Abstract: The present study was aimed at finding out the genetic control and heritability of days to flowering at application and non application of nitrogen environments (Dp and Ds) and its related stress tolerance indices based on half diallel crosses of six spring oil seed rape varieties and their 15 F2 progenies. Significant mean squares of general and specific combining abilities (GCA and SCA) were observed for Dp, Ds, Mean Productivity (MP), Geometric Mean Productivity (GMP), tolerance index (TOL), stress tolerance index (STI) and Stress Susceptibility Index (SSI), indicating importance of additive and non-additive genetic effects for them. Significant ratio of GCA to SCA mean square and high narrow-sense heritability estimates were exhibited for Dp, Ds, MP, GMP and STI, indicating the prime importance of additive genetic effects. Significant correlation among Dp, Ds, MP, GMP and STI for GCA effects of parents and SCA effects of crosses, indicating the efficiency of these stress indices for improving days to flowering. On compare to SCA effects, most of crosses had significant high parent heterosis for Yp, Ys and all the stress indices, therefore, selection the better combinations based on heterosis will be more effective than SCA effect.
INTRODUCTION
Oil seed rape (Brassica napus L.) is one of the most important oil seed in Iran and other parts of the world because of its high oil quality and adaptability to different diverse climatic conditions. A number of investigations showed that nitrogen fertilizers gave substantial oil seed rape yield increases even in diverse and contradicting conditions (Yasari and Patwardhan, 2006; Zhang et al., 2009) and also its time and amount applications have important effect on yield associated traits such as phenological traits (Danesh-Shahraki et al., 2008; Fageria and Baligar, 2005; Rathke et al., 2005).
Flowering is the most critical stage influencing the yield of oilseed rape. The unset of flower initiation can have strong influence on flower, pod and seed number (Diepenbrock, 2000; Downey and Rimmer, 1993; Faraji et al., 2008). Habekotte (1997) used a sensitivity analysis within a crop growth model to study options for increasing seed yield in winter oilseed rape. The most promising crop type for high seed yield combined late maturity with early flowering (Downey and Rimmer, 1993). Heterosis is commercially exploited in rapeseed (Brassica napus L.) and its potential use has been demonstrated in turnip rape (B. rapa L.) and Indian mustard (B. juncea L.) for most agronomic traits (Teklewold and Becker, 2005; Zhang and Zhu, 2006). Early flowering in brassica can provide adequate time for grain formation process and can certainly cause early maturity and higher yields; therefore negative heterosis is desirable for flowering. Significant negative mid-parent and better-parent heterosis were reported for days to 50% flowering and physiological maturity (Nassimi et al., 2006). In this study mid parent heterosis for days to 50% flowering and physiological maturity ranged from -0.04 to -2.78 and -0.01 to -3.06, respectively and also its high parent heterosis for these traits ranged from -0.92 to -2.78 and -0.01 to -4.08, respectively. Significant correlation was reported between post-anthesis duration with some of important agronomic traits including number of pods per plant, 1000 seed weight and oil yield (Marjanovic-Jeromela et al., 2007).
Various studies on spring cultivars of oilseed rape have shown the important role of GCA and SCA effects for days to flowering and also it be a highly heritable character determined by genes that exhibit some degree of dominance (Huang et al., 2010). Likewise, studies with winter cultivars of this species (Amiri-Oghana et al., 2009; Sabaghnia et al., 2010) showed both additive and dominance gene effects to have a significant role in the inheritance of flowering time. Significant negative GCA and SCA effects were reported for days to flowering (Teklewold and Becker, 2005).
Several yield-based stress indices have been developed that may be more applicable to work on environmental stress tolerance such as drought tolerance (Golabadi et al., 2006; Nasri et al., 2006; Saba et al., 2001), salinity tolerance (Rameeh et al., 2004; Rezai and Saeidi, 2005) and temperature tolerance(Porch and Jahn, 2001). Rosielle and Hamblin (1981) defined stress tolerance (TOL) as the differences in yield between the stress (Ys) and non-stress (Yp) environments and Mean Productivity (MP) as the average yield of Ys and Yp. Fischer and Maurer (1978) proposed a Stress Susceptibility Index (SSI) of the cultivar. Fernandez, (1992) defined a new advanced index (STI = stress tolerance index) which can be used to identify genotypes that produce high yield under both stress and non-stress conditions and geometric mean (GMP). The geometric mean is often used by breeders interested in relative performance since drought stress can vary in severity in field environment over years (Ramirez-Vallejo and Kelly, 1998). The optimal selection criterion should distinguish genotypes express uniform superiority in both stress and non stress environments from the genotypes that are favorable only in one environment. Among the stress tolerance indicators, a larger value of TOL and SSI represent relatively more sensitivity to stress, thus a smaller value of TOL and SSI are favored. Selection based on these two criteria favors genotypes with low yield potential under non-stress conditions and high yield under stress conditions. Saba et al. (2001) reported that due to negligible narrow- sense heritability estimates of SSI and TOL for drought tolerance, these stress indices are not useful for selection of drought tolerant genotypes in plant breeding programs. They also reported that TOL and SSI were not strongly correlated to MP, GMP and STI. On the other hand, selection based on STI and GMP will be resulted in genotypes with higher stress tolerance and yield potential will be selected (Fernandez, 1992).
Although diallel analysis are frequently used in oil seed rape breeding to assess genetic parameters for yield associated traits and yield-based stress indices but in a few studies were focused on nitrogen deficiency stress effects and its stress indices. The objectives of this study were therefore, (1) to identify general and specific combining abilities and narrow-sense heritability for nitrogen deficiency stress indices among a set of adapted cultivars and (2) relation ship among nitrogen stress tolerance indices and days to flowering of oil seed rape cultivars and their F2 progenies at nitrogen application (Dp) and non application of nitrogen (Ds) environments.
MATERIALS AND METHODS
Six spring oil seed rape cultivars including RGS-003, Option500, RW008911, RAS-3/99, 19H and PF7045/91 were crossed in half diallel method during 2004-05. In order to produce F2 progenies, fifteen F1s were selfed at Biekol Agriculture Research Station, located in Neka, Iran (13°, 53' E longitude and 43°36' N latitude, 15 m above sea level) during winter 2005-06. F2 progenies and 6 parents were grown in a randomized complete block design with four replications at two experiments including N0: without nitrogen as stress condition and N+ : 150 kg nitrogen per hectare as non stress condition during 2006-07. The plots related to each experiment were consisted of four rows 5 m long and 40 cm apart. The distance between plants on each row was 5 cm resulting in approximately 400 plants per plot which were sufficient for F2 genetic analysis in each experiment. The soil was classified as a deep loam soil (Typic Xerofluents, USDA classification) contained an average of 280 g clay kg-1, 560 g silt kg-1, 160 g sand kg-1 and 22.4 g organic matter kg-1 with a pH of 7.3. Soil samples were found to have 45 kg ha-1 (mineral N in the upper 30 cm profile. Fertilized experiment (N+) received 150 kg ha-1 N as Urea (50 kg N at planting time, beginning of stem elongation and at initial of flowering stage) while unfertilized experiment (N0) received no N. All the plant protection measures were adopted to make the crop free from insects. Seed yield (adjusted to kg ha-1) was recorded based on three middle rows of each plot.
The stress tolerance indices were determined using the equations including stress intensity: SI=1-(μs/μp), tolerance index: TOL = Dp-Ds, stress susceptibility index: SSI = [1-(Ds/Dp)]/SI, stress tolerance index: STI = (Dp.Ds)/(μp)2, mean productivity: MP = (Ds+Dp)/2 and geometric mean productivity: GMP = (Ds.Dp)0.5, respectively. Ds and Dp are number of days to flowering of all genotypes per trial under stress and non-stress conditions, respectively and also μs and μp are the mean of this trait for all the genotypes per trial under stress and non-stress conditions, respectively.
Diallel analysis of variance (two way ANOVA) for the parents and their crosses was based on Griffing’s method 2, model 1 for fixed genotypes (Griffing, 1956). The analysis was performed using the diallel-SAS program written by Zhang and Kang (1997). The CORR procedure of SAS was used to estimate correlations among the traits. A t-test was used to test whether the GCA and SCA effects were different from 0. For each hybrid and each stress tolerance index, the difference between hybrid and the mean of high parents was computed. A Least Significant Difference (LSD) was used to test whether these differences were different from 0 (Mather and Jinks, 1982).
RESULTS
Combining ability analysis of variance: Diallel analysis based on Griffing's method two with B mixed model revealed significant mean squares of general and specific combining ability (GCA and SCA, respectively) for days to flowering at N+ (Dp) and N0 (Ds) and also its nitrogen stress tolerance indices including Mean Productivity (MP), Geometric Mean Productivity (GMP), tolerance index (TOL), Stress Tolerance Index (STI) and Stress Susceptibility Index (SSI) indicating the importance of additive and non additive genetic effects for them (Table 1). Significant ratio of GCA to SCA mean squares and high narrow sense heritability estimates were observed for Dp, Ds, MP, GMP and STI.
General combining ability of the parents: Significant positive GCA effects of Dp, Ds, MP, GMP and STI were observed for RW008911 and PF7045/91 and also these GCA effects were significant and negative for RGS003 (Table 2). Significant GCA effects of MP, GMP and STI were displayed for parents which had significant GCA effect for Dp and Ds. Non of parents had significant GCA effects of TOL and SSI, therefore selection of the parents based on these stress tolerance indices will be less efficient than MP, GMP and STI. Significant positive correlation of GCA effect was detected between Dp and Ds and also between these traits and MP, GMP and STI, indicating selection based on MP, GMP and STI make improving Dp and Ds (Table 3).
| Table 1: | Analysis of variance for Dp and Ds, MP, GMP, TOL, STI and SSI based on griffing’s method two with mixed-B model in six parents of oil seed rape and their 15 F2 progenies |
| *and **: Significant at 0.05 and 0.01 probability levels, respectively. GCA: General combining ability. SCA: Specific combining ability. Dp and Ds: Days to flowering at N+ and N0, respectively, MP: Mean productivity.GMP: Geometric mean productivity. TOL: Tolerance index, STI: Stress tolerance index, SSI: Stress susceptibility index | |
| Table 2: | Estimates of GCA effects for Dp and Ds, MP, GMP, TOL, STI and SSI in six parents of B. napus L |
| * and **: Significant at 0.05 and 0.01 probability levels. Dp and Ds: Days to flowering at N+ and N0, respectively, MP: Mean productivity.GMP: Geometric mean productivity. TOL: Tolerance index, STI: Stress tolerance index, SSI: Stress susceptibility index | |
| Table 3: | Pearson correlation coefficient among GCA effects of parents and SCA effects of crosses for days to flowering at N+ (Dp) and N0 (Ds) and the other stress tolerance indices |
| **: Significant at 0.01 probability levels. Dp and Ds: Days to flowering at N+ and N0, respectively, MP: Mean productivity.GMP: Geometric mean productivity, TOL: Tolerance index, STI: Stress tolerance index, SSI: Stress susceptibility index | |
Specific combining ability of the crosses: Significant negative SCA effect of Dp was observed for the crosses including RW008911 x Option 500, RGS 003 x Option 500 and Option 500 x PF7045/91 and these combinations were suitable for reducing days to flowering at N+ (Table 4). The crosses RAS-3/99 x RW008911 and Option 500 x PF7045/91 with significant negative SCA effect of Ds were detected as good combinations for decreasing days to flowering at N0. Significant negative SCA effect of MP, GMP and STI were detected for Option 500 x PF7045/91. Significant negative SCA effects of TOL and SSI were exhibited by RAS-3/99 x RGS003 and RW008911 x Option 500 and also these stress indices were significant and positive for RAS-3/99 x PF7045/91.
| Table 4: | Estimates of SCA effects for Dp and Ds, MP, GMP, TOL, STI and SSI in the half diallel crosses of six parents of B. napus L. |
| * and **: Significant at 0.05 and 0.01 probability levels, respectively. Dp and Ds: Days to flowering at N+and N0, respectively, MP: Mean productivity.GMP: Geometric mean productivity. TOL: Tolerance index, STI: Stress tolerance index and SSI: Stress susceptibility index | |
| Table 5: | High parent heterosis estimates for Dp and Ds, MP, GMP, TOL, STI and SSI in 15 F2 progenies of B. napus L. |
| * and **: Significant at 0.05 and 0.01 probability levels, respectively. Dp and Ds: Days to flowering at N+ and N0, respectively, MP: Mean productivity.GMP: Geometric mean productivity. TOL: Tolerance index, STI: Stress tolerance index, SSI: Stress susceptibility index | |
The same sign (positive or negative) of TOL and SSI SCA effects were exhibited by all the combinations. The result of Pearson correlation coefficient among SCA effects of crosses for days to flowering at N+ (Dp) and N0 (Ds) and its related stress tolerance indices is presented in Table 3. Significant positive correlations were observed among Dp, Ds, MP, GMP and STI but Dp and Ds had not significant correlation with the TOL and SSI.
Heterosis: Data presented in Table 5 for heterosis of the crosses showed that out of 15 crosses, 8 crosses exhibited significant negative heterosis for Dp, Ds, MP, GMP and STI. Significant negative high parent heterosis for days to flowering was displayed for most of crosses at N+ and N0, indicating that most of crosses had less days to flowering on compare to their parents. The crosses including RGS003 x Option500, RGS003 x PF7045/91 and Option500 x PF7045/91 and RW008911 x RGS003 with high significant negative heterosis for Dp, Ds, MP and GMP had less days to flowering in compare to the other combinations. Significant negative heterosis of TOL and SSI were detected for RW008911 x RGS003 and RW008911 x Option500 and also the heterosis of these stress indices were significant and positive for RAS-3/99 x PF7045/91.
DISCUSSION
Significant ratio of GCA to SCA mean squares and high narrow sense heritability estimates for Dp and Ds indicated the importance of additive genetic effects for this trait at N+ and N0. Similarly, in various studies on spring cultivars of oilseed rape have shown the important role of GCA and SCA effects for days to flowering and also it be a highly heritable character determined by genes that exhibit some degree of dominance in spring cultivars of oil seed rape (Nassimi et al., 2006) and also winter cultivars of this species (Amiri-Oghana et al., 2009; Sabaghnia et al., 2010). Among stress tolerance indices, MP, GMP and STI had high narrow sense heritability estimates, indicating the prime importance of additive genetic effects for these stress tolerance indices but for TOL and SSI, non significant GCA to SCA mean squares and negligible narrow- sense heritability estimates indicating the importance of non-additive genetic effects. Therefore for breeding program based on selection method, for improving days to flowering uses of MP, GMP and STI more be efficient than TOL and SSI. Selection based on STI and GMP will be resulted in genotypes with higher stress tolerance and yield potential will be selected (Fernandez, 1992). These results are similar to the earlier finding of Saba et al. (2001) who reported that due to negligible narrow- sense heritability estimates of SSI and TOL for drought tolerance, these stress indices are not useful for selection of drought tolerant genotypes in plant breeding programs.
Significant positive and negative GCA effects of parents for days to flowering were displayed at N+ and N0. Huang et al. (2010) and Sabaghnia et al. (2010) reported significant positive and negative GCA effects for days to flowering. RGS003 with significant negative GCA effect of Dp and Ds makes decreasing in days to flowering at N+ and No in combination to the other parents. The most promising crop type of oil seed rape for high seed yield combined late maturity with early flowering (Downey and Rimmer, 1993), therefore RGS003 with significant negative of GCA effects is favored. Days to flowering has important effect on yield and yield associated traits (Diepenbrock, 2000; Faraji et al., 2008; Habekotte, 1997), so selection of the parents with suitable GCA effects has important role for seed yield improving. Non- significant correlation of GCA effect was observed between SSI and TOL and also between each of these stress tolerance indices and Dp, Ds, MP, GMP and STI showed that the heritable and non heritable indices were not affected by the same locus of chromosome and had not any correlation. Saba et al. (2001) reported that TOL and SSI were not strongly correlated to MP, GMP and STI.
The crosses including RAS-3/99 x RGS 003 and RW008911 x Option 500 with significant negative SCA effect more be affected by stress environment and also due to significant positive SCA effect of TOL and SSI for RAS-3/99 x PF7045/91, stress environment made increase days to flowering for this combination. Teklewold and Becker (2005) reported similar results of SCA effects for days to flowering. Significant negative SCA effect of TOL and SSI for RW008911 x Option 500 indicating of more nitrogen stress effects of days to flowering for this combination. Significant positive correlations among Dp, Ds, MP, GMP and STI for GCA and SCA effects, indicating selection based on MP, GMP and STI make improving Dp and Ds and also TOL and SSI had not significant correlation with Dp and Ds. Non significant of TOL and SSI GCA effects showed that these indices were affected by non additive genetic effects in the parents. Significant SCA effects of MP, GMP and STI were observed for crosses which had one parents with significant GCA effect for Dp and Ds.
Early flowering in brassica can provide adequate time for grain formation process and can certainly cause early maturity and higher yields; therefore, negative heterosis is desirable for flowering. In compare to SCA effects, most of the crosses had significant negative heterosis for Dp, Ds and the other related stress tolerance indices except TOL, therefore recognizing of the suitable combinations for improving days to flowering based on high parent heterosis is more easier than SCA effects. Significant negative mid-parent and better-parent heterosis were reported for days to 50% flowering and physiological maturity (Nassimi et al., 2006). Due to significant positive correlation of days to flowering with days to maturity (0.53*), most of crosses were more early maturity than their parents. The crosses including RGS003 x Option 500, RGS003 x PF7045/91 and Option500 x PF7045/91 and RW008911 x RGS003 with high significant negative heterosis for Dp, Ds, MP and GMP had less days to flowering and most of these crosses had at least one parent with significant negative GCA effect for Dp, Ds and other stress tolerance indices. Similarly, significant correlation was reported between post-anthesis duration with some of important agronomic traits including number of pods per plant, 1000 seed weight and oil yield (Marjanovic-Jeromela et al., 2007). Among the stress tolerance indices, TOL and SSI had less significant high parent heterosis for the crosses. With compare to SCA effects most of crosses had significant negative high parent heterosis for Dp and Ds, MP, GMP and STI, therefore, recognizing of the suitable combinations for improving days to flowering based on high parent heterosis is more easier than SCA effects.
In conclusion, based on this study, it seemed that SSI and TOL were not useful indices to select for nitrogen stress tolerant genotypes in breeding programs. Due to negligible heritability estimates for SSI and TOL, selection based on these stress indices would not have sufficient gain for improving days to flowering. Significant correlation among Dp, Ds, MP, GMP and STI for GCA effects of parents and SCA effects of crosses, indicating efficiency of these stress indices for improving days to flowering. Due to significant high parent heterosis of Dp, Ds and the related stress tolerance indices for most of the crosses, detecting the superior crosses based on heterosis is more easier than SCA effects of the crosses.
ACKNOWLEDGMENTS
The author wish to thanks from Agricultural and Natural Resources Research Center of Mazandaran and Seed and Plant Institute Improvement (SPII) for providing genetic materials and facility for conducting the experiment.