Abstract: To estimate heterosis over mid-parent and better-parent of F1 hybrids in Brassica napus L. an experiment was conducted during 2004-05 and 2005-06 using 8x8 full diallel crosses. All the 56 F1 hybrids and their parents were planted in a randomized complete block design with three replications. Out of 56 hybrids, positive mid-parent and better-parent heterosis were found in 44 and 32 hybrids for number of pods/raceme, in 27 and 19 hybrids for number of pods/plant, in 27 and 15 hybrids for pod length, in 26 and 20 hybrids for number of seeds/pod, in 50 and 39 crosses for 1000-seed weightand in 21 and 07 crosses for yield/plant. However, significant positive mid-parent and better-parent heterotic effects were recorded in 44 and 32 hybrids for pods/raceme, in 26 and 19 hybrids for pods/plant, in 11 and 09 hybrids for pod length, in 21 and 20 hybrids for seeds/pod and in 07 and 04 crosses for 1000-seed weight. Better-parent heterosis reached to 66.67% for pods/raceme, 60.85% for pods/plant, 16.03% for pod length, 27.27% for seeds/pod, 54.62% for 1000-seed weight and 1.02% for grain yield. Among parents, NUR1, NUR4, NUR5, NUR7, NUR8 and NUR9 proved to be superior when used as parents in most of the hybrid combinations. Crosses NUR1xNUR7, NUR1x NUR9, NUR5x NUR4, NUR4xNUR9, NUR3xNUR9 and NUR8xNUR9 were best for yield associated traits and their further utilization in breeding programmes would be useful for developing high yielding genotypes.
INTRODUCTION
The shortage of edible oils in the country offers great challenges to the plant breeders. The domestically produced edible oil hardly meets 30% of the national demand whereas remaining 70% is met through the import by spending huge foreign exchange (Anonymous, 2005). Furthermore the gap between local production and consumption is continuously widening at the rate of 13% annually (Razi, 2004). This alarming situation is pushing plant scientists to explore new ways and means not only to enhance per unit yield upto the maximum potential of the existing edible oil crops but also to introduce new high yielding crops like canola.
The success of hybrid breeding is the reason for its expansion in all most all major fields of agricultural plants and animals. Heterosis is defined as the superiority of a hybrid over its parents (Pourdad and Sachan, 2003) and is considered as a quick, cheap and easy method in increasing crop production (Pal and Sikka, 1956). Different researchers reported substantial heterosis in major oilseed Brassica (Teklewold and Becker, 1991; Leon and Becker, 1995; McVetty, 1995) that stimulated a worldwide interest for developing hybrid cultivars. In Canada, China and Europe, hybrids are becoming the major cultivar types in B. napus cultivation (Dianrong, 1999; Frauen et al., 2003). Identifying parental combination with strong yield heterosis is the most important step in developing hybrids (Diers et al., 1996; Becker et al., 1999; Melchinger, 1999). The level of genetic diversity between parents has been proposed as a predictor of F1 performance and heterosis (Moll et al., 1965; Falconer and Mackay, 1996). As one of the most important sources of edible oil, Brassica napus is grown worldwide. At present, hybrid cultivars have higher productivity than conventional ones and their seed quality (contents of erucic acid and glucosinolates) has also been greatly improved (Fu, 2000). Therefore, the development of commercial F1 hybrids of Brassica species has attracted considerable interest in breeders. F1 hybrids produced from crosses between different varieties had high heterosis for yield traits in rapeseed (Brandle and McVetty, 1990; Banks and Beversdorf, 1994; Ali et al., 1995; Esch and Wricke, 1995; Starmer et al., 1998).
Keeping in view the importance of edible oil and its alarming situation in the country the present study was aimed to evaluate eight Brassica napus L. genotypes for heterotic effects and identify their potential hybrids for developing new genotypes.
MATERIALS AND METHODS
The experiment was conducted at NWFP Agricultural University, Peshawar during 2004-2006. Eight Brassica napus L. genotypes viz., NUR1, NUR2, NUR3, NUR4, NUR5, NUR7, NUR8 and NUR9 (NUR for National Uniform Rapeseed Yield Trail) were crossed manually (hand emasculation and pollination) in all possible combinations (direct and reciprocals) in a 8x8 diallel fashion during the Rabi season 2004-05.
In 2005-06, the parents and their 56 F1 hybrids were grown in a randomized complete block design with three replications in field conditions. All the F1 hybrids and their parent lines were randomly assigned to experimental units/plots. Each plot comprised two rows of 4 m length with space of 1 m between rows whereas seeds were planted 30 cm apart.
The data were collected on yield associated traits like number of pods main/raceme, number of pods/plant, pod length, number of seeds/pod, 1000 seed weight and seed yield/plant. The analyses of variance was computed according to Steel and Torrie (1980) and the percent increase (+) or decrease (-) of F1 cross over mid-parent as well as better parent was calculated to observe heterotic effects for all the parameters. The estimate of heterosis over the mid-parent and better parent was calculated using the procedure of Matzinger et al. (1962). The difference of F1 mean from the respective mid parent and better parent value was evaluated by using a t-test according to Wyne et al. (1970).
RESULTS
Mean square values presented in Table 1 showed that highly significant differences (p≤0.01) were observed among mean values of all the yield associated traits studied in this experiment viz., number of pods main/raceme, number of pods/plant, pod length, number of seeds/pod, 1000 seed weight, seed yield/plant in Brassica napus L. genotypes.
Number of pods main/raceme: Since number of pods main raceme-1 is considered major yield associated trait and more number of pods can contribute in higher grain yield therefore, positive heterosis is desirable for this trait. Effects of heterosis over mid parent (Table 2) showed that out of 56 crosses, 45 crosses showed positive heterosis for number of pods/main raceme-1 where data ranged from 0.06 to 78.57%. Among these crosses, 44 crosses showed significant positive effects, where the maximum effects were recorded for cross NUR8xNUR9. Positive heterosis over better parent was recorded for 33 crosses where effects ranged from 0.01 to 66.67%. Significant positive heterosis over better parent was recorded in 32 crosses with maximum effects being observed in NUR8xNUR9.
Number of pods/plant: Number of pods/plant is the most important factor of grain yield in brassica crop therefore more pods/plant would certainly results in greater yield per unit area, hence, positive heterosis is desirable for number of pods/plant. Heterosis effects over mid parent concerning to number of pods plant-1 indicated that out of 56 crosses, 27 crosses exhibited positive heterosis and the values ranged from 0.10 to 78.30%. Among these crosses, 26 crosses, presented significant positive heterosis over mid parent, where the maximum positive value was recorded in cross NUR1xNUR7. Heterosis effects over better parent for number of pods/plant, demonstrated that out of 56 crosses, positive effects were noted in 19 crosses where values ranged from 0.09 to 60.85%. Significant positive heterosis over better parent was exhibited by 19 crosses with maximum positive values being observed in cross NUR1xNUR7 (Table 2).
Pod length: Longer pod would contain more and bulky seeds which would directly contribute in higher yields; therefore, positive heterosis is useful for pod length. Heterosis effects showed that 27 crosses presented positive effects over mid parent for pod length and the values ranged from 0.02 to 33.96%. Of these crosses, 11 crosses showed significant positive effects where the maximum estimate was observed in cross NUR7xNUR9. Positive heterosis over better parent was noted in 15 crosses and values ranged from 0.03 to 16.03%. Significant positive effects were identified for 9 crosses where the maximum positive value was noted for NUR5xNUR4 (Table 2).
| Table 1: | Mean squares for number of pods main/raceme, number of pods/plant,
pod length, number of seeds/pod, 1000 seed weight, seed yield/plant in Brassica
napus L. genotypes |
| ** = Significant at p≤0.01% probability level | |
| Table 2: | Heterotic effects for number of pods main/raceme, number
of pods/plant, pod length, number of seeds/pod, 1000 seed weight, seed yield/plant
in Brassica napus L. genotypes |
| *MP = Mid Parent, BP = Better Parent | |
Number of seed/pod: Since number of seed/pod directly contributes towards grain yield therefore, positive heterosis is desirable. Data showed that out of 56 crosses, 26 crosses displayed positive heterosis over mid parent and the effects ranged from 0.02 to 31.82%. Of these crosses, significant positive heterosis over mid parent was recorded for 21 crosses where the maximum value was observed in cross NUR5xNUR4. Of total 56 crosses, positive heterosis over better parent was noted in 20 crosses, where data ranged from 0.04 to 27.27%. Significant positive effects were recorded for 20 crosses where the maximum positive value was recorded for cross NUR3xNUR9 (Table 2).
1000 seed weight: Greater seed weights certainly contribute towards higher production therefore; positive heterosis is desirable for 1000 seed weight. Heterosis over mid parent for 1000 seed weight (Table 2) showed that out of 56 crosses, 50 crosses exhibited positive heterosis. The data for crosses showing positive heterosis over mid parent ranged from 0.01 to 66.18%. Of these crosses, significant heterosis was noted in only 07 crosses where the maximum positive value (66.18%) was recorded in cross NUR7xNUR9. Data regarding heterosis over better parent for 1000 seed weight showed that out of 56 crosses, positive values were recorded in 39 crosses where data ranged from 0.16 to 54.62%. Significant positive effects were noticed for 4 crosses with the maximum value being observed in cross NUR1xNUR9.
Seed yield/plant: Like other crops seed yield/plant is considered to be the main contributor towards grain yields per unit area in brassica; therefore, positive heterosis is considered useful for selecting hybrids for seed yield/plant. Effects of heterosis over mid parent for grain yield/plant showed that out of 56 crosses, only 07 crosses presented positive but negative heterosis and the values ranged from 0.00 to 1.53%. However the maximum heterosis over mid parent was recorded for crosses NUR3xNUR9, NUR4xNUR9 and NUR8xNUR9. Regarding heterosis over better parent, positive but non significant effects were observed for 07 crosses where data ranged from 0.00 to 1.02 and the maximum effect was recorded in cross NUR8xNUR9.
DISCUSSION
Number of pods/plant, pod length, number of seeds/pod, 1000 seed eight are major yield associated component and are directly linked with yield in brassica. Therefore significant positive heterosis over both mid parent and better-parent is desirable for these traits for the enhancement of yield in a line. Significant positive values for heterosis were recorded in different F1 hybrids for the above mentioned traits. The F1 hybrids that surpassed their respective parents could be an asset for breeders to develop high yielding genotypes. Present results are similar to the earlier findings of Satwinder et al. (2000), who reported that F1 generations expressed significant heterosis for number of primary branches, number and length of pods, seeds/pod, yield/plant and oil content. Similarly Krzymanski et al. (1997) found significant heterosis for seed yield, oil content and some flowering traits. Their report showed that the mean heterosis for seed yield over the mid-parental mean was 24.71%. Present results are also confirmed by the findings of Srivastava and Rai (1997) who reported highest value of better-parent heterosis for seed yield. Similarly the superiority of F1 hybrids for seed yield/plant was also reported by Davik (1997). Hu et al. (1996) reported significant positive effects of heterosis for plant height and seed yield/plant. Jorgensen et al. (1995) also found high positive heterosis for primary and secondary branches, pods on main shoot, seeds/pod, seed yield and oil content. Present results are also confirmed by Fray et al. (1997), who reported significant heterotic estimates for seed yield, primary branches and siliquae/plant. Shen et al. (2005) reported that pods/plant had the highest heterosis, while 1000 seed weight had the lowest heterosis. Similarly other reports indicated that F1 hybrids produced from crosses between different varieties had high heterosis for yield traits and no significant heterosis for seed oil content in rapeseed (Starmer et al., 1998). Among the yield traits, siliques/plant had the greatest heterosis, followed by seeds/silique and 1000 seed weight (Fu, 2000). Jorgensen and Andersen (1997) reported strong positive heterosis for dry matter yield with high yielding F1s. However, Cheung et al. (1997) reported that better parent heterosis was rather low and the majority of hybrids were generally inferior compared to their better parent. The reason for the difference could be difference genotype and the environmental conditions.
ACKNOWLEDGMENTS
The first author was a recipient of M.Sc. (Honors) fellowship from USAID AUP-Peshawar. This project was sponsored by HEC-AUP project for 2004-2006.