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PAPER PRESENTED AT INTERNATIONAL WORKSHOP
ON INCREASING WHEAT YIELD POTENTIAL,
CIMMYT, OBREGON, MEXICO, 20–24 MARCH 2006
Genetic progress in yield potential in wheat: recent
advances and future prospects
M. J. FOULKES
1
*, J. W . S N A PE
2
,V.J.SHEARMAN
1
, M.P. REYNOLDS
3
,
O. GAJU
1
AND R. SYLVESTER-BRADLEY
4
1
Division of Agricultural & Environmental Sciences,University of Nottingham,Sutton Bonington Campus,
Leicestershire LE12 5RD,UK
2
John Innes Centre,Crop Genetics Department,Colney Lane,Norwich NR4 7UH,UK
3
CIMMYT,Int. Apdo.,Postal 6-641,06600,Mexico City D.F.,Mexico
4
ADAS Boxworth R & D Centre,Boxworth,Cambridgeshire CB3 8NN,UK
(Revised MS received 30 November 2006; First published online 25 January 2007)
SUMMARY
Knowledge of the changes in physiological traits associated with genetic gains in yield potential is
essential to improve understanding of yield-limiting factors and to inform future breeding strategies.
Recent advances in genetic yield potential and associated physiological changes in wheat (Triticum
aestivum L.) are reviewed. Genetic gains in yield potential worldwide have been both positively
correlated with harvest index (HI) and above-ground dry matter (AGDM), with more frequent
reports of yield progress associated with biomass since about 1990. It is concluded that an important
aim of future breeding will be the increase of biomass production while maintaining the present
values of HI. In winter wheat recent biomass progress has been positively associated with pre-anthesis
radiation-use efficiency (RUE) and water-soluble carbohydrate (WSC) content of stems at anthesis.
Present results in two doubled-haploid (DH) populations show a positive linear relationship between
stem WSC and grain yield in the UK environment. Results from various investigations worldwide
in recent years have demonstrated that biomass increases have been associated with particular
introductions of alien genes into wheat germplasm, e.g. the 1BL.1RS wheat-rye translocation and
the 7DL.7Ag wheat-Agropyron elongatum translocation. Present results confirm a positive effect of
1BL.1RS on harvest biomass in two DH populations in the UK. The future prospects for identifying
physiological traits to raise yield potential are considered with particular reference to winter wheat
grown in northwestern Europe. It is proposed that optimized rooting traits, an extended stem-
elongation phase, greater RUE, greater stem WSC storage and optimized ear morphology will be
important for breeding progress in yield potential in future years.
INTRODUCTION
The present paper is organized into three sections.
Firstly, the evidence is reviewed for traits associated
with recent genetic gains in yield potential from in-
vestigations of historic sets of cultivars in high-output
environments worldwide. Secondly, the contribution
of particular alien genes to biomass productivity and
grain yield potential in recent years is considered.
Thirdly, the prospects for identifying physiological
traits for raising yield potential in future years are
considered. The paper examines evidence from wheat
investigations worldwide in the initial review sections,
and then focuses on future prospects for identifying
traits for raising yield potential with particular refer-
ence to winter wheat grown in N.W. Europe.
* To whom all correspondence should be addressed.
Email: john.foulkes@nottingham.ac.uk
Journal of Agricultural Science (2007), 145, 17–29. f2007 Cambridge University Press 17
doi:10.1017/S0021859607006740 Printed in the United Kingdom
GENETIC GAINS IN YIELD
POTENTIAL: STUDIES ON HISTORIC
SETS OF CULTIVARS
Knowledge of the changes in physiological traits
associated with genetic gains in yield potential is
essential to improve understanding of yield-limiting
factors and to inform future breeding strategies as
well as more basic research at the molecular level.
Historically, increases in on-farm yields over time in
high-output environments have been attributed about
half to plant breeding and half to husbandry (Silvey
1986; Austin et al. 1989). The genetic gain in yield
potential in high-output wheat systems worldwide
since the mid 1960s has been approximately 1 % per
year, e.g. in Mexico (Waddington et al. 1986 ; Sayre
et al. 1997), Argentina (Abbate et al. 1998), Italy
(Canevara et al. 1994), France (Brancourt-Hulmel et
al. 2003) and the UK (Austin et al. 1989 ; Shearman
et al. 2005). Some studies suggest that wheat yields
in many countries may be asymptotically approach-
ing a ceiling, e.g. Calderini & Slafer (1998). However,
the reasons for this are complex and may involve
agronomic and economic factors, and there remains a
general consensus that raising on-farm yields is likely
to remain an important objective of wheat production
systems in high potential environments.
Studies of the physiological basis of reported
genetic gains since the 1960s have generally shown
grains/m
2
(usually due to grains/ear) and harvest
index (HI) to be most closely correlated with grain
yield; in part contributed by the semidwarf cultivars,
introduced in the 1960s and 1970s (e.g. Waddington
et al. 1986; Austin et al. 1989; Slafer et al. 1994;
Donmez et al. 2001; Brancourt-Hulmel et al. 2003).
Improved HI has been associated with altered par-
titioning in favour of the ear with the introduction of
the Rht semidwarf alleles leading to more grains/m
2
in
the modern cultivars. Higher grain number, in turn,
resulted mainly from more grains/ear, and further
studies have found that semidwarf wheat cultivars
showed more fertile florets per ear as a consequence
of increased assimilate partitioning to the ear during
the pre-flowering period (Fischer 1983 ; Calderini et al.
1995; Miralles et al. 1998). In most studies on historic
sets of cultivars the grains/ear DM ratio (i.e. number
of grains per gram of ear DM) has changed little with
breeding, although genetic progress in this physio-
logical component was reported in one recent inves-
tigation in Argentina (Abbate et al. 1998). Because of
the systematic increase in partitioning, modern culti-
vars have already reached HI that are approaching
a theoretical upper limit of c.0
.62, as estimated by
Austin (1980). It is clear, therefore, that one of the
aims of future genetic improvement should be the
increase of biomass production while maintaining
the present values of biomass partitioning (Slafer &
Andrade 1991; Calderini et al. 1999). In northwestern
Europe, the theoretical upper limit of c.0
.62 for HI
may soon be reached, as there are reports of winter
wheats already achieving close to this, e.g. 0.61 for
Consort in the UK (Spink et al. 2000). HIs for wheat
in the UK are generally of the order of 0.50–0.55,
hence possibilities for small improvements in par-
titioning probably still exist. However, future gains
in yield will increasingly depend upon achieving
greater biomass production, whilst maintaining HI.
Although most investigations of genetic yield
progress have failed to show positive correlation be-
tween above-ground biomass at harvest and grain
yield (e.g. Waddington et al. 1986; Austin et al. 1989;
Slafer et al. 1994; Brancourt-Hulmel et al. 2003), a
few investigations have shown biomass to be posi-
tively associated with yield progress (e.g. Siddique
et al. 1989; Donmez et al. 2001; Shearman et al.
2005). In the investigation of Shearman et al. (2005)
the physiological basis of the improved biomass
production was examined (Table 1). For eight winter
wheat cultivars introduced from 1972 to 1995 in the
UK, there were significant genetic changes over time
and correlations with grain yield for pre-anthesis
RUE (0.012 g/MJ/yr) and WSC content of stems
and leaf sheaths at anthesis (4.6 g/m
2
/yr). Thus yield
progress was based on a combination of improved
growth rate in the pre-anthesis period driving in-
creases in grains/m
2
and a larger source for grain
filling through increases in stem soluble carbo-
hydrate reserves. The results of Shearman et al. (2005)
showed no genetic progress in individual grain
weight, and in general there has been little change in
grain weight with breeding in recent decades world-
wide (e.g. Calderini et al. 1999; Brancourt-Hulmel
et al. 2003).
Alongside improvements in source size pre-anthesis
to increase grains/m
2
, it is possible that breeders
may eventually need to take further steps to increase
assimilate supply post-anthesis to maintain current
rates of genetic gains in yield potential. In this re-
spect, greater accumulation of stem WSC reserves
may be beneficial, providing this is not competitive
with ear growth during the rapid ear-growth phase
(Blum 1998; Shearman et al. 2005). Improvements in
grain sink strength have also been shown to increase
post-anthesis RUE by alleviation of feedback inhi-
bition of photosynthesis in spring wheat in Mexico
(Reynolds et al. 2005). In summary, it seems likely
that breeders may need to select increasingly for traits
which are associated with more or less simultaneous
increases in source and sink size to maintain current
rates of yield progress in high-output environments
in future years. The remaining sections of this paper
initially will review the evidence for contributions of
alien genes to biomass productivity and yield poten-
tial in hexaploid wheat in optimal environments and
then consider the prospects for identifying traits for
raising yield potential in future years with particular
18 M.J.FOULKES ET AL.
reference to winter wheat grown in northwestern
Europe.
CONTRIBUTION OF ALIEN GENES
TO BIOMASS PRODUCTIVITY
AND YIELD POTENTIAL
There is considerable evidence that biomass increases
have been associated with particular introductions
of alien genes into wheat germplasm in the last
decades, e.g. the 1BL.1RS wheat–rye translocation
in Mexico (Villareal et al. 1998) and in the Great
Plains (Carver & Rayburn 1994) and the 7DL.7Ag
wheat–Agropyron elongatum translocation in Mexico
(Reynolds et al. 2001). The 1BL.1RS wheat trans-
location has been extensively used in wheat breeding
programmes across the world (Lukaszewski 1990;
Rajaram et al. 1990). In spring wheat, genes located
on the fragment of rye chromosome have been re-
ported to determine wide adaptation (Rabinovich
1998) and high yield potential (Rajaram et al. 1990 ;
Villareal et al. 1995, 1998). In winter wheat, reported
effects in optimal environments depend on genetic
background, but are generally either positive or
neutral on biomass and grain yield. Rajaram et al.
(1983), examining a range of cultivars worldwide, re-
ported 1BL.1RS to be positively associated with yield
potential in hard red winter wheats. In a comparison
of near-isogenic lines in several genetic backgrounds,
Carver & Rayburn (1994) showed an overall yield
benefit with 1BL.1RS of 9–10 % associated with a
biomass increase of 11–12 %. Moreno-Sevilla et al.
(1995a) similarly evaluating progeny of a cross
Siouxland/Ram in Nebraska found the 1BL.IRS
class was 9% higher yielding than the 1B class. On
the other hand, McKendry et al. (1996) and Moreno-
Sevilla et al. (1995b) found no consistent differences
in biomass or grain yield between 1BL.1RS and 1B
lines.
Winter wheat cultivars incorporating 1BL.1RS
were first released in the UK in the late 1980s, e.g.
Haven released in 1990. The translocation was orig-
inally introgressed to incorporate race-specific disease
resistance genes on the rye fragment. Since then most
of the disease resistances have been overcome, but
there may be a residual benefit of the translocation
for yield potential. To date, 1BL.1RS has been re-
stricted to feed and biscuit-making wheats, since
genes determining high-molecular-weight glutenin
sub-units required in bread-making processes are
located on the short arm of chromosome 1B. The
effects of the 1BL.1RS translocation on biomass,
yield and associated physiological traits in three DH
winter wheat populations in the UK have been
quantified in Tables 2 and 3. The results indicated
1BL.1RS increased harvest biomass on average by
55 g/m
2
in a Drake-sibrWelton population (P<
0.05; Table 3), with average increases of 27 and
30 g/m
2
in a BeaverrSoissons and a RialtorSpark
population, respectively, which were not significant
(Table 2). However, in one year out of two, 1BL.1RS
increased biomass by 66 g/m
2
in the BeaverrSoissons
population (P<0.01). In eight of the nine DH
experiments, the effect of 1BL.1RS on grain yield was
neutral, with a positive effect of 1BL.1RS on grain
yield in the BeaverrSoissons population in 2001
(P<0.01).
In the Drake-sibrWelton population, the physio-
logical basis of the increased harvest biomass was
investigated, and found to be associated with greater
pre-anthesis biomass (P<0.01; Table 3). Greater pre-
anthesis biomass was produced from a similar green
canopy area (Table 3) and light extinction coefficient
at ear emergence (data not shown) consistent with
Table 1. Grain yield (850 g DM/kg), harvest above-ground dry matter (AGDM), harvest index (HI), radiation-
use efficiency (RUE
PAR
)from GS31 to GS61, stem water-soluble carbohydrate (WSC)at GS61+75 xCd
(base temp. 0 xC)and flag-leaf area (FLA)for eight winter wheat cultivars. Values represent means across 1997
and 1998; except for FLA measured in 1999, adapted from Shearman et al.(2005)
Cultivar (year of release)
Grain yield
(t/ha)
AGDM
(t/ha) HI
RUE
PAR
(g/MJ)
Stem WSC
(g/m
2
)
Flag-leaf area
GS39 (cm
2
)
Maris Huntsman (1972) 8.75 17.85 0.423 2.33 244 38.2
Avalon (1980) 9.43 17.29 0.467 2.34 255 39.7
Norman (1981) 10.23 17.98 0.484 2.43 268 33.4
Galahad (1983) 9.56 17.05 0.477 2.50 294 29.7
Riband (1989) 10.64 18.35 0.495 2.41 257 24.7
Haven (1990) 11.40 19.31 0.503 2.63 391 25.7
Brigadier (1993) 11.23 18.97 0.504 2.47 284 24.4
Rialto (1995) 11.31 20.14 0.478 2.64 364 22.1
Mean 10.32 18.37 0.479 2.47 295 29.7
S.E.D., D.F.0
.428, 28 0.541, 28 0.0210, 28 0.090, 28 19.2, 28 2.38, 14
Yield potential of wheat 19
the suggestion of Shearman et al. (2005) that pre-
anthesis RUE may be enhanced by 1BL.1RS. Effects
of 1BL.1RS on flowering were small (1 day’s delay
to GS61 with 1BL.1RS, data not shown) and could
not account for the differences in pre-anthesis growth
observed. The observation of 1 day’s delay in flower-
ing with 1BL.1RS is consistent with findings of
McKendry et al. (1996) and Villareal et al. (1994).
The average biomass increase in the Drake-
sibrWelton population of 55 g/m
2
(2.5%) is at the
lower end of the range of positive responses found
in previous studies worldwide of c. 3–10 % in high-
output environments. In general, the additional bio-
mass contributed to straw rather than grain at harvest
in the present experiments. This raises the question of
whether the rye chromosome translocation is useful
for enhancing yield potential in northwestern Europe.
It can be speculated that grain yield in these exper-
iments was predominantly sink limited and the ad-
ditional pre-anthesis biomass may have contributed
to excess source. If so, 1BL.1RS may contribute
greater yield stability rather than yield potential per se
in northwestern Europe.
Other examples of the successful application of
wide crossing to introduce alien genes into the hexa-
ploid genome include the 7DL.7Ag wheat-Agropyron
elongatum translocation which was introduced into
spring wheat at the International Center for Maize
and Wheat Improvement (CIMMYT) in Mexico in
the 1990s (Singh et al. 1998). This translocation was
associated with significant increases in biomass and
yield over control lines. In this case, the physiological
Table 3. Ear number/m
2
, green area index and above-ground DM (AGDM)at ear emergence (GS55); and grain
yield (850 g DM/kg)and AGDM at harvest, for groups* of 1BL.1RS and 1B DH lines derived from Drake-sib
(1BL.1RS)rWelton (1B)at Sutton Bonington, Nottinghamshire. Values at harvest represent means across six
experiments (two experiments in each of 1997/8, 1998/9 and 1999/2000). Values at GS55 represent means across
three of the six experiments (one experiment in each of 1997/8, 1998/9 and 1999/2000). Each field experiment
included three replicates; methods for growth analysis were as described by Gay et al.(1998)
1BL.1RS Group
GS55 Harvest
Ears/m
2
Green area
index AGDM (g/m
2
) AGDM (g/m
2
)
Grain yield
(t/ha)
1BL.1RS 648 7.43 1081 2206 12.44
1B 635 7.27 1047 2151 12.49
¡1BL.1RS VR, D.F.=11
.58 NS 0.99 NS 7.03, P<0.01 14.6, P<0.001 1.3NS
* 11 1BL.1RS and 11 1B lines.
Table 2. Grain yield (850 g DM/kg), harvest above-ground DM (AGDM)and stem water-soluble carbohydrate
(WSC)at GS61+75 xCd (base temp. 0 xC)for groups *,#of 1BL.1RS and 1B DH lines derived from Beaver
(1BL.1RS)rSoissons (1B)and Rialto (1BL.1RS)rSpark (1B)at Gleadthorpe, Nottinghamshire. Each field
experiment included three replicates ; methods for growth analysis were as described by Gay et al.(1998)
2001 2002
Year ¡1BL.1RS
Yearr
¡1BL.1RS1BL.1RS 1B 1BL.1RS 1B
BeaverrSoissons
AGDM (t/ha) 16.02 15.36 12.91 13.04 P<0.05 NS P<0.05
Grain yield (t/ha) 9.10 8.87 7.48 7.63 P<0.05 NS P<0.01
Stem WSC (g/m
2
) n/a$n/a 322 327 NS
RialtorSpark
AGDM (t/ha) 17.10 16.80 n/a n/a NS
Grain yield (t/ha) 9.16 9.14 n/a n/a NS
Stem WSC (g/m
2
) 349 298 n/a n/a P<0.001
* BeaverrSoissons 2001 (harvest year): 16 1BL.1RS and 17 1B lines ; 2002: 24 1BL.1RS and 22 1B lines.
#RialtorSpark 2001: 13 1BL.1RS and 13 1B lines.
$n/a=not assessed.
20 M.J.FOULKES ET AL.
basis of the increases in grain yield (13 %), biomass
(10%) and grains/m
2
(15%) was associated with an
increased partitioning of biomass to ear growth at
anthesis (13%), a higher grain number per ear, and
higher RUE and flag-leaf photosynthetic rate during
grain filling (Reynolds et al. 2001). During the last
few decades, CIMMYT has developed an important
programme of wide crosses and hybridization be-
tween tetraploids (e.g. Triticum turgidum) and wild
diploid relatives (e.g. Triticum tauschii; Mujeeb-Kazi
et al. 1996). Over the last ten years the proportion of
wheat lines that CIMMYT has distributed to rainfed
wheat improvement programmes around the world
using its international nursery systems that are syn-
thetic wheat derivatives has increased from approxi-
mately 0.1to0
.5. These synthetic wheat derivatives
provide a wide range of variability for biotic and
abiotic stresses, e.g. drought resistance (Reynolds
et al., in press). In addition, evidence is building from
CIMMYT’s global wheat yield trials that synthetic
derivatives can also contribute to yield potential
in well-watered, highly productive environments
(Ogbonnaya et al. 2006). In a comparison of
synthetic-derived wheat with recurrent parents, final
biomass and kernel weight were larger under both
irrigated and moisture-stressed conditions (Reynolds
et al., in press). In the last decade, particular in-
trogressions have been associated with higher
yield in newly released winter wheat cultivars in the
UK and include introductions from Triticum durum
var. dicoccoides (e.g. Shamrock and Gulliver). In
summary, it seems likely that pre-breeding to intro-
gress alien genes from wide crosses worldwide into
hexaploid wheat will continue to be an important
avenue for raising yield potential in future years.
GENETIC GAINS IN YIELD
POTENTIAL OF WHEAT:
FUTURE PROSPECTS
In the following discussion, priority traits for yield
potential will be considered with particular reference
to winter wheat grown in northwestern Europe.
However, with the exception of the rooting traits
discussed, it is also anticipated that the identified
traits will be relevant to enhancing yield potential in
spring wheat. As outlined above, in order to raise
yield potential in future years wheat breeders should
aim to increase grains/m
2
, while also increasing as-
similate availability (source) for grain filling. In this
respect, the most effective traits for future progress
will be those that principally increase biomass. When
predicting traits to raise yield potential, it is informa-
tive to refer to benchmarks for key traits for a modern
cultivar in a high potential environment to consider
how traits may need to be altered to achieve yield
gains in future years. Crop development, canopy
growth and dry weights of organs, and components
of grain yield of the modern UK cultivar Consort
(released 1995) were reported by Sylvester-Bradley
et al. (2005) and provide such a set of benchmarks.
These benchmarks were median values for crops
sown in early October and grown under optimal
conditions at three UK sites in each of two seasons.
Taking the median value across the six site/seasons,
Consort reached onset of stem extension (GS31)
on 11 April and flowered (GS61) on 11 June (Fig. 1).
At onset of stem extension, green area index had
reached 2.1 and subsequently peaked at 6.8, eight
days after flag-leaf emergence. Above-ground bio-
mass accumulated at flowering was 11.9 t/ha, in-
creasing to 19.6 t/ha at harvest. The combine grain
yield was 11.1 t/ha (850 g DM/kg) and the HI was
0.52. Stem WSC was maximal at GS61+12 days at
2.4 t/ha declining to 0.4 t/ha by the end of the grain
filling (GS87) on 1 August. Grain yield, therefore,
equated to post-flowering growth plus 0.7 of the
maximal accumulated stem reserves. The yield com-
ponents were 479 ears/m
2
, 48 grains/ear and grain
weight of 43 mg. Therefore, in identifying traits for
further progress, the aim is to raise attainable yields
significantly above the current values of c. 11 t/ha.
The present authors propose that the following
candidate traits offer promise for improving yields in
future years in this way: (i) optimized rooting traits,
(ii) extended duration of stem-elongation period, (iii)
higher RUE, (iv) greater stem soluble carbohydrate
reserves and (v) optimized ear morphology. The fac-
tors involved in extrapolating relationships between
these traits and higher yield potential levels and their
potential usefulness as selection criteria in wheat
breeding programmes are now discussed.
Rooting traits
The genetic gains in above-ground biomass in recent
years, assuming no change in root:shoot ratio or
8
7
6
5
4
3
2
1
0
Jan Feb Mar Apr May Jun Jul Aug
Green area index
25
20
15
10
5
0
Above-ground DM
Ear DM
Green area index
GS61
GS31
Dry matter (t/ha)
Fig. 1. Green area index, above-ground dry matter and ear
dry matter for winter wheat cv. Consort grown at three sites
in harvest years 1996, 1997 and 1998. Values are the median
from the six reference crops.
Yield potential of wheat 21
water-use efficiency, could result in yield being
increasingly limited by ability to access water and/or
N in future years. In northwestern Europe, virtually
all winter wheat is grown under rain-fed conditions.
Therefore, some attention should be focused on op-
timizing rooting systems for most efficient water and
nutrient capture. The present authors have recently
developed a quantitative model examining the effects
of altering rooting traits, including cumulative root
length density distribution with depth and specific
root weight, on water and N uptake (King et al.
2003). A review of the literature was conducted
and two summarizing concepts were adopted, one
describing the cumulative proportion of the total root
length (Y) to any depth (d), with
Y=1xbd(1)
and the other describing the potential resource cap-
ture (PRC) as a proportion of potentially available
resource (water or N over the full season) in relation
to root length density (L
v
) at anthesis, in the same
volume of soil:
PRC=1xek(Lv)(2)
The parameter bdescribes cumulative root distri-
bution with depth, and kis termed the resource
capture coefficient and equals 2.0 for water and
nitrogen.
A sensitivity analysis of water capture to changes
in root traits and water-use efficiency was performed
using the model to assess the potential of root
traits for enhancing below-ground resource capture
(Fig. 2). Likely genetic ranges in the values of the
main parameters were inserted into the model, and
values of yield were calculated in each case. Figure 2
shows economic outputs (£/ha) as affected by water
capture (mm), for variation in respective traits. In
summary, results showed that bhad a profound effect
on water capture, more uniform root distribution
with depth conferring greater water capture. More
even distribution of roots with depth may therefore
be an important component for raising yield potential
in rain-fed winter wheat, and sub-traits favouring
this pattern of rooting might be further investigated.
In addition, specific root weight was shown to be
an influential trait, with lower specific root weight
(thinner roots) leading to improved yields. Although
most winter wheat crops in northwestern Europe are
currently not routinely subjected to drought it seems
likely that in future years, when yield gains must in-
creasingly be driven by biomass and climate change
will bring more frequent summer droughts (Hulme
et al. 2002), that water availability may limit yields
across a wider range of soil types. A similar sensitivity
analysis for N capture revealed that band specific
root weight were again important traits (King et al.
2003).
Duration of stem-elongation phase
Prospects for increasing radiation interception in the
pre-anthesis period to increase grains/m
2
will mainly
be by extending the phase duration in high-output
environments, since interception during the rapid
ear-growth phase is already close to maximal in
modern cultivars (e.g. Foulkes et al. 2001 a). Several
studies in spring wheat point to the potential ad-
vantages of increasing the proportion of thermal
time to anthesis accounted for by the stem-elongation
period to increase anthesis biomass and favour
ear partitioning hence increase grains/m
2
(Reynolds
et al. 2005; Slafer et al. 2005). Extending the stem-
elongation period has similarly been proposed as a
strategy to increase yield potential in winter wheat
(Sylvester-Bradley et al. 2005). Delaying flowering
significantly in northwestern Europe could increase
grain losses associated with late harvesting, so
advancing GS31 while maintaining GS61 is the
preferred pattern of development. An extended stem-
elongation period should simultaneously favour
greater ear biomass, stem WSC and crown root bio-
mass at flowering. There is some empirical evidence
for the importance of growth during this pheno-
phase to UK wheat yields. For example, to explore
the dominant climatic influences on UK wheat
800
600
400
–1·0 0·0 1·0
Normalized range
Return (£/ha)
Fig. 2. The sensitivity of the economic return to variations
in water capture during grain filling, as affected by individual
traits. The value of each trait was adjusted singly over a
range normalized to zero at its mid-point. ¾, root distri-
bution with soil depth (b,0
.955–0.985) ; m, dry matter
partitioning to roots (0.05–0.20) ; %, resource capture
coefficient (k, 1–3 cm
2
); &, shoot dry weight at anthesis
(0.8–1.4 kg/m
2
); ^, specific root weight (2.5–7.5 g/km); ^,
water-use efficiency (4–6 g/m
2
/mm), adapted from King
et al. (2003).
22 M.J.FOULKES ET AL.
yields, Sylvester-Bradley et al. (2005) have related
annual deviations from the linear trend in national
average winter wheat yields between 1978 and 2002
to weather data for each cropping year (September
to August). Of the 12 parameters examined, the
photo-thermal quotient (an index of incident radi-
ation per unit thermal time) during April showed
the second highest correlation with yield (Table 4).
Since onset of stem elongation typically occurs in UK
winter wheat around the beginning of April, this
points to the critical importance of growth during
this phase in determining attainable yield in the UK
environment.
Additional evidence for the relevance of this period
is available from a field investigation in which a
positive linear relationship between the duration of
the stem-elongation period and each of stem WSC
accumulation (P<0.001) and grain yield (P<0.01)
was evident amongst a set of 30 modern UK winter
wheat cultivars examined (Foulkes et al. 1998;
Fig. 3). In summary, there are some crude indicators
that extending the stem-elongation period may offer
an avenue for simultaneously increasing post-anthesis
source and sink size hence yield potential in the
UK environment. Since there has been no systematic
change in phenology with recent UK breeding
(Shearman et al. 2005), this pattern of development
would offer a novel strategy for raising yield poten-
tial. The length of developmental phases depends
on their sensitivity to photoperiod (daylength) and
Table 4. Weather parameters resulting from Window-Pane analysis* of UK wheat yields: parameters having
significant individual correlations with deviations from the linear trend in national average yields over the last 25
or 40 years, adapted from Sylvester-Bradley et al.(2005)
Weather parameter and summary period Start
Duration
(days)
Correlation coefficient (r)
1978–2002 1960–2002
AMinimum temperature (grass) 15 Sep 30 0.39 0.26#
BPhoto-thermal quotient (PTQ) 20 Oct 10 0.55 0.30#
CWind-speed 25 Oct 30 x0.49 x0.32$
DRainfall 4 Nov 20 x0.51 x0.49
ESun 8 Jan 20 0.24 0.32
FWind-speed 8 Jan 20 0.47 0.24$
GMinimum temperature (grass) 2 Apr 30 x0.47 x0.24#
HWind-speed 2 Apr 30 x0.40 x0.39#
IPhoto-thermal quotient (PTQ) 2 Apr 30 0.53 0.10#
JMean temperature 1 May 30 x0.39 x0.20#
KMinimum temperature (air) 21 Jun 20 x0.26 x0.54#
LWind-speed 16 Jul 10 x0.49 x0.35$
* Coakley et al. (1982).
#Omitting four seasons from 1967 to 1970 when data were unavailable.
$Omitting 11 seasons from 1960 to 1970 when wind data were unavailable.
500
400
300
200
100
40
0
45 50 55 60 65
Days from GS31 to GS61
40 45 50 55 60 65
Days from GS31 to GS61
(a)
Stem WSC (g/m2)
Grain yield (t/ha)
11·5
11·0
10·5
10·0
9·5
(b)
Fig. 3. Linear regression of duration of stem-elongation
period on (a) water soluble carbohydrate in stems and
leaf sheaths at GS61+5d (y=8.53xx171.26, R
2
=0.28,
D.F.=28, P<0.001) and (b) combine grain yield (y=
0.048x+7.96, R
2
=0.17, D.F.=28, P<0.01) for 30 Rec-
ommended List winter wheat cultivars examined at Headley
Hall, Yorkshire in 1996–7, adapted from Foulkes et al.
(1998).
Yield potential of wheat 23
vernalization (exposure to low temperatures) and on
their intrinsic earliness (basic length of a phase). The
genes required to manipulate the stem-elongation
period independently of the whole period to anthesis
have not yet been identified. However, there is some
prospect for doing so, since recent field investigations
have confirmed that the thermal duration of the stem-
elongation period is sensitive to changes in photo-
period imposed after the onset of stem elongation
(Whitechurch & Slafer 2001, 2002). Work is currently
ongoing in Argentina to dissect the genetic basis of
this photoperiod response further in spring wheat
(Miralles & Slafer, in press). Parallel complementary
studies seem justified to establish the physiological
and genetic basis of this trait in winter wheat to
underpin future breeding programmes.
RUE
RUE can potentially be improved by manipulating
traits at the biochemical, cellular, leaf or canopy
levels of organization (Reynolds et al. 2000). Doub-
ling the specificity of Rubisco for CO
2
could theor-
etically increase net photosynthetic rate at saturating
light intensities (A
MAX
) by 20 %. However, attempts
to select for low rates of photorespiration in wheat
have so far met with little success (Evans 1983). With
regard to leaf traits, positive correlations between
N content per unit leaf area and A
MAX
have been
observed in cereals, e.g. amongst rice lines (Cook &
Evans 1983). However, RUE only increases at a
low rate as A
MAX
increases above values about 1 mg
CO
2
/m
2
/s (Monteith 1977), not least because individ-
ual leaves in the canopy generally operate well below
light saturation, so a substantial increase in A
MAX
may be required to give a modest increase in RUE.
At the canopy level, more erect leaf attitude to
reduce the extent of light-saturation of leaves at the
top of the canopy has been shown to increase harvest
biomass (Innes & Blackwell 1983) and stomatal con-
ductance (Araus et al. 1993) in wheat. However, most
modern winter wheat cultivars in northwestern
Europe already have semi-erect or erect flag-leaf
attitude.
From our recent investigation of changes in
physiological traits associated with yield progress
in the UK (Shearman et al. 2005; Table 1), there is
evidence that wheat breeders have inadvertently
selected for higher pre-anthesis RUE in recent years.
The mechanisms responsible for the improvement
of RUE cannot be identified with certainty. Flag
leaves were observed to be generally more erect in the
modern cultivars, but there was no trend in K
PAR
.
Additionally, RUE continued to increase among the
four most recently introduced cultivars, among which
there was no change in flag-leaf attitude, suggesting
it was not a primary determinant of the improved
RUE. Interestingly, modern cultivars had both
smaller flag leaves and greater specific leaf dry weight
(SLW; ratio of dry weight to green leaf lamina area),
i.e. thicker leaves, and there were statistically signifi-
cant correlations with RUE (Fig. 4). The smaller flag-
leaf area may have reduced saturating light intensities
at the top of the canopy and improved distribution
of light to lower leaves ; and greater SLW may have
increased photosynthetic tissues per unit leaf area.
RUE was also found to increase with year of release
in a different set of four cultivars and flag-leaf size
was again negatively correlated with RUE confirming
these relationships (Foulkes et al. 2006). Therefore,
selection for a smaller flag leaf of greater SLW could
offer scope for increasing RUE both pre- and post-
anthesis in future years. Of course, fertile shoots/m
2
might eventually need to be readjusted upwards to
counter any reduction in fractional radiation inter-
ception with smaller flag leaves ; initially, however,
any reduction in season-long interception would be
negligible for canopies of modern winter wheat
45
35
25
15
1995 1975 1985 1995
60504030
(b)
(a)
2·7
2·6
2·5
2·4
2·3
Year of release
Flag-leaf area (cm2)
Specific leaf DW (g/m2)
RUE (g/MJ)
Fig. 4. (a) Change in flag-leaf area at GS39 with year of
release from 1972 to 1995 (y=x0.80x+1612.1, R
2
=0.85,
D.F.=6, P<0.001), (b) linear regression of specific leaf dry
weight (all culm leaves, green fraction) at GS61 on pre-
anthesis radiation-use efficiency
PAR
(RUE) (y=
0.023x+1.43, R
2
=0.64, D.F.=6, P<0.001), for eight winter
wheat cultivars. Values represent means across 1997 and
1998 for specific leaf dry weight and RUE; and in 1999 for
flag-leaf area, adapted from Shearman et al. (2005).
24 M.J.FOULKES ET AL.
cultivars (Fig. 1) according to the ‘ Beer’s law ’ ex-
ponential relationship between green canopy area
and fractional interception (Monsi & Saeki 1953).
Estimating RUE in breeders’ plots is unfeasible, so
further work seems justified to identify sub-traits and
associated ‘smart-screens’ for RUE, including genetic
markers for use in winter wheat breeding pro-
grammes.
Stem soluble carbohydrate reserves
By flowering, reserves of WSC, mostly as fructans,
have accumulated in the stems and leaf sheaths of
the crop. Maximal amounts are accumulated about
nine days after flowering (Austin et al. 1977;
Schnyder 1993; Foulkes et al. 2002). The relative
contribution of stem reserves to grain yield varies
widely depending on environmental conditions and
cultivars (Borrell et al. 1993; Blum 1998; Foulkes
et al. 2002; Ruuska et al. 2006). In general, a re-
duction in current assimilation under post-anthesis
moisture stress will induce greater stem reserve
mobilization to, and utilization by, the grain (Palta
et al. 1994; Yang et al. 2000). Thus, a significant
proportion of reserves are usually retranslocated to
grains under drought to buffer effects of accelerated
senescence (Bidinger et al. 1977; Schnyder 1993).
However, there is evidence for significant deposition
of stem WSC reserves in grains in the absence of post-
anthesis stress in wheat (Gebbing et al. 1999). The
results of Shearman et al. (2005) bear out the poten-
tial importance of stem WSC for grain yield potential
even under favourable post-anthesis conditions. The
amount of stem WSC accumulated varied among 17
UK cultivars in the range 254–447 g/m
2
(Foulkes
et al. 1998) and was positively correlated with yield
(r=0.65). The levels of stem WSC were correlated
over two sites (r=0.64). Similarly, six wheat cultivars
were grown under irrigation at a single site over three
years and their ranking for WSC content was gen-
erally maintained (Foulkes et al. 2002). Ruuska et al.
(2006) reported there were significant and repeatable
differences in WSC accumulation among 22 wheat
genotypes grown in Australia (means ranging from
112 to 213 mg/g dry weight averaged across environ-
ments), associated with large broad-sense heritability
(H=0.90¡0.12). Their results were consistent with
fructan biosynthesis occurring via a sequential mech-
anism that is dependent on the availability of sucrose,
and differences in WSC contents of genotypes were
reported to be unlikely to be due to major mechanistic
differences. Therefore breeding for high stem WSC
should be possible.
The relationship between the amount of stem WSC
measured shortly after flowering and grain yield
under fully irrigated conditions was tested in two
winter wheat DH populations (RialtorSpark and
BeaverrSoissons; Fig. 5). Lines differed in the range
1.71–4.26 and 1.52–4.23 t/ha, respectively (P<0.001).
There was a positive linear relationship between stem
WSC and grain yield in both populations (P=0.07
and P<0.001, respectively; Fig. 5). Although the
genetic differences in stem WSC accumulation in both
populations were associated with differences in each
of stem biomass and % WSC (data not shown), the
proportion of variance in stem WSC accounted for by
stem biomass was greater than that accounted for
by %WSC. These results confirmed the importance
of the contribution of stem reserves to grain yield
potential in populations representative of modern
winter wheat backgrounds even under favourable
post-anthesis conditions. Although stem reserves
were not measured at harvest in the work reported
here, Foulkes et al. (1998) showed these amounts to
be consistently very low (<0.25 t/ha) for 17 winter
wheat cultivars tested in the UK.
Present results confirmed the importance of stem
reserves accumulation for yield potential in winter
wheat grown in northwestern Europe. Some inves-
tigations have suggested that increasing stems re-
serves utilization may be incompatible with increasing
yield potential since ear growth and stem sugars
are competitive sinks in the pre-anthesis period,
12
10
8
6
4
12
10
8
6
4
12345
12345
Grain yield(t/ha)
Grain yield(t/ha)
Stem WSC (t/ha)
Stem WSC (t/ha)
(b)
(a)
Fig. 5. Linear regression of water-soluble carbohydrate
(WSC) in stems and leaf sheaths at GS61+75 xCd (base
temp. 0 xCd) on grain yield (850 g DM/kg) in (a)
RialtorSpark lines in 2001 (y=0.32x+8.10; R
2
=0.13,
D.F.=24, P=0.07) and (b) BeaverrSoissons lines in 2002
(y=0.47x+6.03, R
2
=0.22, D.F.=44, P<0.001).
Yield potential of wheat 25
e.g. Pheloung & Siddique (1991). This would be ex-
pected to be the case if greater stem partitioning was
the sole basis for the higher reserve storage. However,
the present results showed that greater anthesis bio-
mass and a greater proportion of stem biomass allo-
cated to WSC were the primary reasons for the higher
stem WSC rather than greater stem partitioning. This
may be the reason that in the present experiments the
correlation of grains/m
2
with the absolute amount
of stem WSC was not significant in the RialtorSpark
population and actually positive in the Beaverr
Soissons population (r=0.58, P<0.001). In summary,
evidence from a number of investigations suggests
that stem WSC is a trait of high heritability and
selection for high stem WSC is possible. Phenotyping
lines in the field for this trait using the established
anthrone method (Yemm & Willis 1954) is time-
consuming due to the requirement to assess according
to developmental stage and prone to imprecision due
to diurnal fluctuations, so marker-assisted selection
might possibly be considered. Major QTLs associated
with the amount of stem WSC have recently been
identified on chromosomes 1B and 2A and may be
of value with regard to future marker development
(Foulkes et al. 2001b).
Optimized ear morphology
To complement increases in grain sink strength
achieved through raising anthesis biomass, novel
large-ear phenotype (LEP) traits (e.g. long rachis,
high spikelet number) may offer scope for increasing
ear partitioning and/or the ratio of grains to ear
DM. Restructured plant types exploiting heterosis
were developed by Ricardo Rodriguez at CIMMYT
during the 1980s, based on a novel genetic combi-
nation of germplasm, including Agrotriticum
(Canada), Polonicum (Poland), Morocco (Morocco),
and semidwarf (Mexico) wheats (Rajaram &
Reynolds 2001). These wheats have intermediate til-
lering capacity (up to ten tillers), long ears (300 mm)
and high ear fertility (up to 200 grains). These sink-
related characters have already been introgressed
into advanced (F
7
+) spring wheat breeding lines at
CIMMYT. Recent research has begun to examine
the physiological basis of the higher grains per ear
in selected advanced LEP lines (Gaju et al. 2006).
Results on single plants in controlled-environment
conditions indicated 3–4 more spikelets per ear to be
initiated in the novel material associated with a longer
thermal duration for spikelet primordium production
(Gaju et al. 2006). Field results in Mexico confirmed
a greater ear index (ratio of ear to above-ground
biomass) at anthesis and potential individual grain
weight in novel lines. However, there was a reduction
in ears/m
2
of c. 50–100. It is encouraging that these
results suggest that increased spikelet number and
potential individual grain weight may be partially
independent of reduced tillering in the novel
lines. Therefore it is possible that there may be
synergies between more fertile ears in the CIMMYT
material and improved stem WSC and RUE exhi-
bited in elite winter wheat germplasm in northwestern
Europe.
Exploitation of the LEP trait in the longer term will
depend on maintaining lodging resistance. Recent
work in the UK on the development of a lodging-
proof ideotype (Berry et al. 2004) is therefore relevant
in this respect. It suggests that breeders should select
for wide, deep root plate spread and wide stems with
increased material strength of the stem wall. A further
consideration in bread-making cultivars will be the
negative relationship that is usually observed between
yield and grain protein concentration. For bread-
making cultivars, it will be important in the new plant
types to identify and select for traits associated with
favourable departures from the grain yield to protein
concentration relationship.
Agricultural policies in the EU and other developed
economies take an increasingly holistic view of the
productivity of agriculture, and it appears that crop
production will have to contend with demand for in-
creased resource-use efficiency to minimize environ-
mental impacts. Improving water and N uptake
through optimizing root traits and raising RUE will
help in this respect. However, it is likely other genetic
and agronomic solutions will be needed alongside
those discussed in this paper to moderate N fertilizer
requirements and increase water-use efficiency.
We thank UK government Department of Environ-
ment, Food and Rural Affairs for funding projects
OC9602, CC0370, AR9008 and IS0210 and the PhD
studentship of Vicky Shearman. We thank University
of Nottingham and CIMMYT for funding the PhD
studentship of Reshmi Gaju. We also thank Syngenta
Seeds (New Farm Crops) for use of the Drake-sibr
Welton DH population.
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