A method for screening salt stress tolerance in Indian mustard (Brassica juncea) (L.) Czern & Coss at seedling stage

Abstract

Fifty-nine diverse Brassica juncea (Indian mustard) genotypes were used to find an effective screening method to identify salt tolerance at the germination and seedling stages. Salinity stress limits crop productivity and is difficult to simulate on farms, hindering parental selection for hybridization programmes and the development of tolerant cultivars. To estimate an optimum salt concentration for screening, seeds of 15 genotypes were selected randomly and grown in vitro at 0 mM/L, 75 mM/L, 150 mM/L, 225 mM/L, and 300 mM/L concentrations of NaCl in 2 replications in a complete randomized design. Various morphological parameters, viz., length of seedling, root and shoot length, fresh weight, and dry weight, were observed to determine a single concentration using the Salt Injury Index. Then, this optimum concentration (225 mM/L) was used to assess the salt tolerance of all the 59 genotypes in 4 replications while observing the same morphological parameters. With the help of Mean Membership Function Value evaluation criteria, the genotypes were categorized into 5 grades: 4 highly salt-tolerant (HST), 6 salt-tolerant (ST), 19 moderately salt-tolerant (MST), 21 salt-sensitive (SS), and 9 highly salt-sensitive (HSS). Seedling fresh weight (SFW) at 225 mM/L was found to be an ideal trait, which demonstrates the extent to which B. juncea genotypes respond to saline conditions. This is the first report that establishes a highly efficient and reliable method for evaluating the salinity tolerance of Indian mustard at the seedling stage and will facilitate breeders in the development of salt-tolerant cultivars.

Figure 1

Germination of the genotypes #7 and #10 at various levels of salt stress concentration.

Figure 2

The germination of #15 genotype under different salt stress concentration on the seventh day.

Figure 3

Selection of optimum salt concentration for evaluating salt tolerance ability of 59 Brassica juncea genotypes when salt injury index (SII) was reduced to 50% of the control. Linear fit between SII of each parameter against different NaCl concentrations (A) SII of germination rate vs NaCl (mM/L) (B) SII of Root length (RL) vs NaCl (mM/L) (C) SII of Shoot length (SHL) vs NaCl (mM/L) (D) SII of Seedling length (SL) vs NaCl (mM/L) (E) SII of Seedling fresh weight (SFW) vs NaCl (mM/L) (F) SII of Seedling dry weight (SDW) vs NaCl (mM/L) (F) Average SII vs NaCl (mM/L).

Figure 4

Hierarchical clustering of 59 Brassica juncea genotypes based on their mean membership function values (Mean MFV) where HSS Highly salt stressed, SS Salt stressed, MST Moderately salt tolerant, ST Salt tolerant, HST Highly salt tolerant.

Figure 5

Classification of 59 Brassica juncea genotypes on the basis of mean membership function value (mean MFV). (A) Categorization of 59 Brassica juncea genotypes according to salt tolerance based on mean MFV. (B) Comparison between control (0 mM/L) and stress condition (225 mM/L) at various salt tolerance categories where HSS Highly salt stressed, SS Salt stressed, MST Moderately salt tolerant, ST Salt tolerant, HST Highly salt tolerant.

Figure 6

Linear correlation analysis between Salt tolerance index (STI) of each physiological parameter and Mean MFV. (A) Between STI of Root length and mean MFV; (B) between STI of Shoot length and mean MFV; (C) between STI of Seedling length and mean MFV; (D) between STI of Seedling fresh weight and mean MFV; (E) between STI of Seedling dry weight and mean MFV; (F) between STI of Germination rate and mean MFV. R² Linear is the coefficient of determination.