CSIRO PUBLISHING
Crop & Pasture Science, 2016, 67, 369–380
http://dx.doi.org/10.1071/CP15236
Seed yield of canola (Brassica napus L.) is determined
primarily by biomass in a high-yielding environment
Heping Zhang A,B and Sam Flottmann A
A
B
CSIRO Agriculture, Private Bag 5, PO Wembley, WA 6913, Australia.
Corresponding author. Email: heping.zhang@csiro.au
Abstract. The better performance of hybrid canola compared with open-pollinated triazine-tolerant canola can be
associated with greater biomass and harvest index. We compared several hybrid and open-pollinated canola cultivars in
field conditions to (i) quantitatively analyse yield formation and identify the key drivers of yield formation process;
(ii) investigate biomass accumulation and partitioning and evaluate the relative importance of biomass, harvest index and
yield components. Six elite varieties, two from each of the three types (triazine-tolerant (TT), hybrid TT, and hybrid
imidazolinone-tolerant (IT) or conventional (CV) (hybrid IT/CV)) of canola, were grown under the optimum crop
management in the 3 years from 2009 to 2011 in the high-rainfall zone of south-western Australia. Leaf area, specific
leaf area, light interception, biomass, seed yield and yield components were measured at key growth stages to determine
biomass accumulation, crop growth rate (CGR), radiation-use efficiency and to investigate the relationship between yield,
biomass, CGR, specific leaf area, yield components and harvest index. Hybrid IT/CV canola grew more vigorously with
thicker leaves and greater leaf area, allocated more biomass into leaves, intercepted more radiation, produced higher biomass
in the vegetative stage and maintained its biomass superiority throughout the whole crop cycle. It had radiation-use efficiency
of 1.74 g MJ m–2 photosynthetic active radiation, 28% higher (P < 0.001) than TT canola (1.41 g MJ m–2 photosynthetic
active radiation) and 16% higher (P < 0.001) than hybrid TT canola (1.52 g MJ m–2 photosynthetic active radiation). The
average CGR for hybrid IT/CV canola (12.1 g m–2 day–1) was 32% higher than that of TT canola (9.2 g m–2 day–1) from
budding to the beginning of pod filling. Hybrid IT/CV canola produced 38% higher seed yield than TT canola in favourable
growing conditions (2009, 2011). However, there was no yield difference between the hybrid IT/CV, hybrid TT, and
TT canola in the drought year (2010). The number of pods m–2 and seeds m–2 was highly associated with biomass at
vegetative, budding, flowering, podding and maturity and CGR from budding to podding. High yield in hybrid canola was
attributed mainly to higher biomass from each phenological phase from the vegetative stage to maturity and not to improved
harvest index.
Additional keywords: biomass, hybrid canola, harvest index, open-pollinated canola, partitioning.
Received 16 July 2015, accepted 25 January 2016, published online 21 April 2016
Introduction
Canola (Brassica napus L.) is an important agricultural crop
grown primarily for oil production. The global area of the oilseed
crop has increased from just over 7 million ha in 1965 to
27 million ha in 2005 (Berry and Spink 2006). It also provides
a valuable break crop in rotation with cereals in many of the major
food production systems (Angus et al. 1991; Seymour et al.
2012). In Australia, the area sown to canola rose from 150 000 ha
in 1991 to 1.8 million ha in 2008–2012. The average yield of
canola in Australia is relatively low (1.2 t ha–1) compared with the
rest of the world. Although canola has been expanding across
the traditional wheatbelt as a break crop for the wheat-based
farming system, canola is mainly grown in the high rainfall zone
(HRZ, annual rainfall of 450–750 mm) of southern Australia. The
potential yield is estimated to be 3–4 t ha–1 in this region but onfarm yield is about half of the potential (Zhang et al. 2006).
Therefore, yield can be significantly improved through selecting
Journal compilation CSIRO 2016
better adapted varieties (e.g. longer-season cultivars) (Riffkin
et al. 2012) and improved agronomy such as early sowing (Farré
et al. 2002; Brill et al. 2016; Kirkegaard et al. 2016) and high N
input (Hocking et al. 1997a, 1997b).
Open-pollinated (OP) triazine-tolerance (TT) canola is
widely adopted in Australian farming systems (Zhang et al.
2016) because of the cheap cost of seed and robust weed
(brassicaceous weeds and ryegrass) control the system
provides. However, ongoing self-pollination in OP TT canola
can produce inbred plants that may display lowered fitness or
vigour as compared with their non-inbred counterparts, and
therefore result in low yield potential. In contrast, the progeny
from crosses of genetically distinct parents has hybrid vigour
(heterosis). Hybrid canola varieties are taller, more vigorous,
establish a denser canopy, and are more competitive with weeds
than OP varieties and therefore have greater biomass and yield
advantage of up to 20% compared with the standard varieties in
www.publish.csiro.au/journals/cp
370
Crop & Pasture Science
H. Zhang and S. Flottmann
Canada and Australia (Harker et al. 2000; Kirkegaard et al.
2012), and therefore high-yield potential. In Canada, hybrid
varieties dominate Canadian canola acreage with no significant
OP varieties since 2001 (Brewin and Malla 2012). Compared
with Canada, adoption of hybrid canola in Australia is low with
<15% of the acreage being attributed to hybrids (predominately
Roundup Ready hybrids) (Zhang et al. 2016). Despite the
considerably higher yield potential of hybrids, there has been
little research evaluating the potential of hybrid canola and
identifying what attributes are responsible for high yield in
Australia.
The published data on canola adaptive traits in Australia
largely focus on drought tolerance in the low to medium
rainfall environment in Australia (Richards and Thurling 1978;
Wright et al. 1995; Gunasekera et al. 2006) and genotype by
environment interaction (Gunasekera et al. 2006; Zhang et al.
2013). Little attention has been paid to capturing high-yield
potential in the HRZ and identifying which traits contribute
to high yield in this area apart from flowering time (Cullis
et al. 2010; Riffkin et al. 2012; Zhang et al. 2013). In crops
like rice and maize, biomass and harvest index (HI) are two
important traits contributing to high-yield performance of
hybrids compared with in-bred lines and have been considered
as target traits in breeding programs. For example, comparison
of crop growth of high-yielding hybrid and inbred rice suggests
that hybrids accumulated more biomass before anthesis or the
whole crop cycle (Bueno and Lafarge 2009). Hybrids partitioned
biomass more efficiently among organs and therefore expressed
higher HI at maturity (Virmani et al. 1982; Laza et al. 2003;
Bueno and Lafarge 2009). In contrast, genetic improvement
in grain yield of North American maize (Zea mays L.) hybrids
during the past three to five decades has attributed mainly (85%)
to increased biomass production whereas the improvement
of HI contributed 15% of yield gains (Tollenaar 1989, 1991).
The contribution of biomass and HI to high yield depends on
weather conditions. Biomass accumulation is reported to be the
main driver under favourable conditions and HI under suboptimal
conditions in rice (Laza et al. 2003). For canola, little research has
been devoted to understand to what extent the yield advantage of
hybrid canola are attributed to the increased biomass production
and biomass partitioning (HI) or the combination of both, and
under what conditions are these traits better expressed.
Investigation into the processes responsible for yield
superiority of hybrid over OP canola could lead to identify
promising traits for high-yield potential. It is well known that
TT canola has a restriction on photosynthesis compared with
other canola, producing 20% less biomass and yield (Robertson
et al. 2002). However, heterosis of hybrids allows the crop to
quickly accumulate higher biomass early in the growing season
compared with OP cultivars (Brandt et al. 2007). A longer
vegetative growth stage and late flowering in canola also
allows crop to accumulate more biomass in a longer period
(Thurling 1974). Furthermore, it is not known if this increased
biomass is accompanied by an increase in partitioning to grain
leading to high HI. The answers to these questions could help
breeders to identify physiological traits that can be bred for and
the bottlenecks that need to overcome and growers to manage
the key drivers of yield agronomically.
The objectives of this study were to (i) quantitatively analyse
and identify the key drivers of yield formation; and (ii) evaluate
the relative importance of biomass, biomass partitioning, and
yield components in high-yielding canola crops.
Materials and methods
The experiments were conducted from 2009 to 2011 near
Kojonup (–33.888, 116.778), Western Australia. Eight spring
canola cultivars (Brassica napus L.) were sown at earliest
sowing opportunity (20 May 2009, 21 May 2010, and 20 May
2011) at Kojonup, Western Australia. The eight cultivars were
classified herbicide tolerance (HT) as TT canola (CB Argyle/
Thunder, Tornado), hybrid TT canola (CB Jardee, Hyola 751TT,
CB-HY125), and hybrid imidazolinone tolerance (IT) (Pioneer
46Y78) and hybrid conventional canola (CV) (Hyola 50)
(Table 1). Only six were grown each year (Table 1). As hybrid
IT and hybrid CV canola do not have a yield penalty associated
with TT and we consider hybrid IT/CV as one group hereafter.
Most varieties have an intermediate phenology, begin flowering
within 3–4 days of each other whereas Pioneer 46Y78 and
Hyola 751TT flower slightly later (8–12 days) (Table 1). The
crop was sown to achieve 40 plants m2 by adjusting seeding
rates according to one-thousand-seed weight and the germination
rate. A randomised experimental design was used. The treatments
were replicated four times each year. The plot size was 20 m by
1.44 m. In all 3 years, the crop was managed under close to the
optimum agronomic conditions by supplying with 120 kg N ha–1
split as 20 kg N ha–1 at sowing, 50 kg N ha–1 at 4–6 leaves stage,
and 50 kg N ha–1 at budding to flowering. The initial soil-available
N in the 0–120-cm soil profile at sowing was 120–130 kg/ha in all
3 years. Because of the different HT of varieties, all canola were
Table 1. Canola genotypes and cultivars used in the field experiments and their pollination type, herbicide tolerance (HT) groups and flowering time
OP, open-pollinated; TT, triazine-tolerant; IT, Clearfield; CV, conventional
Genotype
CB Argyle
CB Jardee
CB-CBHY-125
Hyola 50
Hyola 751 TT
Pioneer 46Y78
Thunder TT
Tornado
OP/
hybrid
HT
group
OP
Hybrid
Hybrid
Hybrid
Hybrid
Hybrid
OP
OP
TT
TT
TT
CV
TT
IT
TT
TT
Inclusion of cultivars
2009
2010
2011
ü
ü
ü
ü
–
ü
–
ü
–
ü
–
ü
ü
ü
ü
ü
–
ü
–
ü
ü
ü
ü
ü
2009
Flowering date
2010
2011
30 Aug. 2009
29 Aug. 2009
6 Sept. 2009
6 Sept. 2009
–
10 Sept. 2009
–
29 Aug. 2009
–
7 Sept. 2010
–
2 Sept. 2010
10 Sept. 2010
10 Sept. 2010
7 Sept. 2010
7 Sept. 2010
–
24 Aug. 2011
–
24 Aug. 2011
31 Aug. 2011
6 Sept. 2011
24 Aug. 2011
24 Aug. 2011
Biomass and seed yield of canola
Crop & Pasture Science
treated with conventional canola in weed management in all
3 years.
The 3 years represented three different growing seasons; 2009
was close to the average year with annual (506 mm) and growing
season rainfall (455 mm), not far from the long-term averages but
with extremely low October rainfall (10 mm) (Table 2) and 2010
represented a dry year (20th percentile) with annual (346 mm) and
growing-season rainfall (257 mm) (1 in 5 years). Lack of rain in
September and October severely constrained crop growth during
the pod and seed setting period and seed filling. Rainfall in 2011
was above the long-term average with an even distribution. No
frost damages were observed to flowers and pods in all 3 years.
before maturity. The main stems and individual branches of these
three plants were harvested separately and carefully to prevent
seed loss. All samples were dried, weighed and threshed by hand.
The biomass required to produce one pod was calculated by
dividing the biomass by the number of pods of the three plants.
Average seed weight was measured by dividing the total seed
weight by the total number of seeds of the three plants. The
number of seeds m–2 was calculated by dividing the seed yield by
the average seed weight. The number of pods m–2 was calculated
by dividing aboveground biomass by the biomass required to
produce one pod.
Light interception and radiation-use efficiency
Photosynthetic active radiation (PAR) was measured above the
canopy and at ground level at major crop growth stages using a
linear ceptometer. Light interception fraction was related to leaf
area index (LAI) using Beer’s law:
Crop sampling
Crop growth stages were scored according to a standardised growth
stage scale developed by Sylvester-Bradley and Makepeace
(1984). Crop samples were collected using a quadrat of 0.54 m2
(0.5 m length of row from six adjacent 180 mm-apart rows) at the
vegetative growth stage (eight leaves stage), flowering (50%) (4.5)
and the end of flowering/beginning of pod filling (5–10% flowers
remaining) (5.8). For the simplicity, the end of flowering/beginning
of pod filling is referred as podding thereafter. Five plants were
randomly subsampled and partitioned into leaves, stems, and pods.
Leaf area and pod area was measured using a leaf area meter. All
samples were dried to constant weight in a forced-draught oven at
608C and weighed. Specific leaf area was calculated by dividing
leaf area by leaf biomass. The changes of biomass between the two
consecutive sampling dates were used to calculate the crop growth
rate (CGR) during the sampling period. For the linear growth phase
from budding to podding, the slopes of regression lines of biomass
against day after sowing were taken as the average CGR.
At harvest, plant sample from an area of 1.08 m2 was collected
from each plot to determine seed yield and HI. All plant samples
were dried in a forced-draught oven at 608C for 96 h and weighed
and threshed. Grain yield was reported at a 5% moisture content
basis. The HI was calculated as the ratio of seed yield to biomass
from 1.08-m2 samples. In addition, three plants from each plot
were randomly selected, tagged and the number of pods from each
branch including the main stem of the three plants was counted
f ¼ 1 lnðkLAIÞ
where k is the extinction coefficient. The value of k is taken as
0.75 for LAI and 0.5 for pod area index (Justes et al. 2000).
The calculated f at the days when leaf area and pod area were
measured was significantly (P < 0.01) related to the measured
light interception rates five times in 2011 (r2 = 0.75) (data
not shown). Cumulative intercepted PAR was calculated by
multiplying the 50% of daily incoming solar radiation by f for
each day. Radiation-use efficiency was calculated as the slope of
the linear regression of accumulated biomass against cumulative
intercepted PAR (Robertson et al. 2002). Midday photosynthetic
rates of fully expanded leaves were measured in the field in
2011 using a portable photosynthesis system (CIRAS-2, PP
Systems International, Amesbury, USA) on a sunny day at the
budding stage.
Statistical analyses
A nested (HT/Variety) ANOVA within years and a nested
three-way (HT/Variety*Year) ANOVA between years were
performed for biomass at major crop development stages,
grain yield, yield components, and associated traits using
Table 2. Monthly rainfall (R, mm), the minimum (Tmin) and maximum (Tmax) temperatures at Kojonup, Western Australia in
2009, 2010 and 2011 and the long-term average rainfall
Month
371
R
2009
Tmax
Tmin
R
2010
Tmax
Tmin
R
2011
Tmax
Tmin
Long-term
average R
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
10
19
19
2
29
153
103
59
71
10
30
1
30.2
28.6
25.9
24.2
20.6
15.0
14.1
15.2
15.0
21.2
26.1
29.1
14.0
13.3
11.5
9.8
6.1
6.5
5.4
6.1
5.5
8.7
10.2
10.6
2
16
42
16
43
33
65
40
17
20
40
13
31.0
31.1
27.0
22.9
18.8
15.9
15.0
15.7
19.3
22.9
26.8
24.6
12.9
14.2
13.7
9.9
7.0
4.6
4.2
4.6
4.4
6.1
10.9
11.2
77
7
25
40
42
94
88
119
74
43
60
55
30.7
28.8
28.5
24.1
19.5
16.2
14.6
16.5
17.3
21.3
23.7
27.2
14.2
15.6
13.5
10.5
7.7
7.2
5.5
6.0
5.9
9.0
10.4
13.5
13
16
22
32
68
91
88
75
53
42
23
15
Total
May–Nov.
506
455
–
–
–
–
346
257
–
–
–
–
723
520
–
–
–
–
538
440
372
Crop & Pasture Science
H. Zhang and S. Flottmann
GENSTAT version 13. The means were compared using the l.s.d.
of the means, calculated from standard errors of the difference
of the means using corresponding degrees of freedom. The
ANOVA for individual years indicated that there were no
significant differences between cultivars within the three HT
groups of canola for most of the plant parameters investigated.
Therefore, the results are mainly presented comparing the three
HT groups of canola.
Regression lines were fitted for the data in each year in order
to find genetic variations in the traits examined. This allows the
confounding impact of season to be excluded. When the data
showed similar responses across the years, the data points were
pooled and a single regression line was used. In order to detect
the differences in a given parameter or trait between canola
genotypes, linear regression with the canola HT groups as a
group factor was employed to test the significant difference of
slopes and intercepts.
canola, with significantly higher values by podding (early
October). Hybrid IT/CV canola shed leaves quicker than TT
canola and favoured pod growth. Nevertheless, when the sum of
pod area and leaf area were considered, hybrid IT/CV had higher
leaf and pod area index than TT canola at the beginning of pod
filling. Hybrid IT/CV canola had lower SLA than hybrid TT and
TT canola at the vegetative and budding stages, indicating that
it had thicker leaves than TT canola at these stages (Fig. 2).
Light interception and radiation-use efficiency
As a result of the LAI difference, hybrid IT/CV canola intercepted
significantly more radiation than hybrid TT and TT canola early
in the growing seasons but the difference in the fraction of
radiation interception gradually became insignificant as the
crop approached greater than 90% interception at flowering
and when LAI was greater than 2.0 (Fig. 1). From germination
to podding, cumulative PAR interception was 20–25% greater
for hybrid IT/CV than for TT canola and 10% more than hybrid
TT canola. The linear regression of cumulative biomass
against cumulative intercepted PAR accounted for more than
94% of the variance of biomass. The difference in radiation-use
efficiency was highly significant (P < 0.001) between the HT
groups. Hybrid IT/CV canola had radiation-use efficiency of
1.74 g MJ–1 PAR, 28% higher (P < 0.001) than TT canola
(1.41 g MJ–1 PAR) and 16% higher (P < 0.001) than hybrid TT
canola (1.52 g MJ–1 PAR) (Fig. 3).
Results
Leaf area and specific leaf area (SLA)
The LAI differences were much greater (P < 0.05) between than
within the HT groups in all 3 years. During the vegetative growth
period (early August), hybrid IT/CV canola had double the LAI
of TT canola (Fig. 1). The LAI reached a maximum at flowering
(late August/early September) with higher (P < 0.05) LAI in
hybrid IT/CV canola than TT and hybrid TT canola (Fig. 1).
However, TT canola maintained LAI longer than hybrid IT/CV
6
(a)
(c)
(b)
5
LAI/PLAI
4
F
F
3
P
2
F
P
HIT/CV
1
P
HTT
TT
Fraction of PAR interception (%)
0
100
(d)
(e)
(f )
80
60
HIT/CV
HTT
TT
40
20
1/7/09
1/8/09
1/9/09
1/10/09
1/11/09 1/7/10
1/8/10
1/9/10
1/10/10
1/11/10 1/7/11
1/8/11
1/9/11
1/10/11
1/11/11
Date
Fig. 1. Leaf area index (LAI), pod and leaf area index (PLAI) (a, b, c) and the fraction of photosynthetic active radiation (PAR) intercepted by three types of
canola (d, e, f ) at Kojonup, Western Australia in 2009 (a, d), 2010 (b, e), and 2011 (c, f). F indicates flowering and P represents the beginning of pod filling/the
end of flowering. HIT/CV: hybrid imidazoline-tolerant/hybrid conventional canola; HTT: hybrid TT canola; TT: triazine-tolerant canola. The bars indicate the
l.s.d. values at P < 0.05.
Biomass and seed yield of canola
Crop & Pasture Science
2000
300
(a)
HIT/HCV
HTT
TT
250
SLA (cm2 g–1)
373
Pioneer 46Y78
Hyola 50
CB Jardee
CB-CHYB-125
CB Argyle
Thunder
1600
200
1200
150
800
100
400
50
0
1/7/09
0
2009
2010
2011
Year
1/8/09
1/9/09
1/10/09
1/11/09
1/12/09
1/10/10
1/11/10
1/12/10
1/10/11
1/11/11
1/12/11
1600
1200
1000
(b)
Pioneer 46Y78
Hyola 50
Jardee
Hyola 751 TT
Tornado
Thunder TT
1200
Biomass (g m–2)
Fig. 2. Specific leaf area at the budding stage of hybrid imidazoline-tolerant
(HIT) and conventional (HCV) canola, hybrid TT (HTT) canola, and triazinetolerant (TT) canola, at Kojonup, Western Australia in 2009, 2010, and 2011.
The bars indicate the l.s.d. values at P < 0.05.
800
Biomass (g m–2)
400
HIT/CV
HTT
TT
800
0
1/7/10
600
1/8/10
1/9/10
1600
400
(c)
200
1200
0
0
100
200
300
400
500
600
Pioneer 46Y78
Hyola 50
CB Jardee
Hyola 751 TT
Thunder
Tornado
700
Intercepted PAR (MJ m–2)
Fig. 3. Relationship between cumulative biomass from the vegetative
stage to the begining of pod filling and cumulative photosynthetic active
radiation (PAR) intercepted by the canola canopy. Fitted regression lines are:
HIT/CV, y = 44.0 + 1.85x, R2 = 0.93; HTT, y = 30.6 + 1.58x, R2 = 0.94;
TT, y = 3.6 + 1.42x, R2 = 0.94.
Crop growth rates, biomass accumulation and its partitioning
Biomass increased linearly from budding (early August) to
podding (the beginning of October) (Fig. 4). Canola continued
to accumulate significant amounts of biomass after podding in
2009 and 2011 but not in 2010 when the crop was under terminal
water stress. Significant CGR and biomass differences occurred
mainly between the canola HT groups whereas the differences
within a particular group were small. At the budding stage,
hybrid IT/CV canola accumulated twice (P < 0.001) as much
biomass as TT canola as a result of a greater LAI (Fig. 1)
and in 2011 had higher (P < 0.001) photosynthesis rates
(44.5 mmol m–2 s–1 for hybrid IT/CV vs 36.3 mmol m–2 s–1 for
TT canola and 34.1 mmol m–2 s–1 for hybrid TT canola). The mean
CGR for hybrid IT/CV canola was 2.1 g m–2 day–1 compared with
800
400
0
1/7/11
1/8/11
1/9/11
Date
Fig. 4. Biomass accumulation of hybrid IT/CV (Pioneer 46Y78 and
Hyola 50), hybrid TT (CB Jardee, CB-CHYB-125, and Hyola 751TT) and
TT (CB Argyle, Thunder and Tornado) canola at Kojonup, Western Australia.
The triangles on the x-axis indicate flowering time for different cultivars.
The bars indicate the l.s.d. values at P < 0.05.
1.2 g m–2 day–1 for TT canola (Fig. 5). During the linear phase of
the growth, linear regression of biomass against days after sowing
with the canola type as a group factor accounted for 94% of
variance and showed significant differences in the slopes (CGR)
between the canola types. The largest difference was observed
between the hybrid IT/CV and TT canola. The CGR of hybrid
374
Crop & Pasture Science
H. Zhang and S. Flottmann
(a)
15
n.s.
10
5
Crop growth rate (g m–2 day–1)
0
(b)
Hybrid/CV
15
Hybrid TT
TT
10
5
n.s.
0
(c)
15
10
n.s.
5
0
Sow-buding Budding-Fl
Fl-Podding Seed filling
Fig. 5. Crop growth rate at different growth stages for hybrid IT/CV, hybrid
TT and open-pollinated TT canola in (a) 2009, (b) 2010, and (c) 2011 at
Kojonup, Western Australia. Sow: sowing; FL: flowering.
IT/CV canola (12.1 g m–2 day–1) was 32% higher than that of TT
canola (9.2 g m–2 day–1). By podding, hybrid IT/CV canola had
the highest biomass, followed by hybrid TT and TT canola
(Fig. 4). The superiority of hybrid IT/CV canola in biomass
production was maintained at maturity. Hybrid IT/CV canola
had significantly higher (P < 0.01) biomass than hybrid TT and
TT canola whereas hybrid TT canola had higher biomass than TT
canola in 2011 and similar biomass in 2009 and 2010 (Table 3).
Biomass at maturity was highly correlated with CGR from
budding to podding (Fig. 6a) and to LAI at vegetative stage
(R2 = 0.84–0.93, P < 0.05) and at flowering (R2 = 0.78–0.98,
P < 0.05) (data not shown), but decreased with increased SLA
at budding (Fig. 6b). These relationships were insignificant in the
drought year 2010.
The pattern of biomass partitioning differed between hybrid
IT/CV and TT canola. At the budding stage, hybrid IT/CV canola
allocated significantly higher proportion of biomass into leaves
(0.75 vs 0.69) and less into stem (0.25 vs 0.30) than TT canola
(Fig. 7). By flowering, the proportion of stem and leaves in all
canola was reversed compared with that at budding. Hybrid IT/
CV canola allocated more biomass into stem (0.74 vs 0.66) and
less in leaves (0.30 vs 0.33) than TT canola as the investment in
stems increased. At the beginning of pod filling, hybrid IT/CV
canola had allocated more (0.64 vs 0.57) biomass to stem than TT
canola, less (0.02 vs 0.05) to leaves and to pods (0.30 vs 0.34).
By maturity, hybrid IT/CV canola had higher proportion of
biomass in stem and pod wall than TT canola (0.72 vs 0.69),
resulting in a lower proportion in seed (0.28 vs 0.31) than TT
canola. The HI was also affected by the temperature during podfilling period. All genotypes had higher HI in the cooler year
(2011) than in the warmer years (2009 and 2010) (Table 3).
Seed yield
The effect of cultivars within the HT groups was not significant
and the effect of the HT types was highly significant for seed yield,
biomass, the number of pods and seeds m–2, and HI. Therefore,
the results presented here focus on the difference between the
canola HT groups and its interaction with the growing-season
conditions (Year). On average, hybrid IT/CV canola produced
344 g m–2, higher (P < 0.01) than OP TT canola (265 g m–2)
and hybrid TT canola (303 g m–2) (Table 3). However, the
performance of canola depended on the growing-season
conditions. In 2009 and 2011, the average yield of two hybrid
IT/CV canola cultivars was 374 g m–2 and 407 g m–2, consistently
higher than that of two OP TT canola cultivars (279 and
284 g m–2) by 34% and 43%, respectively. No yield difference
was observed among three HT groups in 2010 when the
low rainfall severely constrained the performance of hybrid
IT/CV and hybrid TT canola (Table 3). Hybrid TT canola outperformed TT canola only in 2011 and hybrid IT/CV canola yield
better than hybrid TT canola only in 2009. The higher yield for
hybrid IT/CV canola was mainly from higher biomass (1443 vs
951 g m–2 in 2009 and 1217 vs 800 g m–2 in 2011), more pods m–2
(7010 vs 5650 in 2009 and 7111 vs 3900 in 2011), and seeds m–2
(115 vs 89 103 in 2009 and 104 vs 68 103 in 2011). The higher
yield was not from HI, seeds pod–1 and seed weight (Table 3).
In fact, hybrid IT/CV canola has lower (P < 0.05) HI than TT
canola in all 3 years. Compared with 2009 and 2011, no yield
difference between the three HT groups of canola in 2010
suggests there was significant interaction between the year and
the HT groups on seed yield, the number of pods and seeds m–2
(Table 3).
Determinants of yield and yield components
The CGR during the linear growth phase and biomass at different
growth stages significantly affected the two most important yield
components: the number of pods m–2 and seeds m–2 (Fig. 8).
In 2009 and 2011, both traits were positively correlated with
biomass at the vegetative stage (r2 = 0.88–0.95, P < 0.01), at
podding (r2 = 0.47, P < 0.05) (Fig. 8a), and at maturity
(r2 = 0.84–0.94, P < 0.05). Seed yield (Fig. 8c) and the number
of pods m–2 (Fig. 8b) and seeds m–2 (Fig. 8d) were positively
correlated with CGR during the linear growth phase in 2009 and
2011. Again, these relationships were not significant in 2010.
Biomass and seed yield of canola
Crop & Pasture Science
375
Table 3. Seed yield, biomass, harvest index (HI), pods m–2, seed pod–1, and thousand-seed weight (TSW) and oil content of
canola groups in Kojonup, Western Australia in 2009, 2010, and 2011
HT groups
Yield
(g m–2)
Biomass
(g m–2)
HIT/HCV
HTT
TT
Mean
l.s.d. (0.05)
374
295
279
316
48.5
1423
1053
951
1142
127
HIT/HCV
HTT
TT
Mean
l.s.d. (0.05)
253
244
233
243
n.s.
HIT/HCV
HTT
TT
Mean
l.s.d. (0.05)
HIT/HCV
HTT
TT
l.s.d. (0.05) for Year
l.s.d. (0.05) for HT
l.s.d. (0.05) for Year HT
1600
HI
Pods
m–2
Seeds
pod–1
Seeds
103 m–2
TSW
(g)
Oil
(%)
2009
0.26
0.28
0.29
0.28
0.021
7010
4949
4995
5651
731
16.0
19.0
18.0
18.0
2.2
115.2
95.8
89
100
20.8
3.27
3.09
3.2
3.18
n.s.
42.4
40.9
42.3
41.8
0.50
996
843
802
881
89
2010
0.25
0.29
0.29
0.28
0.015
5191
4648
3841
4650
578
15.0
16.0
17.0
16.0
n.s.
71.5
72.1
62.5
68.7
n.s.
3.54
3.43
3.76
3.58
0.22
43.5
41.3
43.8
42.9
1.30
406
370
284
353
39.6
1277
1028
800
1035
114
2011
0.32
0.36
0.35
0.34
0.016
7111
5593
3899
5534
822
15.0
17.5
17.8
16.8
2.1
104
94
68
89
10.7
3.92
3.93
4.25
4.03
0.14
47.0
45.8
47.5
46.8
0.50
344
303
265
23.6
23.6
41
1232
976
851
65
65
112
3-year average
0.28
6437
0.31
5064
0.31
4245
0.013
459
0.013
459
n.s.
795
15.5
17.7
17.4
1.3
1.3
n.s.
97
87
73
8.2
8.2
14.2
3.58
3.48
3.73
0.17
0.17
n.s.
44.3
42.6
44.5
0.5
0.5
n.s.
(a)
(b)
r 2 = 0.76
2
r = 0.87
1400
2009
Biomass (g m–2)
2010
r 2 = 0.96
2011
1200
r 2 = 0.80
r 2 = 0.45
1000
800
r 2 = 0.90
600
4
8
12
Crop growth rate (g m–2 day–1)
16
100
150
200
250
300
SLA (cm g–2 )
Fig. 6. Relationships between biomass at maturity and (a) crop growth rate during the linear growth phase
and (b) specific leaf area (SLA) at budding stage.
Yield was related to biomass at maturity (r2 = 0.87–0.96,
P < 0.01) (Fig. 9a), budding (r2 = 0.62–0.71, P < 0.05),
P < 0.05),
and
podding
flowering
(r2 = 0.56–0.86,
2
(r = 0.83–0.99, P < 0.01). As expected, yield was positively
correlated with the number of pods and seeds per unit area
(Fig. 9c, d) although these relationships were insignificant in
the drought Year 2010. However, HI tended to decrease with
increased biomass, in particular in 2009 (r2 = 0.74, P < 0.05)
376
Crop & Pasture Science
H. Zhang and S. Flottmann
Budding
Flowering
1.00
0.75
a a
a
b
a
b
Hybrid IT/CV
Hybrid TT
0.50
TT
a a
Proportion
0.25
b
a
a
b
0
Podding
Maturity
1.00
a
0.75
b b
a
b b
0.50
b b
a
a
b b
0.25
a a
b
0
Leaf
Stem
Pod
S&P wall
Seeds
Leaf
Stem
Pod
S&P wall
Seeds
Plant organ
Fig. 7. Biomass partitioning into leaf, stem, pod, stem and pod wall (S and P wall) and seeds for hybrid Clearfield/
Conventional (IT/CV), hybrid and open-pollinated triazine-tolerant (TT) canola at budding, flowering, podding and
maturity. The data are the means of 3 years. Means with different letters are significantly different between the HT
groups at P = 0.05.
(Fig. 9b). The yield gains from higher biomass at maturity were
significantly greater than the yield loss from lower HI when
biomass was higher. More importantly, neither did the number
of seeds per pod decrease with increased number of pods m–2
within the observed range (4500–7500 pods m–2) (Table 3), nor
seed weight decreased with increased seed number.
Discussion
The key outcome of this study has been to demonstrate the
importance of high biomass accumulation for each phenological
phase to achieve high seed yield in the high-yielding environment
of southern Australia. This is reflected in the Australian canola
breeding program, which has focussed on using phenology
differences to match the length of growing seasons in different
environments and more recently hybrid vigour to accumulate high
biomass. Compared with elite OP TT canola, hybrid IT/CV canola
produced significantly higher biomass, set up more pods m–2 and
consequently produced 38% higher seed yield under favourable
conditions. The higher seed yield in hybrid IT/CV canola was
attributed solely to the high biomass production as lower HI were
observed compared with TT canola. This finding is similar to the
findings for hybrid rice and maize in which high biomass has
been reported to be the main driver of yield under favourable
conditions (Tollenaar 1991; Laza et al. 2003; Bueno and Lafarge
2009). It is known that the TT trait has ~20% yield penalty
as a result of reduced radiation efficiency (Beversdorf et al.
1988; Robertson et al. 2002). Hybrid IT/CV canola grew more
vigorously, allocated more biomass into leaves early in the growing
season, and produced twice as much biomass as OP TT canola at
the budding stage. Furthermore, hybrid canola maintained its
biomass superiority throughout the whole crop cycle, resulting
in 50–100% more biomass at flowering and 40% more biomass at
maturity than OP TT canola and converted its biomass superiority
into a seed yield advantage under favourable growth conditions.
Higher biomass production of hybrid IT/CV canola was due to
a combination of the larger leaf area (Fig. 1), lower SLA (thick
leaves) (Fig. 2), higher radiation-use efficiency (Fig. 3), and higher
photosynthetic rates than OP TT and hybrid TT canola early in the
growing season and the greater pod area later in the growing season.
Biomass and seed yield of canola
10.0
Crop & Pasture Science
(a)
r 2 = 0.79
(c)
r 2 = 0.97
r 2 = 0.71
7.5
r 2 = 0.37
5.0
Yield (g m–2)
Pod number (×103 m–2)
400
300
r 2 = 0.015
200
2009
2009 & 2011
2.5
100
2010
2010
2011
0
0
250
500
750
1000
4
1250
Biomass at podding (g m–2)
(b)
120
r 2 = 0.47
7.5
r 2 = 0.29
5.0
2009 & 2011
2.5
8
12
16
Crop growth rate (g m–2 day–1)
2010
Seed number (×103 m–2)
10.0
Pod number (×103 m–2)
377
r 2 = 0.71
(d)
r 2 = 0.15
80
40
2009 & 2011
2010
0
0
4
8
12
16
Crop growth rate (g m–2 day–1)
4
8
12
16
Crop growth rate (g m–2 day–1)
Fig. 8. Relationships among pod number, biomass at podding, seed number, yield and crop growth rate during
the linear growth stage (budding to podding) in 2009, 2010 and 2011.
The ability to maintain the growth superiority from early vigour
in hybrid canola is in contrast to wheat in which the advantage of
early vigour declined as the crop developed (Whan et al. 1991;
Botwright et al. 2002). As the major difference in biomass was
developed before pod filling, agronomic management strategies
such as N application early in the growing season may also be
beneficial to stimulate early growth.
The potential traits to support high biomass accumulation
include early vigour, greater LAI, thicker leaves at vegetative
growth and budding stages, and greater CGR from budding
to podding. The high correlations of biomass at maturity to
biomass at early growth stages suggest that early vigour is an
essential trait for higher final biomass and yield in the
HRZ. Normalised difference vegetation index is a surrogate
for biomass accumulation that can be recorded rapidly and
repeatedly with inexpensive equipment (Cowley et al. 2014).
Breeders may use this non-destructive method to select for
vigorous genotypes that are more likely to have higher grain
yields. Longer vegetative growth period and late flowering could
be used as selection criteria for high biomass in canola breeding
program in the high-yielding environments (Riffkin et al. 2012).
This study showed that hybrid IT/CV canola maintained not only
greater leaf area but also thicker leaves than TT canola. The
thicker leaves (lower SLA) in hybrid IT/CV canola at the budding
stage were also related to the higher biomass and yield.
The thicker leaves usually have more ribulose biphosphate
carboxylase per unit surface area and a greater photosynthetic
capacity per unit area (Evans and Poorter 2001; Poorter et al.
2009). Our results are in contrast to hybrid rice, in which greater
biomass was achieved by higher SLA (thinner leaves) (Laza et al.
2003; Peng and Khush 2003). This difference may be related to
the difference between dicot and monocot species.
The CGR and biomass accumulation from budding to
podding are the driving forces in determining two major yield
components (the number of pods and seeds) and therefore yield.
This is in agreement with other studies on winter oilseed crops
in temperate European climate. For example, the number of
seeds m–2 is determined during a critical phase for pod and
seed abortion lasting ~3008C after mid-flowering, which is
equivalent to 20–35 days in the field situation (Mendham et al.
1981). Habekotté (1993) showed that potential and actual pod
density were linearly related to cumulative biomass production
of the crop at the onset of flowering and at podding. More
importantly, we found that the number of pods m–2, seed m–2
378
Crop & Pasture Science
H. Zhang and S. Flottmann
0.40 (b)
r 2 = 0.87
(a)
r 2 = 0.96
400
300
HI
Yield (g m–2)
0.35
r 2 = 0.29
200
2009
r 2 = 0.39
0.30
0.25
r 2 = 0.61
2010
r 2 = 0.72
2011
0.20
100
400
800
1200
400
1600
800
Biomass (g m–2)
r 2 = 0.99
(c)
1200
1600
Biomass (g m–2)
(d)
400
r 2 = 0.82
400
r 2 = 0.73
Yield (g m–2)
Yield (g m–2)
r 2 = 0.72
300
r 2 = 0.79
200
100
300
r 2 = 0.52
200
100
2
4
6
8
10
Pod number (×103 m–2)
25
50
75
100
125
Seed number (×103 m–2)
Fig. 9. Relationship between (a) seed yield, (b) harvest index (HI) and biomass at maturity, (c) between yield
and pod, and (d) seed number in canola in 2009, 2010, and 2011.
and seed yield were correlated with biomass at the vegetative
stage, highlighting the importance of acquiring resources early in
the season that will later be turned into yield. Therefore, selecting
for fast accumulation of biomass should improve yield potential
for the high-yielding environments.
The concern with increased biomass is whether this increase
would lead to a decrease in HI, resulting in no yield advantage or
even lower yields, in particular under the terminal water-stressed
conditions. Hybrid IT/CV canola maintained its superiority in
achieving high biomass in all conditions, and produced the
highest yield in the favourable growing conditions (2009 and
2011), but tended to have lower HI than other types. This suggests
that there is trade-off between higher biomass and lower HI. The
trade-off favours achieving high biomass because the yield gain
from higher biomass was significantly greater than the yield loss
from a small change of HI resulting from the higher biomass.
In maize and rice, the increase of yield from hybrid cultivars was
attributed to the increase of both biomass and HI (Virmani et al.
1982; Tollenaar 1991; Bueno and Lafarge 2009). It is surprising
to note that hybrid canola did not increase HI compared with TT
canola in our study. In wheat and rice, an increase in HI has been
recognised as a major factor contributing to high yield since
the green revolution (Peng et al. 1999; Reynolds et al. 1999).
Breeding selection for hybrid maize in the United States had
already brought maize plants to an approximation of the 40–50%
HI (Duvick 2005). The lower HI or at best similar HI in hybrid
IT/CV canola to TT canola might be related to the 25-cm-taller
plants of the hybrids and high proportion of DM was allocated
in stem at maturity (Fig. 7). In maize, plant height of hybrids
has been reduced through the breeding cycles (Duvick 2005).
This suggests that it may be possible to improve HI by further
reducing the height of hybrid canola through breeding.
The failure of hybrid IT/CV canola to convert higher biomass
into yield advantage in dry conditions suggests that there were
significant HT group by environment interactions in the yield
responses to the environment conditions. The deciles for 2010
rainfall were 2 (only 2 in 10 years receiving less than 267 mm
growing-season rainfall) and therefore unlikely to represent the
HRZ. However, the rainfall received in 2010 is typical for the
traditional wheatbelt in the low rainfall area and therefore
the results may have implications for this region. Lack of yield
advantage of hybrid IT/CV and hybrid TT canola over TT canola
Biomass and seed yield of canola
in 2010 indicates that the traits (early vigour, greater CGR and
high biomass) related to high yield in the HRZ might not be useful
for drought- and heat-prone low rainfall areas of south-western
Australia. Different traits to those in the HRZ are needed for low
rainfall areas and dry years in the HRZ. Early flowering is the
foremost important trait to achieve stable and high yield in the low
rainfall areas by escaping drought (Zhang et al. 2013). Higher
water-soluble carbohydrates stored in stem and pod wall made
higher contribution to yield under the drought condition than
under the favourable conditions (H. Zhang and S. Flottmann,
unpubl.). In rice and maize, hybrids produce higher yield not only
under favourable growing conditions, but also show yield
advantage under drought environments and high temperature
environments (Tollenaar and Wu 1999). This study showed
that the superiority of hybrid was constrained by the water
availability and that no yield advantage was observed in the
dry season (2010). In fact, excessive biomass production before
pod setting might have exacerbated water stress during seed
filling and reduced HI. In low rainfall areas, restraining crop
growth before podding and conserving water for seed filling could
improve HI of canola.
Conclusion
In the high-yielding environment such as the HRZ of southern
Australia, yield of canola was mainly determined by biomass
accumulation and associated with more pods and seeds and
to a less extent by HI, seeds per pod and seed weight. Hybrid
canola grew more vigorously, allocated more biomass into leaves,
produced higher biomass in vegetative stage from hybrid
heterosis. More importantly, it maintained its superiority in
biomass throughout the whole crop cycle and enabled hybrid
canola to produce more pods and seeds and therefore higher seed
yield. Therefore, it is suggested that efforts to increase yield in
canola should be directed to increase biomass production during
the period of pod and seed setting through both breeding for early
vigour, thick leaves, fast CGR and agronomic management early
in the growing season in the high-yielding environment.
Acknowledgements
The authors would like to acknowledge research funding support (Project
CSP000128) from the Australian Grains Research and Development
Corporation and the Commonwealth Scientific and Industrial Research
Organisation. We acknowledge Dr Jens Berger for his comments on an
early draft of the paper and Mr Sam Henty, Mr Marthin Slabber, and
Mr Jacob Joyce for help in collecting field data, and Vince Lambert for
managing the field experiments and Peter and Anna Macleay, Ben and
Emmalyn Webb for their generous provision of the experimental sites.
Pioneer Hybrid, Pacific Seeds and Canola Breeders Western Australia are
acknowledged for providing seeds used in this work.
References
Angus JF, Vanherwaarden AF, Howe GN (1991) Productivity and break crop
effects of winter-growing oilseeds. Australian Journal of Experimental
Agriculture 31, 669–677. doi:10.1071/EA9910669
Berry PM, Spink JH (2006) A physiological analysis of oilseed rape
yields: past and future. The Journal of Agricultural Science 144,
381–392. doi:10.1017/S0021859606006423
Beversdorf WD, Hume DJ, Donnellyvanderloo MJ (1988) Agronomic
performance of triazine-resistant and susceptible reciprocal spring
Crop & Pasture Science
379
canola hybrids. Crop Science 28, 932–934. doi:10.2135/cropsci1988.
0011183X002800060012x
Botwright TL, Condon AG, Rebetzke GJ, Richards RA (2002) Field
evaluation of early vigour for genetic improvement of grain yield in
wheat. Australian Journal of Agricultural Research 53, 1137–1145.
doi:10.1071/AR02007
Brandt SA, Malhi SS, Ulrich D, Lafond GR, Kutcher HR, Jonston AM (2007)
Seeding rate, fertilizer level and disease management effects on hybrid
versus open pollinated canola (Brassica napus L.). Canadian Journal of
Plant Science 87, 255–266. doi:10.4141/P05-223
Brewin DG, Malla S (2012) The consequences of biotechnology: a broad
view of the changes in the Canadian canola sector, 1969 to 2012.
AgBioForum 15, 257–275.
Brill RD, Jenkins ML, Gardner MJ, Lilley JM, Orchard BA (2016) Optimising
canola establishment and yield in south-eastern Australia with hybrids
and large seed. Crop & Pasture Science 67, 409–418.
Bueno CS, Lafarge T (2009) Higher crop performance of rice hybrids than
of elite inbreds in the tropics: 1. Hybrids accumulate more biomass
during each phenological phase. Field Crops Research 112, 229–237.
doi:10.1016/j.fcr.2009.03.006
Cowley RB, Luckett DJ, Moroni JS, Moroni JS, Diffey S (2014) Use of
remote sensing to determine the relationship of early vigour to grain
yield in canola (Brassica napus L.) germplasm. Crop & Pasture Science
65, 1288–1299. doi:10.1071/CP14055
Cullis BR, Smith AB, Beeck CP, Cowling WA (2010) Analysis of yield
and oil from a series of canola breeding trials. Part II. Exploring variety
by environment interaction using factor analysis. Genome 53, 1002–1016.
doi:10.1139/G10-080
Duvick DN (2005) The contribution of breeding to yield advances in maize
(Zea mays L.). Advances in Agronomy 86, 83–145. doi:10.1016/S00652113(05)86002-X
Evans JR, Poorter H (2001) Photosynthetic acclimation of plants to growth
irradiance: the relative importance of specific leaf area and nitrogen
partitioning in maximizing carbon gain. Plant, Cell & Environment 24,
755–767. doi:10.1046/j.1365-3040.2001.00724.x
Farré I, Robertson MJ, Walton GH, Asseng S (2002) Simulating phenology
and yield response of canola to sowing date in Western Australia using
the APSIM model. Australian Journal of Agricultural Research 53,
1155–1164. doi:10.1071/AR02031
Gunasekera CP, Martin LD, Siddique KHM, Walton GH (2006) Genotype
by environment interactions of Indian mustard (Brassica juncea L.) and
canola (B. napus L.) in Mediterranean-type environments I. Crop growth
and seed yield. European Journal of Agronomy 25, 1–12. doi:10.1016/
j.eja.2005.08.002
Habekotté B (1993) Quantitative-analysis of pod formation, seed set and
seed filling in winter oilseed rape (Brassica napus L.) under field
conditions. Field Crops Research 35, 21–33. doi:10.1016/0378-4290
(93)90133-8
Harker KN, Blackshaw RE, Kirkland KJ, Derksen DA, Wall D (2000)
Herbicide-tolerant canola: weed control and yield comparisons in
western Canada. Canadian Journal of Plant Science 80, 647–654.
doi:10.4141/P99-149
Hocking PJ, Kirkegaard JA, Angus JF, Gibson AH, Koetz EA (1997a)
Comparison of canola, Indian mustard and Linola in two contrasting
environments. 1. Effects of nitrogen fertilizer on dry-matter production,
seed yield and seed quality. Field Crops Research 49, 107–125.
doi:10.1016/S0378-4290(96)01063-5
Hocking PJ, Randall PJ, DeMarco D (1997b) The response of dryland canola
to nitrogen fertilizer: partitioning and mobilization of dry matter and
nitrogen, and nitrogen effects on yield components. Field Crops Research
54, 201–220. doi:10.1016/S0378-4290(97)00049-X
Justes E, Denoroy P, Gabrielle B, Gosse G (2000) Effect of crop nitrogen
status and temperature on the radiation use efficiency of winter oilseed
rape. European Journal of Agronomy 13, 165–177. doi:10.1016/S11610301(00)00072-1
380
Crop & Pasture Science
H. Zhang and S. Flottmann
Kirkegaard JA, Sprague SJ, Hamblin PJ, Graham JM, Lilley JM (2012)
Refining crop and livestock management for dual-purpose spring canola
(Brassica napus). Crop & Pasture Science 63, 429–443. doi:10.1071/
CP12163
Kirkegaard JA, Lilley JM, Brill RD, Sprague SJ, Fettell NA, Pengilley GC
(2016) Re-evaluating sowing time of spring canola (Brassica napus L.) in
south-eastern Australia—how early is too early? Crop & Pasture Science
67, 381–396.
Laza RC, Peng SB, Akita S, Saka H (2003) Contribution of biomass
partitioning and translocation to grain yield under sub-optimum
growing conditions in irrigated rice. Plant Production Science 6,
28–35. doi:10.1626/pps.6.28
Mendham NJ, Shipway PA, Scott RK (1981) The effects of delayed sowing
and weather on growth, development and yield of winter oil-seed rape
(Brassica napus). The Journal of Agricultural Science 96, 389–416.
doi:10.1017/S002185960006617X
Peng SB, Khush GS (2003) Four decades of breeding for varietal
improvement of irrigated lowland rice in the international rice research
institute. Plant Production Science 6, 157–164. doi:10.1626/pps.6.157
Peng S, Cassman KG, Virmani SS, Sheehy J, Khush GS (1999) Yield
potential trends of tropical rice since the release of IR8 and the
challenge of increasing rice yield potential. Crop Science 39,
1552–1559. doi:10.2135/cropsci1999.3961552x
Poorter H, Niinemets U, Poorter L, Wright IJ, Villar R (2009) Causes
and consequences of variation in leaf mass per area (LMA): a metaanalysis. New Phytologist 182, 565–588. doi:10.1111/j.1469-8137.2009.
02830.x
Reynolds MP, Rajaram S, Sayre KD (1999) Physiological and genetic
changes of irrigated wheat in the post-green revolution period and
approaches for meeting projected global demand. Crop Science 39,
1611–1621. doi:10.2135/cropsci1999.3961611x
Richards RA, Thurling N (1978) Variation between and within Species of
Rapeseed (Brassica campestris and B. napus) in response to drought
stress. 2. Growth and development under natural drought stresses.
Australian Journal of Agricultural Research 29, 479–490. doi:10.1071/
AR9780479
Riffkin P, Potter T, Kearney G (2012) Yield performance of late-maturing
winter canola (Brassica napus L.) types in the high rainfall zone of
southern Australia. Crop & Pasture Science 63, 17–32. doi:10.1071/
CP10410
Robertson MJ, Holland JF, Cawley S, Potter TD, Burton W, Walton GH,
Thomas G (2002) Growth and yield differences between triazine-tolerant
and non-triazine-tolerant cultivars of canola. Australian Journal of
Agricultural Research 53, 643–651. doi:10.1071/AR01159
Seymour M, Kirkegaard JA, Peoples MB, White PF, French RJ (2012)
Break-crop benefits to wheat in Western Australia – insights from over
three decades of research. Crop & Pasture Science 63, 1–16. doi:10.1071/
CP11320
Sylvester-Bradley R, Makepeace RJ (1984) A code for stages of development
in oilseed rape (Brassica napus L.). Aspects of Applied Biology 6,
399–419.
Thurling N (1974) Morphophysiological determinants of yield in rapeseed
(Brassica campestris and Brassica napus). 1. Growth and morphological
characters. Australian Journal of Agricultural Research 25, 697–710.
doi:10.1071/AR9740697
Tollenaar M (1989) Genetic-improvement in grain-yield of commercial
maize hybrids grown in Ontario from 1959 to 1988. Crop Science 29,
1365–1371. doi:10.2135/cropsci1989.0011183X002900060007x
Tollenaar M (1991) Physiological basis of genetic improvement of maize
hybrids in Ontario from 1959 to 1988. Crop Science 31, 119–124.
doi:10.2135/cropsci1991.0011183X003100010029x
Tollenaar M, Wu J (1999) Yield improvement in temperate maize is
attributable to greater stress tolerance. Crop Science 39, 1597–1604.
doi:10.2135/cropsci1999.3961597x
Virmani SS, Aquino RC, Khush GS (1982) Heterosis breeding in rice
(Oryza sativa L.). Theoretical and Applied Genetics 63, 373–380.
doi:10.1007/BF00303911
Whan BR, Carlton GP, Anderson WK (1991) Potential for increasing
early vigor and total biomass in spring wheat. 1. Identification of
genetic improvements. Australian Journal of Agricultural Research
42, 347–361. doi:10.1071/AR9910347
Wright PR, Morgan JM, Jessop RS, Cass A (1995) Comparative adaptation
of canola (Brassica napus) and Indian mustard (Brassica juncea)
to soil-water deficits – Yield and yield components. Field Crops
Research 42, 1–13. doi:10.1016/0378-4290(95)00013-G
Zhang H, Turner NC, Poole ML, Simpson N (2006) Crop production in
the high rainfall zones of southern Australia – potential, constraints
and opportunities. Australian Journal of Experimental Agriculture 46,
1035–1049. doi:10.1071/EA05150
Zhang H, Berger JD, Milroy S (2013) Genotype environment interaction
studies highlight the role of phenology in specific adaptation of canola
(Brassica napus) to contrasting Mediterranean climates. Field Crops
Research 144, 77–88. doi:10.1016/j.fcr.2013.01.006
Zhang H, Berger JD, Seymour M, Brill R, Herrmann C, Quinlan R, Knell G
(2016) Relative yield and profit of Australian hybrid compared with openpollinated canola is largely determined by growing-season rainfall. Crop
& Pasture Science 67, 323–331.
www.publish.csiro.au/journals/cp