioned by two nuclear genes. The plants carrying ms 5 ms5 or ms4 in heterozygous condition (MS4 ms4) or homozygous condition (MS4 MS4) result in male sterility.
B. Gene Cytoplasmic Male Sterility
Jones (1950) and Lamprech (1951) reported that the male sterility is due to interaction between dominant nuclear gene and cytoplasm. It is not clearly that how many genes are involved, but according to Timin et al. (1981), the brown anther male sterility is conditioned by dominant and recessive genes (MS1 MS 1 MS2 MS2) in (s) cytoplasm, while the petaloid type is conditioned by three dominant genes (MS3 MS3 MS4 MS4 MS 5 MS5) in (s) sterile cytoplasm.
Hybrid Seed Production:
Hybrid seed production using male sterility needs location of male fertile plants (maintainer) which in successive generations will yield all male sterile progenies. Such plants should be homozygous dominant for one or more MS loci, should be free from epistasis. In the heterosis breeding programme 3 lines are used, namely the male sterile line (A), male fertile sister line (B), and the pollinator line (C) which is male fertile and has a good combining ability with the male sterile line. By the use of suitable cytoplasm and one or more gene it has proven possible to facilitate production and maintenance of hybrids. But there are some difficulties
- Firstly seed behavior is not uniform.
- During seed set seeds of a higher order umbel start growth and ripen later than those of a lower order umbel resulting in low seed germineability and slow seedling growth.
- Large scale maintenance of hybrids has not been very successful because of loss of vigour of inbred components or instability in sterile/maintainer line.
- Narrow genetic base of modern carrots.
- Two CMS systems suffer from instability of male sterility under specific conditions viz., high temperature, dry conditions and long days.
- The development of male sterile line and maintain lines is a very laborious work due to dominant state of male sterility. High degree of homozygosity in the male sterile and maintainer lines is recommended but 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 have been recommended (Stein and Nothangel, 1995). Three way hybrids viz. Spartan Classic, Spartan Premium, Spartan Winter have been developed. All the three hybrids are early maturing, smooth rooted, moderately resistant to Cercospora carotae and rusty root (Pythium spp.) The male sterile and pollinator lines are grown in alternate rows of 4:1 or 8:2 and the F1 hybrid seed is harvested from female line only. Rouging of off types and pollen bearers in female line is very important. Flowering of male and female parents should have synchronization for flowering. Five to six bee colonies/ha is beneficial in pollination. The F1 hybrids are early in maturity and have uniformity in root shape, size and colour resulting in higher number of marketable roots and higher yield than the open pollinated variety. In carrot 3 way or single cross hybrid are made using male sterile line as one of the parent. Hybrids can also be developed with high carotene content as breeding lines in USA are available now having very high carotene of 5000 ppm. Hybrids in carrot are not popular due to problems in the utilization of male sterile lines and production of hybrid seed.
Hausche , P.E. and Gabelman , W.H. (1963). Digenic control of male sterility in
2. Stein, M., Nothnagel , Th. (1995). Some remarks on carrot breeding - review. Plant Breeding. 114 : 1-11.
3. Timin I H.1981,Tsitplazmatich, Muzhsk , sterl’nos,I Seleksiya rast.Kiev, Ukrainian.
4. Welch , J.E. and Grimball , E.L. (1947). Male sterility in carrot . Science .106 :398.
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