After reading this article you will learn about:- 1. Origin of Leek 2. Botany of Leek 3. Genetics 4. Breeding Goals 5. Hybridization Techniques 6. Breeding Methods 7. Sources of Disease Resistance.
Origin of Leek:
The Allium species are not evenly distributed within the northern hemisphere, since most of them occur in Old World. A belt containing many species stretches from the Mediterranean basin to Iran and Afghanistan.
A region of specially high species diversity occurs in Turkey and the Irano Turanian floristic region i.e. Iran, Afghanistan, Soviet Middle-Asia (including Kazakhstan), and West Pakistan. The number of species decreases away from this centre of diversity. Leek is thought to have been derived from the sand leek (A. ameloprasum) which grows wild in southern Europe, north Africa and the Middle East.
Botany of Leek:
Leek is larger than onion. Like garlic, the leaf blades are dorsiventrally flattened rather than being radial. The leaf bases of leek may store some reserves as they do in onion but they do not thicken into a bulb. The cultivated leek is a tetraploid (4x = 32) whereas the onion is diploid (2n = 16). Whether the leek is autotetraploid or allotetraploid is difficult to determine with certainty.
Despite tetraploidy, leek is cytologically stable which it needs to propagate sexually. During early stages of meiosis (pachytene and diplotene), associations of four chromosomes take place. However, they develop into mainly bivalents at metaphase I.
The bivalents nearly always show a marked localization of the chiasmata or cross-over points near the centromeres of the chromosomes. This gives a cross- shaped configuration for the paired chromosomes at the metaphase I stage. This process although rare, has been found to be normal and orderly one in leek tetraploids.
The localization of the chiasmata. reduces the area of chromosomes where pairing occurs and prevents multivalent formation which usually involves the distal ends of the chromosomes leading to only a negligible amount of recombination between the two parental sets of chromosomes.
This effectively causes major parts of chromosomes to remain undisturbed by recombination and the chromosomes are inherited en bloc or as supergenes. This is one of the possible causes of cytological stability in leek.
Genetics of Leek:
In leek 7-14 different chlorophyll deficiency genes have been reported. The level of self- pollination within the open pollinated populations has been reported to be about 18%.
Leek being tetraploid, several heterozygous states are possible at each locus, for example, AAAa (simplex state of a defective recessive allele, a) which is rarely revealed by selfing, AAaa (the duplex state) which gives 2.8-4.5% aaaa on selfing, and Aaaa (the triplex state) which gives 25-28.7% aaaa on selfing.
After open pollination, the frequency of aaaa genotypes is about 1 -5% so that such alleles always persist in the population and contribute to the well-established loss of vigour which occurs after selfing in leek.
Breeding Goals of Leek:
a. Winter hardiness
b. Resistance to bolting
c. Dark green leaf blade
d. Freedom from bulbing
e. Long shaft (the blanched pseudo-stem formed by the young folder leaf blades) length
f. High yield
g. Resistance to leek rust and yellow stripe virus
Hybridization Techniques of Leek:
The inflorescences (umbles) of both the female and male parents are enclosed in a muslin cloth bag. The individual flowers (protandrous in nature) are emasculated as they open. Pollination is generally carried out a day or two after emasculation when the stigma is feathery.
As flowers open sequentially from the bottom to the top of the umbel, emasculation is done daily to prevent self-pollination of emasculated flowers by pollen from later-opening flowers.
Once sufficient pollinations are made, remaining unopened flowers are trimmed off. Leek can also be propagated -vegetatively by the induction of top-sets and plantlets in the umbel or from the basal cloves formed after flower stalk formation.
Breeding Methods of Leek:
Leek shows considerable inbreeding depression during inbreeding. According to review of Currah (1986), inbreeding depression was about 35% in I, and nearly 40% in I2 generation. This depression was equally strong on seed production capability. Therefore, mass selection has been the most effective breeding method of cultivar improvement. Male sterility is also encountered in I1 generation.
Crosses between two varieties and I1 lines often show heterosis. F2 generations of I1 x I1 crosses could also be used as segregating source populations for selection of desirable genotypes which can again be crossed to reverse the drastic effect of inbreeding. Use of male sterility seems to be a possibility for developing hybrid cultivars of leek, but this shall not be easy to employ as done in onion because of tetraploid nature of leek.
Synthetic cultivars derived from clonal lines selected from an open-pollinated population multiplied by top-sets, could be used to stabilize the parent material and to avoid the problem in leeks of constant change in the seed parents from one season to another.
It is generally agreed that because of its tetraploid nature, the leek needs to retain a high degree of heterozygosity in order to remain vigorous like in potato. The genetic re-modelling of the leek by hybrid breeding seems difficult due to problems of developing inbred lines. Due to its tetraploid nature, leek is capable of carrying a higher load of deleterious genes without visible effects than the onion.
It suffers severely on selfing. However, this need not be a barrier to improvement by mass selection or by crossing cultivars with different desirable characters and selecting from among the F2,s and F3,s.
New techniques, such as anther culture might enable fertile diploid lines to be produced (double haploids) which would allow the elimination of gross defects before using selected lines to produce hybrid or synthetic cultivars.
Meanwhile, a few of the ways to get rid of few genetic defects of the leek could be:
(i) Seed grading including specific gravity methods
(ii) Multi-block raising methods which might allow weak plants to be completely suppressed by vigorous crossed plants
(iii) A return to seedling grading.
Sources of Disease Resistance of Leek:
Leek stripe virus:
Siegfried, Deverrieres, Autumn Giant, Trimphatur, Ligina, Siegfried Frost, Winto.