Journal of Crop Breeding

Extended Abstract
Background: Germination is a sensitive process for plant growth and achieving optimal performance. Environmental stresses, including salinity, are the most important limiting factors of plant production, and humans are forced to deal with these stresses through various management practices. Rapeseed (Brassica napus L.), one of the most important oil plants, is relatively tolerant to salinity and is the best choice for saline and sodium soils, so that its resistance to salinity is equal to that of barley. Rapeseed cultivars tolerate salinity of slightly more than 7 dS/m. However, this plant, like most other crops, is sensitive to salinity in the initial stage of seedling establishment. Although the seeds of canola genotypes show different abilities to maintain their metabolic activities under saline conditions, germination and certain stages of growth are more susceptible to damage due to salt stress. Therefore, it is essential to evaluate salinity tolerance in the early stages of growth, especially germination. In this regard, the present study was carried out to evaluate and select salinity-tolerant genotypes of different Brassica species (Brassica spp.) in the germination stage.
Methods: In the present study, the salinity tolerance of 100 genotypes of five Brassica species was evaluated using a factorial experiment based on a completely randomized design in three replications. The first factor included Brassica genotypes (60 genotypes of B. napus L., 15 genotypes of B. nigra L., 15 genotypes of B. juncea L., 5 genotypes of B. rapa L., and 5 genotypes of B. carinata L.). The second factor was five levels of salinity with sodium chloride, including zero (control), 5, 10, 15, and 20 dS/m. After disinfection with 1.5% sodium hypochlorite, healthy seeds were transferred to sterile Petri dishes with a layer of Whatman filter paper No. 1. The diameter of the Petri dishes used in this experiment was 8 cm, and each Petri dish contained 25 seeds. After placing the seeds in Petri dishes, depending on the desired treatments, 5 mL of distilled water or sodium chloride solutions with potentials of 5, 10, 15, and 20 dS/m were placed in a germinator at 22 ̊C, and the number of germinated seeds was counted at 12-hour intervals until the number of germinated seeds was fixed. Traits such as length, weight, and dry weights of roots, stems, and plants were measured after germination. Finally, the beginning, end, and uniformity of germination, germination percentage, germination speed, germination index, seedling length index, and allometric coefficient were calculated in different treatments. After the experiment and data collection, cluster analysis was done at different salinity levels using SPSS software (version 22) and the Ward’s method. Then, the best group of genotypes was selected at each salinity level by analyzing the variance and comparing the mean between the groups using the least significant difference (LSD) test at the 5% probability level.
Results: During the experiment, 20 genotypes were removed due to the low potential of some genotypes, and data analysis was done for 80 genotypes. Based on the dendrograms obtained from cluster analysis at salinity levels of zero, 5, and 15 dS/m, the studied genotypes were divided into three groups and were placed into four groups at salinity levels of 10 and 20 dS/m. In general, based on the comparison of the average between the groups resulting from the cluster analysis, the first group at salinity levels of 0, 5, and 15 dS/m and the fourth group at salinity levels of 10 and 20 dS/m were selected as the best groups. Then, variance analysis and mean comparison were performed between genotypes in the mentioned groups. Based on the results of cluster analysis, comparing the average of the groups and the analysis of the genotypes of the top groups, significant differences were observed between the genotypes in most of the measured traits. As such, 15 genotypes (including codes 141, 306, 328, 336, 346, 367, 446, 483, 509, 517, 693, 767, 831, 850, and 860) at all investigated salinity levels were always placed in the top group, and, therefore, they were selected as the superior genotypes of brassicas.
Conclusion: In total, the results of this experiment showed that 15 out of all the investigated genotypes were always placed in the top groups at all levels of salinity stress. Seven genotypes of B. napus L., 6 genotypes of B. juncea L., and 2 genotypes of B. rapa L. were present among the selected genotypes. There were no genotypes of B. nigra L. The results indicated that these genotypes had a high ability to germinate and produce strong seedlings in both normal and saline conditions and could be used for further studies and breeding programs. The genome of the diploid species B. rapa L. is common to both allotetraploid species B. napus L. and B. juncea L., but it is absent in the genome of the allotetraploid species B. carinata L. and the diploid species B. nigra L. Accordingly, it can be concluded that the relative tolerance to salinity stress in the Brassica genus originates from the genome of B. rapa L.