Citrus development still a far cry
By Fatima Usman,
Muhammad Usman, M. Mumtaz Khan and Muhammad Sarwar Khan
Citrus, a member of family Rutaceae is a rich bounty of
edible fruits of different species like mandarins, oranges,
grapefruit, lemons and limes with minor categories like
tangerines, pummelos and tangelos etc.
The taxonomists are perplexed as how to classify various
kinds of citrus existing since antiquity. However, the
genetic and molecular biology techniques have revealed that
there are probably only three true species namely mandarins,
pummelos and citron.
Citrus stands first among tree fruits in the world sharing
103.82 million metric tons of fruit per annum. Brazil is the
largest (19.215 million metric tons) citrus producing
country of the world followed by the United States.
The US leads in yield per hectare and also contributes the
highest share of exports to the world. In Pakistan, citrus
enjoys first position among 30 other fruit crops with
respect to both area and production.
Pakistan stands amongst one of the major citrus producing
countries, having an area of 200,000 hectares with yearly
production of 1.995 million tons. About 95 per cent of total
citrus production is concentrated in Punjab province.
The country lags far behind in yield per hectare of citrus
and has a negligible share in world exports. Amongst other
factors, citrus cultivation is confronted by the lack of
genetic diversity, high number of seeds, low productive age
and yield, alternate bearing, a number of insect, pests and
diseases and huge post harvest losses.
Seedless or nearly seedless cultivars are desired by the
consumers. The Washington Naval Orange and the Marsh
seedless grapefruit have achieved world wide success.
Polyploidy plays a key role in evolving and exploring this
character.
There are a few form of polyploidy available here. We need
to exploit the potential of seedless citrus. It is an utmost
need to have highly diversified populations of citrus germ
plasm for the future.
Organisms having more than two basic sets of chromosomes are
called polyploid. It plays a vital role in the evolution of
crops. Breeders have been manipulating polyploidy for the
investigation of genes and their inheritance as well as a
source of variability for selection.
Citrus have 18 small chromosomes of 2µ in size. There are
several artificial means of creating polyploid which involve
interploid/inter specific hybridization, somatic doubling by
means of certain chemicals such as colchicines and through
endosperm culture.
On the other hand polyploids such as triploids, tetraploids,
pentaploids, hexaploids and octaploids are found
spontaneously in citrus including a tetraploid Hongkong wild
Kumquat, triploid Tahiti lime and tetraploid Triphasia
plants.
Small seeds are found with considerably high frequencies of
polyploids in several polyembryonic cultivars of sweet
orange and tangor. Many other polyploids have been produced
experimentally and some valuable varieties have been evolved
by ploidy manipulation.
Molecular markers have been widely used in the management
and genetic characterization of the plant genetic resources.
For example, to identify nucellar seedlings, to reduce
number of trees maintained by identifying redundancies, to
identify accessions for which the identification had been
lost and to screen pathogens.
Morphological and genetic characterization of naturally
occurring and induced polyploidy could help improve citrus,
both through breeding and biotechnological approaches.
Keeping in view these objectives the citrus polyploids of
natural and induced origin were characterized for both
morphological and physiological aspects. The basic studies
were carried out at the Institute of Horticultural Sciences,
University of Agriculture, Faisalabad and molecular studies
were carried out in the Chloroplast Biotechnology (A biosafe
GM plants technology) Lab., National Institute for
Biotechnology and Genetic Engineering (NIBGE), Faisalabad.
The citrus cultivars selected for the present study were
amongst the cultivars with high number of seeds. The seed
number and size was found to be cultivar dependent.
Significant differences were observed for underdeveloped
seeds per berry in ascending order as Kinnow (12.23) >
Musambi (11.20) > Feutrell's Early (9.50) > Succari (8.82)
while for appearance of number of shrivelled seeds, Kinnow
appeared at the top.
Overall among the four cultivars explored, Kinnow showed the
highest number of seeds per berry followed by Musambi,
Succari and Feutrell's Early. The different plant sides have
not shown any significant impact on the development of
different types of seeds.
In vitro germination behavioural studies revealed mandarins
better than sweet oranges. These diploid and polyploid
groups were also evaluated through stomatal studies to
confirm the ploidy and were subjected to morphological
characterization.
Better plant growth was found in the ploids of Sweet orange
cultivars than the mandarins. Citrus cultivars Kinnow and
Musambi were high in spontaneous triploids while Kinnow and
Succari were high in tetraploids.
Identification of citrus polyploids and hybrids is usually
performed through various morphological markers; however, it
gets very difficult when mother parents have a convenient
dominant character.
Therefore, the genetic analysis of the citrus polyploids was
performed through simple sequence repeat loci analysis. Use
of the SSR molecular markers confirmed the genetic
variability of the morphologically distinct spontaneous and
colchicines induced ploids; and interploid hybrids.
These studies reflect a comparatively efficient and steady
progress towards creating and exploring the available gene
pool of citrus. Using the SSR molecular markers based
genotyping with morphological selection and cytological
screening of putative polyploids appeared as a much improved
strategy in identification of zygotics, nucellars, hybrids
as well as polyploids at seedling stage.
Consistency in such studies will ultimately help to develop
citrus germ plasm bank by saving cost, reducing time and to
select a wide range of citrus polyploids for diversification
of production base.
Courtesy: The DAWN
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