This experiment was performed during three crop years at the Agriculture and Natural Resources Research Centre in Iran in 2017–2020. In the first year, the experiment included the following 39 rapseed varieties: (1) Talaieh, (2) Sarigol, (3) Zarfam, (4) Zafar, (5) Delgan, (6) Ahmadi, (7) Hyola401, (8) Hyola 60, (9) pp-401015E, (10) T98007, (11) Talaye, (12) SLM046, (13) Geronimo, (14) Modena, (15) Opera, (16) Symbol, (17) KS-11, (18) Colvert, (19) Ks-7, (20) Okapi, (21) Licord, (22) Orient, (23) Option500, (24) H-19, (25) Shiralee, (26) San-14, (27) San-12, (28) SPN178, (29) SPN179, (30) SPN180), (31) SPN182, (32) SPN183, (33) SPN190, (34) SPN184, (35) SPN192, (36) SPN193, (37) RGS003, (38) Dalgon, and (39) SAN56 was carried out in two randomized complete block designs (designs 1 and 2) with three replicates under two irrigation conditions, design 1; 0.631 dSm− 1 and design 2; 8.7 dSm− 1.
In the second year, five salinity-tolerant cultivars as lines (SPN178, 179, 180, 192, 193) and three salinity-sensitive cultivars as testers (RGS003, DALGON, SAN56), selected from the first-year experiment, were crossed based on the line × tester crossing method.
In the third year, in order to evaluate hybrids and parents in terms of different traits, 15 F1s along with their parents (A total of 23 genotypes) were sown in two randomized complete block designs (design 1 and design 2) with three replications under two irrigation conditions, normal (NG) and salinity stress (SSG) growth conditions. In the NG experiment, plants were irrigated with normal, natural irrigation water (EC = 0.631 dSm–1), and in the SSG experiment, plants were irrigated with saline water (EC = 8.7 dSm–1). The source of irrigation water for the two designs, in the first and third year, was two wells of water with 0.63 dSm− 1 and 8.7 dSm− 1 salinity, respectively. In the three years of the experiment, seeds were sown in four 2m rows in each plot with row-to-row intervals of 0.25 m and a density of 80 plants per m2 for a total area of 2 m2. The fertilizers were applied at the rates of 100: 50: 90 kg/ha of N: P: K, respectively. At the maturity stage, 20 plants were randomly selected from two middle rows of four rows in each plot to record data of F1 plants for the twelve characters.
Trait measurements
Yield and yield components:
In the first year, only the seed yield of the genotypes was measured, and in the third year, in addition to yield, yield components and some physiological traits were also measured. Yield components included plant height (PH), pod length (PL), number of seeds per pod, pods per plant (NSP and NPP), 1000-kernel weight (TKW), and grain yield per plant (GY).
Physiological character
Number of days to 50% flowering and maturity (NDF and NDM) and leaf relative water content (RWC) were quantified. Leaf relative water content measured in fully expanded leaves as described by (Ravari et al., 2016) and calculated according to followed Eqn:\(RWC \left(\%\right)= \left(\frac{FW-DW}{TW-DW}\right)\times 100\)
Where FW, DW and TW are shoot fresh, dry and turgid weight, respectively.
Statistical analyses
All measurements were done on 20 plant samples in each experimental plot. The mean value of the 20 samples was used for data analyses.
Cluster analysis was performed by unweighted pair group method with arithmetic mean (UPGMA). The first point on the graph that had the highest value of Λ statistic based on the following formula (Λk−1 - Λk) was selected as the cut-off point and the number of clusters equal to K was selected.
Data were subjected to analysis of variance to evaluate the significance of differences between F1 hybrids and their parents. Line × tester analysis (Kempthorne, 1957) was used to estimate general combining ability (GCA) and specific combining ability effects (SCA). The values of broad \({(h}_{B}^{2})\) and narrow sense \({(h}_{N}^{2})\) heritability, gene action, average degree of dominance and stress tolerance index (STI) were calculated from the following equations, respectively:
$${h}_{B}^{2}=\left(\frac{(\text{V}\text{A}+\text{V}\text{D}}{\text{V}\text{A}+\text{V}\text{D}+\text{M}{\prime }\text{e}}\right)\times 100$$
$${h}_{N}^{2}=\left(\frac{\text{V}\text{A}}{\text{V}\text{A}+\text{V}\text{D}+{\text{M}}^{{\prime }}\text{e}}\right) \times 100$$
$$\text{G}\text{e}\text{n}\text{e} \text{a}\text{c}\text{t}\text{i}\text{o}\text{n}=\frac{{MS}_{gca}}{{MS}_{sca}}$$
$$\text{D}\text{e}\text{g}\text{r}\text{e}\text{e} \text{o}\text{f} \text{d}\text{o}\text{m}\text{i}\text{n}\text{a}\text{n}\text{c}\text{e}=\sqrt{\frac{2\text{V}\text{D}}{\text{V}\text{A}}}$$
Where MSgca and MSsca are general and specific combining ability mean square, respectively. Additive and dominance variance VA and VD denote, respectively. Me is mean of error squares, r is replication number and M’e reflects Me divided by r, \({Y}_{s}\) and \({Y}_{p}\) are the grain yield under stress and non-stress conditions, \({\left({\stackrel{-}{Y}}_{p}\right)}^{2}\) is the mean yield over all genotypes under non-stress conditions.
The eight stress tolerance indices were measured using the following expressions:
\(MP={1ex}{\left({Y}_{s}+{Y}_{p}\right)}\!\left/ \!{-1ex}{2}\right.\) (Bouslama and Schapaugh Jr, 1984), \(TOL=\left({Y}_{p}-{y}_{s}\right)\) (Hossain et al., 1990), \(GMP=\sqrt{{Y}_{s}\times {Y}_{p}}\) (Shen et al., 2006), \(SSI=\frac{1-\frac{{Y}_{s}}{{Y}_{p}}}{\frac{{\stackrel{-}{Y}}_{s}}{{\stackrel{-}{Y}}_{p}}}\) (Fischer and Maurer, 1978),
\(YI=\frac{{Y}_{s}}{{Y}_{p}}\) (Gavuzzi et al., 1997), \(YSI=\frac{{Y}_{s}}{{\stackrel{-}{Y}}_{p}}\) (Bouslama and Schapaugh Jr, 1984), \(STI=\frac{{Y}_{s}\times {Y}_{p}}{{\stackrel{-}{Y}}_{P}^{2}}\) (Gavuzzi et al., 1997), \(HM=\frac{2\left({Y}_{s}\times {Y}_{p}\right)}{\left({Y}_{s}+{Y}_{p}\right)}\) (Rosielle and Hamblin, 1981).
Heterobeltiosis was calculated using the difference in the value of each trait from the mean of the superior parent from generation F1.\(Heterobeltiosis=\left(\frac{\text{F}1-\text{H}\text{P}}{\text{H}\text{P}}\right)\times 100\)
Where F1 = First generation, HP = high parent