Agnieszka Dobrzycka1, Joanna Wolko1, Jan Bocianowski2, Kamila Nowosad3 1
Plant Breeding and Acclimatization Institute – National Research Institute, Branch Office in Poznan 2 Poznań University of Life Sciences
3 Wrocław University of Environmental and Life Sciences
Author for correspondence — A. Dobrzycka, e-mail: a.m.dobrzycka@gmail.com
Phenotypic variation of yield related traits
in DH lines and hybrids of winter oilseed rape
(Brassica napus L.)
Zmienność fenotypowa cech struktury plonu w liniach DH
oraz mieszańcach rzepaku ozimego (Brassica napus L.)
Key words: oilseed rape, seed yield, yield-related traits, heterosisAbstract
Oilseed rape (Brassica napus L.) breeding programs are mainly focused on obtaining varieties with high seed yield, which is associated with greater profitability of growing such varieties and increased consumption of vegetable oils. Improved productivity is obtained, among others, by F1 hybrids, in which the effect of heterosis occurs.
The experiment was designed to investigate variation of yield related traits among 60 DH lines, 60 single cross hybrids (CMS×DH), 60 three-way cross hybrids (CMS/DH×Rfo), one CMS ogura line, and one Rfo line. Field experiment with a randomized blocks design included two growing seasons; this article presents data from the first year of observations. Traits evaluated in the field were: duration of flowering, plant height, number of branches per plant, number of siliques per plant, silique length, number of seeds per silique, thousand seed weight, and oil content in seeds. On the collected data the analysis of variance was performed and correlations between the studied traits were examined, considering the groups of genotypes.
Analysis of variance showed statistically significant differences between groups for all eight traits. Flowering duration was the longest in the group of CMS×DH hybrids and the shortest in the Rfo plants. The CMS/DH×Rfo hybrids were characterized by the largest plant height. The lowest height had Rfo restoring line, which also had the largest number of branches and siliques per plant. This line achieves, however, the lowest values for traits such as length of siliques, number of seeds per silique and thousand seeds weight. The highest thousand seed weight was observed in the group of CMS×DH hybrids and in the CMS ogura line. The lowest oil content in seeds was noted in the group of DH lines, and the highest – in the CMS/DH×Rfo hybrids and CMS ogura line.
Słowa kluczowe: rzepak, plon nasion, cechy struktury plonu, heterozja Streszczenie
Hodowla rzepaku (Brassica napus L.) jest skoncentrowana głównie na otrzymywaniu odmian o wysokim plonie nasion, co jest związane z większą opłacalnością uprawy takich odmian oraz
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wzrastającym zapotrzebowaniem na oleje roślinne. Zwiększona produktywność jest uzyskiwana, między innymi, poprzez mieszańce F1 o wysokim efekcie heterozji.
Badania zaplanowano w celu oceny zmienności cech plonotwórczych wśród 60 linii DH, 60 mieszańców pojedynczych (CMS×DH), 60 mieszańców trójliniowych (CMS/DH×Rfo), jednej linii CMS ogura oraz jednej linii Rfo. Doświadczenie polowe w układzie bloków losowych obejmowało dwa sezony wegetacyjne, niniejszy artykuł przedstawia dane z pierwszego roku obserwacji. Cechy oceniane w warunkach polowych to: długość kwitnienia, wysokość roślin, liczba rozgałęzień na roś-linie, liczba łuszczyn na rośroś-linie, długość łuszczyn, liczba nasion w łuszczynie, masa tysiąca nasion oraz zawartość oleju w nasionach. Na uzyskanych pomiarach przeprowadzono analizę wariancji oraz analizę korelacji dla obserwowanych cech, z uwzględnieniem podziału na grupy obiektów.
Wyniki przeprowadzonej analizy wariancji wskazują na istotne statystycznie zróżnicowanie pomiędzy grupami dla wszystkich ośmiu badanych cech. Najdłuższe kwitnienie zaobserwowano w grupie mieszańców CMS×DH, a najkrótsze u roślin linii Rfo. Mieszańce CMS/DH×Rfo charakteryzowały się największą wysokością, a najniższe rośliny występowały w linii Rfo, która jednocześnie miała największą liczbę rozgałęzień oraz łuszczyn na roślinie. Ta linia charakteryzowała się także najkrótszymi łuszczynami, najmniejszą liczbą nasion w łuszczynie oraz najniższą masą tysiąca nasion. Mieszańce CMS×DH oraz linię CMS ogura cechowała największa masa tysiąca nasion. Najniższą zawartość oleju w nasionach odnotowano w grupie linii DH, a najwyższą dla mieszańców CMS/DH×Rfo oraz dla linii CMS ogura.
Introduction
Over the last 30 years, due to breeding for oil and meal quality, oilseed rape has become a very important crop of increasing significance on the international market. Currently, the production of rapeseed is in the second place among oilseeds in the world after soybeans and in season 2014/2015 reached over 63 million tons. In the European Union, the world's largest producer of rapeseed, the yield of seeds in 2015 reached 21.5 million tons, of which 2.7 million tons in Poland (GUS 2015). Rapeseed oil is used in the production of margarine and cooking oil due to its superior nutritional qualities, which include low saturated fat content, high level of monounsaturated fatty acids, and omega-3 and omega-6 fatty acids. Rapeseed is also a source of oil extraction meal (Friedt and Snowdon 2009), which is used as animal feed due to its high content of protein. Oilseed rape seeds are processed not only for oil and feed, but also used for the production of biofuels. Therefore, demand for rapeseed is still growing, and breeding programs are focused mainly on the production of high seed-yield varieties.
Yield is one of the most important and complex traits in crops. Seed yield in oilseed rape is influenced mostly by silique or silique-related traits. Number of siliques per plant, number of seeds per silique and thousand seed weight are three main components which determine seed yield per plant (Chen et al. 2007). Some authors add to this compilation the length of siliques (Li et al. 2014). Other characteristics connected with yield in rapeseed are flowering time, plant height, and number of branches per plant. Yield-related traits are usually controlled by quantitative trait loci (QTLs) and are easily modified by the environment, which
makes effective selection for them more difficult. In B. napus QTL analyses were used to discover the genetic bases of these traits in segregating populations (Cai et al. 2014). A lot of QTLs and epistatic interactions were detected in the genome of oilseed rape, and many of them had pleiotropic effects (Udall et al. 2006; Basunanda et al. 2010; Shi et al. 2011). For many diverse studies in rapeseed, doubled haploid lines are used (Cegielska et al. 2015). Their advantage is that they provide a homogeneous DNA, what makes phenotyping studies easier (Chen et al. 2007; Radoev et al. 2008). Another advantage of DH production is only one cycle of recombination, so that gene combinations found in the parent lines are easier to track in their offspring (Pink et al. 2008).
The purpose of this paper was to present the evaluation of eight yield components, which were studied in 182 genotypes of oilseed rape. These traits were estimated considering the groups of genotypes, in order to confirm variability between groups, what is necessary for further studies planned on this material. The article presents a part of major project, which concerns inheritance of yield and yield-related traits. Plant material designed for this study consists of DH population and two generations of hybrids, whose parents were selected based on large genetic distance. It allows to investigate the genetic mechanism of heritability of observed traits and the occurrence of the heterosis effect. In the article results of the first year of observations are presented. After collecting two-year data, general combining ability (GCA), specific combining ability (SCA), and heterosis effect will be evaluated.
Materials and methods
Plant material
Plant material used in the project includes 182 genotypes: 60 DH lines, 60 single cross hybrids (CMS×DH), 60 three-way cross hybrids (CMS/DH×Rfo), one CMS ogura line, and one Rfo line. CMS and Rfo lines were selected due to the large genetic distance with respect to the DH lines. DH lines were derived from F1
hybrid between two lines: one with high oleic acid content (77.9%), and the second with high oil content (51.9%) and high seed yield. Single cross hybrids were created by crossing 60 DH lines with the CMS ogura line and three-way cross hybrids – by crossing obtained male-sterile single hybrids with restorer (Rfo) line. The plants were pollinated by hand under greenhouse conditions by transferring pollen from the donor plant (DH or Rfo line) to pollinated plant (CMS line or a male-sterile single hybrid, respectively). From each cross an average of 15 grams of seeds were obtained, which was sufficient to set up a two-year field trials.
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Field experiments
Field experiments were conducted in Borowo (52°70’N, 16°46’E), Plant Breeding Strzelce Ltd., Co. – IHAR-PIB Group, and included two growing seasons: 2014/15 and 2015/16. They had a randomized blocks design with three replications and two randomly distributed standards (Monolit and Arsenal). The whole experiment consisted of 594 plots; each of them contained four rows two meters long. Distance between rows was 30 cm. The field management followed standard agricultural practice. This article presents results collected during the first growing season.
Traits evaluated in the field trials were: duration of flowering (DoF), plant height (PH), number of branches per plant (BpP), number of siliques per plant (SpP), silique length (SL), number of seeds per silique (SpS), thousand seed weight (TSW), and oil content in seeds (OC). Plant height was measured on three randomly selected plants from each plot after the end of flowering time. Number of branches per plant and siliques per plant were estimated on 3 well-developed, randomly selected plants from each plot at the green siliques stage. Silique length and number of seeds per silique were estimated on 20 siliques from each plot. Siliques were collected at the stage of mature seeds from the main branch and then dried. Thousand seed weight was estimated from the average of three measurements from the mixed seeds of plants in a plot. Oil content in seeds was measured in three repetitions for each plot using nuclear magnetic resonance method with pulse NMR Oxford MQA 7005 with rapeseed calibration.
Statistical analyses
Statistical analyses were performed considering 6 groups of genotypes: group 1 – DH lines, group 2 – CMS×DH hybrids, group 3 – CMS/DH×Rfo hybrids, group 4 – CMS ogura line, group 5 – Rfo restorer line, group 6 – control group (Monolit and Arsenal varieties).
The normality of the traits distribution was tested using the Shapiro-Wilk test (Shapiro and Wilk 1965). The significance of groups and DH lines was assessed using one-way analyses of variance (ANOVA). Least significant differences (LSDs) for each trait were calculated. Homogeneous groups for the traits were determined on the basis of least significant differences. Parallel coordinate plots (PCPs) are efficient tools for visualizing multiple interactions (Bocianowski et al. 2015). In PCP construction, a given trait is represented on the x-axis. Mean values were used. The y-axes are parallel, have the same length and start with a minimum and maximum. For a particular level of a factor, points on adjacent y-axes are joined by a line, thereby providing a multidimensional characterization of the level of that factor. As many levels of a given factor are plotted on the same PCP, a particular level’s performance can be seen against a background of the whole pool of levels for that factor studied. The relationships between traits were
determined using the simple correlation analysis (Kozak et al. 2008). Relationships of observed traits were presented in the form of the scatterplot matrix (Wnuk et al. 2013). All plots were drawn with statistical package GenStat 17.
Results
All studied traits have a normal distribution. The results of the analysis of variance (ANOVA) indicate a statistically significant (at 0.001 level) influence of the groups of lines on all observed traits (Tab. 1).
Table 1. Mean squares from one-way analysis of variance for observed traits
Średnie kwadraty dla jednoskładnikowej analizy wariancji obserwowanych cech
Source of variations
Źródło zmienności Grupa Group Residual Duration of flowering
Długość kwitnienia m.s. d.f. 42.702*** 5 1,865 192 Plant height
Wysokość roślin m.s. d.f. 104045.9*** 5 123,2 1776 Number of branches per plant
Liczba rozgałęzień na roślinie m.s. d.f. 106.694*** 5 8,118 1776 Number of siliques per plant
Liczba łuszczyn na roślinie m.s. d.f. 3084623*** 5 52671 1776 Silique length
Długość łuszczyny m.s. d.f. 21836.68*** 5 11874 78,22 Number of seeds per silique
Ilość nasion w łuszczynie m.s. d.f. 7250.81*** 5 11874 34,38 Thousand seed weight
Masa tysiąca nasion m.s. d.f. 158.2126*** 5 0,1863 1776 Oil content
Zawartość oleju m.s. d.f. 4187.333*** 5 3,159 588 *** P < 0.001
d.f. — number of degrees of freedom — liczba stopni swobody m.s. — mean squares — średnie kwadraty
All six examined groups of genotypes varied in terms of the observed traits (Tab. 2, Fig. 1). The longest flowering time was observed for CMS×DH hybrids (24.43 days), and the shortest – for Rfo line (23.33 days; this group differed significantly from the other groups) (Tab. 2). The minimum value for the duration of flowering was noticed for CMS×DH and CMS/DH×Rfo hybrids, while the maximum – for DH lines. These three groups showed also the largest range of this trait values (Fig. 1A).
T ab le 2 . M eas u res o f v ar iab il it y o f a n al y zed t rai ts , co n si d er in g t h e g ro u p s o f g e n o ty p es M iar y zm ie nnoś ci anal izow any ch ce ch, z uw zgl ędni eni em gr up ge n ot ypów G roup Gru p a D u ra tio n o f fl o w er in g D ługoś ć kw itn ie n ia [d ays ] P la n t h eig h t W ys oko ść ro śl in [c m] N o. of br anc he s p er p lan t L ic zba roz gał ęz ie ń na roś lini e No . o f si li q u es p er p lan t Li czb a łu szc zy n na roś lini e S iliq u e le n g th D ługoś ć łu szc zy n [m m] No . o f se ed s p er s iliq u e L ic zba nas ion w łu szc zy ni e T hous and seed w ei g h t M as a 1 00 0 nas io n [g] O il c o n te nt Zaw ar toś ć tłu szc zu [%] m ean śre dn ia SD m ean śre dn ia SD m ean śre dn ia SD m ean śre dn ia SD m ean śre dn ia SD m ean śre dn ia SD m ean śre dn ia SD m ean śre dn ia SD DH 24. 35 ab 1 .21 146 .8 d 12. 28 10. 17 bc 2. 97 434 .8 d 216 .8 59. 28 d 9 .48 14. 80 d 6 .34 5. 2 7 b 0 .54 4 3. 28 d 2 .10 C MS× D H 24. 43 a 1 .66 153 .8 c 10. 36 9. 8 0 c 2 .47 465 .3 cd 205 .2 62. 21 b 9 .47 16. 59 c 5 .46 5. 3 9 a 0 .41 45. 77 b 1 .82 C M S /DH× Rfo 23. 85 d 1 .21 163 .3 a 11. 01 10. 06 bc 3 .13 507 .7 b 269 .8 58. 37 d 7 .63 17. 84 b 5 .63 4. 8 5 d 0 .33 47. 00 a 1 .37 CM S 24. 00 c 1 .00 144 .9 e 7 .28 10. 41 ab 2 .02 485 .1 bc 162 .5 60. 80 c 9 .63 15. 73 c 5 .55 5. 4 0 a 0 .24 47. 03 a 1 .48 Rfo 23. 33 e 1 .52 144 .0 e 8 .18 10. 6 7 a 2 .58 574 .0 a 237 .1 43. 25 e 5 .35 13. 82 d 5 .25 4. 1 0 e 0 .37 44. 42 c 1 .96 M o n o lit /A rs en al 24. 17 bc 0 .93 155 .6 b 10. 21 9. 4 4 d 2 .64 387 .8 e 196 .6 66. 42 a 8 .36 21. 54 a 6 .57 5. 0 7 c 0 .37 44. 47 c 1 .34 LS D0. 05 0 .21 1 .66 0 .43 34. 38 1 .32 0 .88 0 .06 0 .27 CV 5. 6 6% 8. 3 8% 28. 57% 49. 80% 15. 55% 36. 69% 9. 7 6% 5. 1 2% S D — st anda rd de v ia ti o n — odc h yl eni e s tan dar d ow e, CV — co ef fic ie n t o f v ar ia tio n — w sp ół cz ynni k zm ie nnoś ci M ean v al ues in co lu m ns m ar ked w ith th e sam e let ter d o no t d iff er si gn ifi can tly st at is tical ly (p ≤ 0 .0 5) Śr edni e war toś ci w pos zc ze gól ny ch kol um nac h oz nac zo ne tą sam ą lit er ą ni e róż ni ą si ę is tot ni e stat ys ty cz ni e (p ≤ 0. 05)
Fig. 1 A–F. Variation of the observed traits, considering the groups of genotypes. A – duration of flowering [days], B – plant height [cm], C – no. of branches per plant, D – no. of siliques per plant, E – silique length [cm], F – no. of seeds per silique — Zmienność obserwowanych cech, z uwzględ-nieniem grup genotypów. A – długość kwitnienia [dni], B – wysokość roślin [cm], C – liczba rozgałęzień na roślinie, D – liczba łuszczyn na roślinie, E – długość łuszczyn [cm], F – liczba nasion w łuszczynie
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Fig. 1 G–H Variation of the observed traits, considering the groups of genotypes. G – thousand seed weight [g], H – oil content in seeds [%] – Zmienność obserwowanych cech, z uwzględnieniem grup genotypów. G – masa tysiąca nasion [g], H – zawartość tłuszczu w nasionach [%]
Considering plant height, CMS/DH×Rfo hybrids were the tallest (163.3 cm), and this value was statistically different from the other groups. The shortest plants (144 cm) were represented by Rfo line (Tab. 2). The lowest values for this trait were observed in the group of single cross hybrids, and the highest – in the group of three-way cross hybrids (Fig. 1B).
In all investigated groups average values of number of branches per plant were on the similar level; although the three-way cross hybrids (CMS/DH×Rfo) had the widest range of observations (Fig. 1C). The largest number of branches was observed for Rfo line (10.67), and the smallest – for control group (9.44), which was statistically different from the other groups (Tab. 2).
The greatest average number of siliques per plant was observed in Rfo line (574), and the lowest in control group (387.8), and these means were statistically different from the rest of the groups (Tab. 2). The three-way cross hybrids had the widest spread of data regarding this trait and also contained genotypes with the highest number of siliques per plant (Fig. 1D). This trait had the largest coefficient of variation – 49.8% (Tab. 2).
With respect to the silique length, groups with maximum and minimum values differ significantly from the others. The control group had the longest siliques (66.42 mm) and the restorer line (Rfo) – the shortest (43.25 mm), and the smallest range of this characteristic (Tab. 2, Fig. 1E). On the other hand, the widest range of this trait presented DH lines and CMS×DH hybrids.
For number of seeds per silique, the smallest range of observations was noted for Rfo and CMS genotypes (Fig. 1F). However, the largest number of seeds was observed in control group (Monolit/Arsenal) – 21.54, and this value is significantly different from the other groups. Rfo line had the lowest number of seeds per silique (13.82) (Tab. 2).
The smallest thousand seed weight had plants of Rfo line – 4.1 g, and this value was statistically different from the other groups (Tab. 2). The greatest thousand seed weight was observed in CMS ogura line (5.4 g) and in CMS×DH hybrids (5.39 g). At the same time, the narrowest range of this trait values was noted in CMS plants, and the widest – in DH population (Fig. 1G).
Regarding oil content in seeds, it was the highest in CMS line (47.03%) and the lowest in DH population (43.28%, this value was significantly different from the other groups) (Tab. 2). Three groups (DH, CMS×DH, CMS/DH×Rfo) had wide range of this trait values and gradual growth of oil content could be observed from generation to generation (Fig. 1H). This trait had the lowest coefficient of variation (5.12%) (Tab. 2).
Parallel coordinate plot (PCP) very clearly presents the relations between the mean values of observed characteristics for each group of genotypes (Fig. 2).
Fig. 2. Parallel coordinate plot of relations between groups of genotypes for eight traits. DoF – duration of flowering, PH – plant height, BpP – no. of branches per plant, SpP – no. of siliques per plant, SL – silique length, SpS – no. of seeds per silique, TSW – thousand seed weight, OC – oil content — Wykres typu “sieć rybacka” przedstawiający zależności między
grupami genotypów dla ośmiu cech. DoF – długość kwitnienia, PH – wysokość roślin, BpP – liczba rozgałęzień na roślinie, SpP – liczba łuszczyn na roślinie, SL – długość łuszczyn, SpS – liczba nasion w łuszczynie, TSW – masa tysiąca nasion, OC – zawartość tłuszczu
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DH population had the high mean value for the duration of flowering and high thousand seed weight, but low plant height and the number of seeds per silique. It had also the lowest oil content in seeds. CMS×DH hybrids were characterized by the longest flowering and the largest thousand seed weight. Three-way cross hybrids were the tallest and had very high oil content. CMS ogura plants were small, but at the same time had the greatest thousand seed weight and the highest oil content. Rfo line had the shortest flowering time and the smallest plant height, silique length, number of seeds per silique, and thousand seed weight, but the highest number of branches and siliques per plant. On the contrary, in the control group number of branches and siliques per plant was the smallest, and the silique length and number of seeds per silique was the highest.
Nine significant correlations (P < 0.001) were observed between the traits (Tab. 3, Fig. 3). Seven of them were positive and were observed between plant height and number of seeds per silique as well as oil content; between number of branches per plant and number of siliques per plant; between number of siliques per plant and oil content; between silique length and number of seeds per silique as well as thousand seed weight; between number of seeds per silique and oil content. On the other hand, thousand seed weight was strongly negatively correlated with plant height and number of seeds per silique.
Table 3.
The Pearson correlation coefficients matrix for the eight traits Macierz współczynników korelacji Pearsona dla ośmiu cech
Trait Cecha DoF PH BpP SpP SL SpS TSW OC DoF 1 PH -0.093 1 BpP -0.055 -0.042 1 SpP -0.054 0.211** 0.728*** 1 SL 0.092 0.103 0.023 0.010 1 SpS -0.086 0.371*** -0.021 0.065 0.352*** 1 TSW 0.174* -0.304*** -0.051 -0.226** 0.280*** -0.273*** 1 OC 0.068 0.594*** 0.079 0.268*** 0.159* 0.352*** -0.209** 1 * P < 0.05; ** P < 0.01; *** P < 0.001
DoF – duration of flowering, PH – plant height, BpP – no. of branches per plant, SpP – no. of siliques per plant, SL – silique length, SpS – no. of seeds per silique, TSW – thousand seed weight, OC – oil content — DoF – długość kwitnienia, PH – wysokość roślin, BpP – liczba rozgałęzień na roślinie, SpP – liczba łuszczyn na roślinie, SL – długość łuszczyn, SpS – liczba nasion w łuszczynie, TSW – masa tysiąca nasion, OC – zawartość tłuszczu
Fig. 3. Scatter plot matrix for relationship between the eight observed traits of the investigated genotypes for six groups of genotypes. The scatter plots provide a visual representation of the correlation among the traits. Abbreviations like in Fig. 2 — Macierz
wykresów rozrzutu zależności pomiędzy ośmioma obserwowanymi cechami dla badanych genotypów w sześciu grupach obiektów. Wykresy rozrzutu przedstawiają wizualnie korelację pomiędzy cechami. Skróty jak w Fig. 2
Discussion
Seed yield and yield-related traits in Brassica napus are complex quantitative traits, easily modified by the environment (Quarrie et al. 2006). In the present study, the phenotypic variability of eight yield-related traits was examined. The 182 genotypes
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and 2 control varieties have been divided into six groups, between which large variation of these traits was noted.
The potential of seed yield depends significantly on flowering time (Xu et al. 2016), which is a complex agronomic trait. Simultaneously, the duration of the flowering period strongly depends on temperature profile. Hence, “dry matter production during the flowering period and thus seed set are also strongly influenced by temperature” (Habekotte 1997). Flowering duration was studied by Ali et al. (2003) on twenty-five winter type rapeseed varieties originating from different sources. It ranged from 21 to 32.25 days, while in our study this trait varied from 21 to 29 days. Ali et al. (2003) stated also that flowering duration has high heritability (0.662).
The mechanism of plant height regulation is an area of concern related to the plant development. Plant height and architecture is influenced by many QTLs associated with plant hormones biosynthesis, which complicates research in this field (Singh and Savaldi-Goldstein 2015). Wang Y. et al. (2016) studied plant height in crosses and backcrosses of normal and dwarf plants of Brassica napus to fine map the dwarf-dominant locus BnDWF1. Regular (non-dwarf) plants had an average height between 139.2 and 166.1 cm. Similarly, in our study, an average values between 144.0 and 163.3 cm were observed.
Significant characteristic which affects the seed yield in oilseed rape is the number of branches. It was one of the six traits investigated by Chen et al. (2007) on DH and immortalized F2 (IF2)populations. Their purpose was to detect QTLs
for number of primary branches, which oscillated between 1.9 and 7.3 in DH lines, and 2.3 to 6.9 in IF2 lines. In our studies number of branches ranged from 9.44
in control group to 10.67 in Rfo plants. Standard deviation calculated for our observations varied from 2.02 to 3.13, while in Chen’s research it was lower (0.7 – 0.8), which indicates greater variability of this trait in our plant material. Shi et al. (2009) also analysed the number of branches as one of the components affecting seed yield. Authors conducted their study on DH population and its parental lines (Tapidor and Ningyou7 cultivars). Depending on environment, this trait ranged from 3.1 to 14.1 branches in DH population and from 4.9 to 12.0 branches in parents.
As one of the most important organs, siliques play a major role in the survival of a species by protecting the developing seeds from biotic and abiotic stresses (Bennett et al. 2011). Among the three yield components of rapeseed, silique number per plant is highly correlated with seed yield, followed by seed number per silique, suggesting that these traits are the major contributors to the yield in rapeseed. Many QTL mapping studies involving silique number and seed number per silique have been reported, but the genetic and molecular bases of both traits still remain unclear. Shi et al. (2015) identified eight QTLs for silique number per plant on a reference genetic population from two sequenced cultivars,
Zhongshuang11 and No.73290, which are significantly different in many traits, particularly yield components. Phenotypic variation of silique number observed by authors in four investigated environments ranged from 175 to 272 for parents and from 134 to 315 for population (mean value 216). Slightly higher number of siliques per plant observed Shi et al. (2009) in their earlier studies in Tapidor and Ningyou7 cultivars (362 and 377, respectively). These data differ significantly from that collected in our study. Mean values for six groups of genotypes varied from 387.8 in control plants to 574 in three-way cross hybrids. Such high mean value in CMS/DH×Rfo hybrids may indicate the occurrence of the heterosis effect.
Many researches are focused on finding quantitative trait loci for seed number per siliques. Yang et al. (2016) identified five QTLs using RIL population created from parents with extremely significant differences for this trait, amounting 11.7 and 21.7 seeds per silique. A major QTL, qSN.A6, was successfully fine-mapped using near-isogenic line (NIL). In recombinant NIL, seed number per silique was 15.9. Similar study was conducted by Wang X. et al. (2016). The number of seeds per silique in four microenvironments varied from 0.6 to 40.4 in DH population, while in our plant material this trait ranged from 5.2 to 27.15 jointly for DH lines and hybrids (average for object in single repetition; data not shown).
Silique length and seed weight are important components influencing the seed yield in rapeseed. Their high heritability makes the selection for them more effective (Zhang and Zhou 2006). Both these traits are quantitatively inherited and controlled by multiple QTLs, mostly with additive effects (Zhang et al. 2011, Yang et al. 2012). Fu et al. (2015) identified QTLs for silique length and seed weight in doubled haploid and DH-derived reconstructed F2 (RC-F2) populations. The
average silique length in the RC-F2 lines was 61.3 mm and an average seed weight
was 3.77 g. These values were higher than in DH lines, which had an average silique length of 56.7 mm and an average seed weight of 3.47 g. Average silique length observed in our material ranged from 43.25 mm in Rfo line to 66.42 mm in control group. In CMS×DH hybrids a slight increase in silique length in relation to the CMS and DH lines was noted (62.21, 60.8 and 59.28 mm, respectively). After crossing a single hybrid with Rfo line (SL = 43.25 mm), a decrease in length to 58.37 mm was noted in three-way cross hybrids. A similar situation was observed for thousand seed weight – a slight increase in the value of this trait was observed in single hybrids (CMS 5.4 g, DH 5.27 g, CMS×DH 5.39 g), and after the
Rfo line (4.1 g) introduction a decrease of the value to 4.85 g (CMS/DH×Rfo) was
noted. These results may indicate high heritability of these traits and prove the importance of parents choice for hybrid breeding. According to Li et al. (2014), silique length is significantly positively correlated with variation in seed weight. Similar correlation was also observed by Fu et al. (2015) in both studied populations (r = 0.49 and r = 0.34 in the DH and RC-F2 populations, respectively;
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The similar relationship was observed in our study – weaker, but positive correlation between silique length and thousand seed weight (r = 0.28) was statistically significant at P < 0.001 level.
The genetic basis of seed weight is also related to oil and protein content (Yang et al. 2012). Oil yield and oil percent in seeds are both quantitative traits, whose expression depends on genotype, environmental effect and genotype – environment interaction (Huhn and Leon 1985). The seed oil content was measured by Wang et al. (2013) by nuclear magnetic resonance (NMR). This method is commonly used in breeding studies, although allows only the measurements of fat and moisture content (Michalski 2005). In Wang’s et al. (2013) research a wide range of oil content was noted in all trials. Two parents ‘N53-2’ and ‘KenC-8’ showed >10% difference in seed oil content, and the average oil content of the DH population ranged from 42.0% to 47.6% in different microenvironments. Those results indicate that seed oil content in B. napus is considerably influenced by environment. Significant differences in seed oil content were observed by Shen et al. (2005) among the F1 hybrids and between F1 progenies and their parents. Oil
content in parental lines ranged from 37.98 to 42.90%, and in hybrids from 38.48 to 44.67%. According to the authors, “F1 hybrids exhibited a heterotic advantage
for seed oil content”. This was confirmed by our observations, where gradual increase in oil content was observed in two groups of hybrids. Mean seed oil content in our plant material varied for parental lines from 43.28% (DH lines) to 47.03% (CMS line), and for hybrids from 45.77 to 47.00%. In Shen’s et al. (2005) studies “seed oil content was slightly positively related with seed yield per plant and its components, except for thousand seed weight”. In our study, we observed significant positive correlations between oil content and plant height, number of siliques per plant, and number of seeds per silique (P < 0.001).
In conclusion, a large variability of studied characteristics was observed between examined groups of genotypes. For most of the traits means for the groups differed statistically from each other. Diversity of the tested plant material will allow, after collecting two-year data, mapping of the observed yield related traits, study of their inheritance across generations and evaluation of the phenomenon of heterosis.
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