| چکیده انگلیسی مقاله |
Introduction
Back saxaul (Haloxylon aphyllum) is a treelike shrub from the Chenopodiaceae family adapted to desert and arid conditions and has long been of interest. Selection of superior and compatible genotypes of this species has a great impact on the development of the country's hawthorn forests. Given the function of this species in stabilizing sand and reducing wind speed, selection traits such as height, trunk diameter, and canopy diameter of shrubs are of great importance. The degree of success in improving these traits and selecting superior rootstocks depends on the extent of genetic variation and their relationship to each other. This study aimed to investigate the amount and nature of genetic diversity and the response to two cycles of selection for the aforementioned traits in Haloxylon aphyllum.
Materials and Methods
In order to genetically improve black saxaul, three cycles (5-year selection with a total of 15 years) were conducted on a black saxaul population with a high genetic variation for shrub height, trunk diameter, and canopy diameter at the Besat Sand Stabilization Station in Qom, Iran. In the first phase, starting in 1997, 27 genotypes were selected from different regions of the country. 40 pots were planted from the seeds of each Shrub. The seedlings were transferred to the main field in early spring and data were recorded in the fifth year. In the second phase (starting in 2005) and the third phase (beginning in 2012), 19 and 9 superior genotypes were selected and their seedlings were planted in a randomized complete block design with three replications, respectively. In all three phases, shrub height, trunk diameter, and canopy diameter were measured in the fifth year. Analysis of variances was performed on the basic population and selected populations in the first and second generations. The genetic and environmental variances were estimated using expected mean squares (EMS, and broad-sense heritabilities (h²b) of the traits were calculated. To ensure the success of selection during two consecutive generations, the real heritability (h2r) was used to predict the response to selection (h2r = R/S), where R is the response to selection and S is the selection differential. Finally, the narrow sense heritability (h2op) was estimated from the regression of parent on offspring during two consecutive generations. The heritability was estimated assuming diploid inheritance of the materials used, random sampling from the basic population, absence of epistasis, and balance of genes in terms of genetic linkage.
Results
The results of the analysis of variance three generations showed significant variations between genotypes in the base population and the selected G1 generation. But, in the G2 generation, there was no significant difference between the selected populations. This result was expected because in the second generation, the number of selected genotypes was reduced to 9 populations and since all of them were selected for their superiorities and therefore, the difference between them was not significant. The estimated broad sense heritability (h2b) based on the analysis of variance between genotypes was high for all three traits in the base population and the G1 generation with values ranging from 0.66 to 0.78. However, in the G2 generation, its value dropped to less than 0.40. The narrow sense heritability (h2op) from the regression of parent on offspring was relatively high (base population and the G1 progeny), with values of 0.94, 0.88, and 0.76, for shrub height, trunk diameter, and canopy diameter, respectively. However, in the second generation, the narrow sense heritability (h2op) of (the parent G1 and the G2 progeny) was relatively low and moderate. The real heritability (h2r) obtained from the ratio of the response to selection to the selection difference was relatively high in the G1 and G2 generations and ranged from 0.45 to 0.96.
Conclusion
After two generations of direct selection, the response to selection was significant and the average of the studied population increased for shrub height (from 188.93 to 205.95 cm), trunk diameter (from 58.05 to 63.78 cm) and canopy diameter (from 219.19 to 246.59 cm). Since the amounts of three heritabilities (h2b, h2op and h2r) were more and less similar, it was concluded that the studied traits are genetically controlled by additive genes and the response of selection for these traits is positive, indicating the possibility of improving breeding new varieties in black saxaul. Also, genotypes P12, P13, P18, P10 and P22 could be planted in windy areas for dust control due to their superiority in terms of shrub height. On the other hand, the use of genotypes P12, P14, P18, P19 and P22 is recommended for stabilizing quicksand in desert areas due to their superiority and appropriate response to selection in terms of trunk diameter and canopy diameter. In this regard, the establishment of seed orchards is necessary for the conservation and sustainable use of selected genotypes in arid and semi-arid areas of the desert margins of Iran. |