Tobacco Breeding Science

Author: Chen Xueping et al. / Country:
Publisher:
Publishing Time: 2002-03-01
Features: 5. Ecological Expression Some traits do not manifest in the original breeding environment and thus are not subject to selection. Once the environmental conditions of the planting area are suitable for the expression of these traits, the genetic diversity among previously existing plants becomes evident. The differences that exist due to different ecological conditions are brought to expression, providing genetic variation for further selection. (II) Trait Identification and Selection Correct identification of traits is the prerequisite for effective selection, and rapid, accurate identification methods are the guarantee for improving selection efficiency. 1. Categories of Trait Identification Methods (1) Direct and Indirect Identification Direct identification refers to the identification based on the direct expression of the target trait, while indirect identification refers to the identification based on the expression of traits related to the target trait, i.e., identifying another trait to indirectly achieve the purpose of identifying the target trait. Effective traits require specific conditions for identification. For example, the baking quality of wheat flour needs to be identified through baking tests to evaluate the volume, shape, color, texture, and flavor of the bread, followed by a comprehensive evaluation. The identification process is complex, whereas the baking quality can be indirectly identified by measuring the sedimentation value, which is related to baking quality. Generally, traits that can be directly identified are ultimately identified through direct identification, while effective generations can be identified through indirect identification. (2) Natural and Induced Identification The resistance or tolerance of crops to pests, diseases, and environmental stresses can be directly identified under natural conditions, which requires that the harmful factors frequently and sufficiently appear in the local experimental site. However, in practice, experimental sites often do not meet this condition. The method of directly identifying the target traits of experimental materials using local natural conditions is called natural identification, while the method of identifying experimental materials by artificially simulating harmful conditions to fully express their characteristics in this regard and obtain identification results is called induced identification. (3) Local and Off-Site Identification These are identification methods divided based on the location of identification. Off-site identification refers to sending experimental materials to other locations where the identification conditions are suitable for identification. Some regions are high-risk areas for certain diseases, and identifying the resistance or tolerance to these diseases in such areas can yield better natural identification results. Some regions are often arid, providing conditions for drought resistance identification. Of course, off-site identification can only identify specific target traits and is often unsuitable for identifying other target traits. 2. Improving Trait Identification and Selection Efficiency The basis of trait selection is trait identification, and the improvement of selection efficiency mainly depends on the improvement of identification efficiency. (1) For traits identified based on external morphology, the uniformity of experimental conditions is very important, primarily referring to the consistency of soil conditions and cultivation measures in the experimental site. It is also necessary to set up controls and repetitions to minimize experimental errors and make the results more reliable. Additionally, it is necessary to adopt appropriate experimental designs to facilitate the statistical analysis of the obtained experimental data. (2) For traits requiring induced identification, the experimental conditions created should be as consistent as possible with natural conditions to ensure that the identification results can basically represent the actual performance that might occur in production. (3) When identifying quality traits and physiological and biochemical traits, it requires rapid, simple, and accurate identification instruments and suitable identification techniques, with a requirement for microquantification of samples. (4) For identification using molecular markers, it requires a relatively reliable linkage map and relatively fast determination techniques. IV. Mixed Selection and Mixed Selection Breeding The selection of natural variation in a variety population, in addition to the above methods, is another basic method of selection, which is mixed selection. (I) Mixed Selection and Population Improvement Individual selection involves isolating unique genotypes from the population, i.e., isolating pure lines; mixed selection involves selecting individuals with basically similar target traits (single plants, single spikes, single bolls) from the population and mixing them for propagation to improve the population. The effect of individual selection is reflected through the performance of its offspring; mixed selection generally cannot separately test the offspring of the selected individuals. A selection method that can separately test the offspring of the selected individuals is called phenotypic mixed selection. The offspring generated by individual selection tend to be homozygous, with genotypes of individuals within the population being largely homogeneous, and genetic diversity being very small. The offspring population generated by mixed selection does not have completely consistent genotypes among individuals, and in the population of self-pollinated crops, it still maintains a certain proportion and degree of heterozygosity. By improving the population through phenotypic mixed selection, the average value of traits to be improved in the population is increased, and the variation is somewhat reduced, but compared to individual selection, it still maintains a relatively high genetic diversity. This method was previously used for population improvement of cross-pollinated crops and has now been introduced by some breeders into selection breeding programs for self-pollinated crops and cross-pollinated crops. (II) Application Value of Mixed Selection Breeding Mixed selection breeding can be applied to improve and purify local varieties. Local variety populations often contain a variety of genotypes, with differences in traits such as maturity, plant type, disease resistance, and production performance, limiting their production application. Through mixed selection of target traits, the limitations of these traits in the improved population can be optimized and made consistent, which can then be used as improved varieties to replace the original varieties. Early local varieties were mostly improved using this method, and successful results were achieved. For cross-pollinated crops, mixed selection breeding can be used to domesticate varieties introduced from different ecological regions and is also suitable for improving varieties that have been infiltrated with foreign or unsuitable germplasm resources. In seed multiplication, mixed selection is mainly used, from simple weeding and elimination to individual selection, line comparison, and mixed propagation for producing original seeds, all of which apply mixed selection methods. Section II Selection Breeding Program (I) Selecting Superior Variants (Individual Plants, Spikes, Bolls) In the field of the original variety population, select superior individuals that better meet the breeding goals. After indoor reselection, eliminate poor individuals, and keep the selected individuals separately for threshing. Record and number their characteristics for future testing of their offspring. (II) Line (Line) Trials The seeds of the selected individual lines from the previous season are planted separately to form lines (lines), which are either pure lines or systems. Set a control line every 9 or 19 lines, planting the original variety or promoted good varieties. Through field and indoor identification, select superior lines. If the target trait of plants within the line shows uniformity and consistency, it can be used as a line for the next season's line comparison trial; if there is still segregation of the target trait within the line, further individual selection is conducted for the next season's line (line) trial. (III) Line Comparison Trials The seeds of the selected lines from the previous season are planted separately in plots and replicated to improve the accuracy of the trial. Set up one control plot for each repetition, planting a standard variety for comparison. Line trials are generally conducted for two years. Based on field and indoor identification results, select 1-2 superior lines compared to the control lines for the next season to participate in regional trials. Lines that show exceptional superiority in the first year's line comparison trial can continue to participate in the second year's line comparison trial while being advanced for seed multiplication to participate in production experiments. (IV) Regional Trials and Production Trials Newly developed lines need to participate in regional trials to determine their suitable regional range, while production trials are conducted to identify their performance under large-scale production conditions. Based on the results of these two trials, variety approval is carried out, and qualified varieties can begin large-scale promotion. To ensure the timely provision of a large number of high-quality seeds, seed fields are set up simultaneously for these trials to accelerate seed multiplication for promotion.