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Abstract
Currently, citrus fruits distributed for industrial purposes in the world are characterized by great diversity, the main reason for which is their high polymorphism. Fruit plants, especially citrus fruits, are characterized with a high ability to give completely differently modified variants of buds, which make it possible to obtain many forms that differ sharply from each other in morphological, biological and economic characteristics. Most of the widely recognized citrus varieties (80%) were isolated from a similar diversity. Therefore, the possibilities of selective activity for citrus crops are unlimited. It can be vividly confirmed with many new varieties and forms of citrus trees bred by Georgian scientists over many decades, which laid the ground for development of the citrus industry in Georgia and the creation of a rich gene pool. The given paper presents the traditional classical methods of selection and the invaluable contribution of Georgian breeders who have worked for almost a century and created a rich genetic material (gene pool) of citrus fruits, most of which, unfortunately, has been destroyed.
It is known that mutation increases the frequency, spectrum, and volume of variability and provides the source material for selection in many directions. Mutations in natural conditions are quite rare, and the induced mutation process makes it possible to cause changes with a greater frequency and in a wider range while such changes cannot be obtained by hybridization. Mutations that occur during induced mutagenesis lead to a change in the genotype which can be expressed in changes in the morphological, physiological, biochemical, and other characteristics of the organism.
The scientific work is based on the results of 30 years of research conducted by the authors, that widely discusses and analyzes the results of many years of research on the effects of chemical mutagens on
seeds and cuttings of the citrus Unshiu. Based on the results obtained and the analysis of literary sources, the opinions are presented about the strong tendency of citrus fruits to mutations.
The work includes and analyzes all literary materials based on many years of scientific research on citrus and other agricultural crops conducted by Georgian and foreign scientists. Mutations, their classification and the processes of their formation are widely and comprehensively discussed, which will be of great help to students of biology and agriculture. It is known that there is no common opinion on the classification of mutations and it is always vague and contradictory. The classification varies depending on the criteria. Therefore, a detailed discussion of this issue in the article will be of great help to students in studying the essence and classification of mutations.
The work contains all the unique materials and a rich gene pool developed by natural and induced mutation as a result of many years of work by Georgian scientists. The morphological features of varieties, forms (clones) and mutants of citrus fruits bred by mutation are described, which is confirmed by literary sources. It is a pity that the specified gene pool (with the exception) was destroyed survived is also not perfect and it is difficult to find and collect them on collection sites.
It has been established that the impact of the mutagen on Unshiu citrus seeds leads to a significant increase in the frequency and spectrum of mutational variability compared to the natural one. The range of variability changes with the dose of the mutagen, exposure conditions, exposure, changes in its physiological state, etc. The impact of the mutagen is especially manifested by sharp changes in morphological traits belonging to the category of intraspecific changes and manifest themselves with a particularly high frequency during 24-hour exposure. The conclusions are made on the basis of data obtained as a result of the impact of various mutagens on seeds and cuttings of Unshiu citrus.
Morphological analysis of forms obtained as a result of induced mutagenesis reveals that Unshiu citrus has not only the ability to create spontaneous mutations (under the influence of mutagenic environmental factors), but also mutations in the species (mandarin) can be caused by artificial mutagenic factors. In case of induced mutations, the diversity of mutant traits is much higher, although most of the mutants are negative and are not essential for practical selection. It should be noted that along with negative mutations, interesting (positive) beneficial mutations arise both from a practical and selective point of view where the most significant part of the traits falls on the ability to form sprouts, biochemical indicators, growth intensity morphological variability of the plant and other economic indicators.
Studies have confirmed that the use of chemical mutagens to citrus fruits is characterized by a number of features, some of which are of both selective and practical importance. Treatment of Unshiu citrus seeds and cuttings with a chemical mutagen are a powerful factor in the artificial regulation of formation processes. Chemical mutagenesis along with other breeding methods can be considered as one of the important methods for obtaining new citrus varieties.
By exposion of the mutagen ethyl nitrosourea (ENU) onto Unshiu citrus seeds it was confirmed that small mutations were detected at all concentrations and effects of the mutagen, however, relatively interesting early-ripening forms with a high sugar content were obtained when exposed to 0.1% and 0.08% solutions (No. 1691, 2027, 5018). As a result of the research, it was found that under the action of 0.08% and 0.05% ENU solution forms were obtained that differed in storage capacity (№№5440 და 1691) and frost-resistant forms were identified with a 24-hour exposure of 0.06% and 0.05% solution (No. 5440 and 1691). When exposed to seeds of Unshiu citrus of various concentrations of the solution of mutagen ENU under various conditions of exposure, it was found that the mutagenic effect manifests itself in the M1 generation during treatment. Changes in the morphological features of seedlings depend on the concentration and exposure of the solution. 24-hour exposure to a 0.2% solution causes dramatic changes in morphological mutations, and weaker concentrations of 0.1%; 0.08%; 0.06%; 0.05% causes micro mutations with a low frequency that are very difficult to detect and fix in subsequent generations.
The paper describes important studies on the effect of pollinators on the yield and seeds of Unshiu citrus. The study found that the pollen of Citrus Ichangensis and Spherical Pompelmoes greatly contributes to the favorable planting of Unshiu citrus and increases growth from 8% to 11% and also promotes seed formation in the fruit. The optimal pollination period for the formation of seeds in citrus Unshiu is considered the period from the beginning of the opening of the buds to the beginning of the withering of the crown petals. Seedlings obtained from Unshiu citrus treated with a chemical mutagen are characterized by the stimulation of a number of quantitative and qualitative (physiological) traits. In the case of using almost all concentrations, early-fruiting mutant forms (№№2027, 5018, 5412, 1965) with a large amount of sugar (№№1752, 5018, 5442) and rich in vitamin C (№№5435, 1965, 5412, 5425, 5440, 1730) were detected. Exposure of mandarin seeds to 0.06% and 0.05% ENU mutagen solutions stimulated the growth of frost-resistant seedlings.
Mutant №1464 was obtained by treating citrus seeds with a highly concentrated (0.2%) solution of the ENU mutagen, which is characterized by a sharp morphological change. During vegetative propagation of mutant №1464, there were no cases of returning to the original form which indicates the development of resistant mutants under the influence of mutagenic factors on Unshiu seeds.
In the M1 generation of seedlings obtained from Unshiu citrus buds treated with various mutagens, changes in plant morphology and growth and development were detected. Mutants are most often presented as chimeras that "return" to their original form as they grow. The primary mutagenic effect in the M1 generation obtained with the treatment of Unshiu citrus cuttings with mutagenic ENU solution is higher (17.3%), than in the case of treatment with 1,4 DAB (15.3%). Among the mutant forms, a tangerine-like form of the Wase group (N457) was identified, which turned out to be interesting for further breeding work.
The largest number of modified forms of Unshiu citrus was obtained with a 36-hour exposure of 1% and 0.5% ENU Mutagen solutions on cuttings of citrus Unshiu. It has been established that the closer the buds are to the mutagenic solution, the higher the mutagenic effect and vice versa. When treated with a 1% ENU solution, the yield of mutations from the 3rd bud is 5 times higher than from the 5th.
As a result of many years of research activity under the influence of various chemical mutagens on seeds and cuttings of Unshiu citrus fruits, about 300 mutant forms were obtained 20 forms (mutants) of which were isolated and studied. The forms obtained as a result of the research were planted at the experimental collection site of the Research Institute of Subtropical Crops and Tea Industry (Tsvermagala). Unfortunately, along with other collections, this collection was in danger. Most of the mutants were propagated by us vegetative on the experimental-collection plot of the Institute of Phytopathology and Biodiversity of the Batumi Shota Rustaveli State University.
The paper presents a description of the mutants obtained as a result of experimental studies most of which have both theoretical and practical significance. Forms (mutants) are characterized by a wide range of economically useful changes: frost resistance, storage capacity, high content of biochemical substances, etc. Some of the mutants are superior in terms of their three-dimensional and biological benefits to Unshiu citrus, which is currently being produced and can be put into production after appropriate procedures. Even more urgent is the need to introduce them into production or use them as starting material in breeding. The role of induced mutagenesis in citrus crops is important not only in the creation of the initial breeding material (gene pool), but also in the process of control and regulation of morphogenesis processes.
An important part of the paper is modern genomic technologies which are an excellent tool in the study of various agricultural crops including citrus fruits. Genetic engineering has provided significant assistance to farmers in solving the problems of industrial farming, among which productivity is especially important and deserves attention. There is a lot of information about CRISPR/Cas technologies that have great potential for obtaining non-transgenic plants, so this direction is relatively promising.
The paper as a reference tool is intended for students of the agrarian direction. The work will also be useful to biology students, all interested persons and Farmers employed in the field of citrus.
