International Journal of Horticulture, 2017, Vol.7, No.22, 180-204
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Hibiscus:
‘Anjali’
African Violet:
‘Double Flowers’, ‘Pink Flowers’, ‘Girl Foliage’, ‘Fantasy Flowers’, ‘Star Shaped Flowers’,
‘Fringed Flowers’, ‘Bustled Foliages’, ‘Coral Pigments’, ‘Yellow Flowers’.
Hosta:
‘
Frances Williams’, ‘Northern Lights’, ‘Platinum Tiara’, ‘Golden Sunburst’, ‘Color Glory’.
6.7 Ploidy breeding in ornamental crops
A crop species with a genetic chromosome number ‘n’ is known as haploid and somatic chromosome is diploid
(2n). A crop species whose somatic chromosome number is the exact multiple of the basic number is called
euploid and the crop species whose somatic number is not an exact multiple of the basic number is called
aneuploids. Depending upon the multiplicity of the basic number, euploids may be monoploid (x), diploid (2x),
triploid (3x), tetraploid (4x), hexaploid (6x), octaploid (8x) and so on. Species above diploids are called
polyploids. Polyploids with same genome number are termed as autopolyploids and those with different
genomes are termed as allopolyploids.
In general, haploids are weak and sterile but other polyploids have larger plant parts, large cell, stomata with
slower growth rate than diploids. Monosomics do not survive in diploid species and nullisomics donot survive in
polyploidy species.
Amphidiploid is an allopolyploid having two copies of each genome present in it. Tetraploids are vigorous with
robust vegetative growth, thicker leaves and bigger flowers. Triploids show the characteristics of both hybrid
vigour and polyploidy vigour.
6.8 Advantages of polyploidy
Ornamental haploids have more decorative value due to small flowers and prolonged blooming.
Triploids have sterile flowers with longer flower life as reported in azaleas, lilies and hyacinths. Triploid
chrysanthemum is used to produce pyrethrum.
Tetraploids have luxuriant vegetative growth. Due to increased flower size, tetraploid cultivars are available
in zinnia, antirrhinum and petunia.
Somatically induced polyploids or allopolyploids contribute heterosis and hybrid vigour.
Interspecific hybridization followed by polyploidization have helped in the evolution and domestication
of many ornamental plants such as rose, chrysanthemum, gladiolus, alstroemeria, lilium, orchids etc.
Meiotic polyploidization through homeologous recombination in the interspecific hybrids is reported in
Alstroemeria
and
Lilium
.
Polyploids are more tolerant to drought, cold, mutagens, herbicides and poor soils.
6.9 Production of polyploids
Tetraploid and higher level of polyploidy are induced using regeneration methods, heat and cold treatment
on germinating seeds and by using chemicals like colchicine, nitrous oxide, oryzaline, trifluralin and
phosphoric amide.
Polyploidy due to somatic mutation may be caused by disruption in mitosis resulting in chromosome
doubling, e.g.
Primula kewensis.
Polyploidy may be produced from the union of unreduced gametes-eggs and sperms that have not
undergone normal meiosis and still have a 2n constitution.
Polyploidy is more frequent at high altitudes, high latitudes, in wet soils and meadows.
Recently,
in vitro
polyploidization is followed to speed up the heterosis breeding in ornamentals, to reduce
the number of aberrant plants and to reduce the time span required for polyploidy production.
Achievements of polyploidy in ornamentals:
Polyploidy has been very much exploited in the evolution of
species and cultivars of various ornamental crops like tulip, dahlia, anthurium, bougainvillea, lily, cacti, primula,
