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 worldwide 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 germplasm 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/interspecific 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 germplasm bank by saving cost, reducing time and to select a wide range of citrus polyploids for diversification of production base.