Crop Science
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Published online 1 January 2005
Published in Crop Sci 45:175-185 (2005)
© 2005 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Related articles in Crop Science
Right arrow Similar articles in this journal
Right arrow Similar articles in Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Web of Science (39)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Shearman, V. J.
Right arrow Articles by Foulkes, M. J.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Shearman, V. J.
Right arrow Articles by Foulkes, M. J.
Agricola
Right arrow Articles by Shearman, V. J.
Right arrow Articles by Foulkes, M. J.
Related Collections
Right arrow Crop Physiology & Metabolism
Right arrow Wheat

Physiological Processes Associated with Wheat Yield Progress in the UK

V. J. Shearmana, R. Sylvester-Bradleyb, R. K. Scotta and M. J. Foulkesa,*

a Division of Agricultural and Environmental Sciences, Univ. of Nottingham, Sutton Bonington Campus, Loughborough, Leicestershire, LE12 5RD, UK
b ADAS Boxworth R & D Centre, Boxworth, Cambridgeshire CB3 8NN, UK

* Corresponding author (John.Foulkes{at}nottingham.ac.uk).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
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. Our objective was to identify physiological traits associated with genetic gains in grain yield of winter wheat (Triticum aestivum L.) in the UK. The growth and development of eight representative cultivars introduced from 1972 to 1995 (one tall rht-D1b cultivar and seven Rht-D1b, formerly Rht2, semidwarf cultivars) was examined in field experiments at Sutton Bonington in 1996–1997, 1997–1998, and 1998–1999. A linear genetic gain in grain yield of 0.12 Mg ha–1 yr–1 (1.2% yr–1) was positively correlated with both harvest index (HI) and aboveground biomass; a quadratic function fitted to the data showed that progress in HI was most apparent during the earlier phase of the 23-yr period, whereas biomass contributed most since about 1983. There was a linear increase across time of 217 grains m–2 yr–1, but no change in grain weight. Significant genetic changes across time and correlations with grain yield were also found for preanthesis radiation-use efficiency (RUE, 0.012 g MJ–1 yr–1) and water soluble carbohydrate (WSC) content of stems and leaf sheaths at anthesis (4.6 g m–2 yr–1). Our results suggest that recent genetic gains in grain yield have been based on a combination of improved growth rate in the preanthesis period, which has driven increases in number of grains per square meter, and a larger source for grain filling through increases in stem soluble carbohydrate reserves.

Abbreviations: AGDM, aboveground dry matter • Amax, light-saturated net CO2 exchange rate • CCS, complete canopy senescence • GAI, green area index • GS, growth stage • HI, harvest index • K, extinction coefficient • LTM, long-term mean • PAR, photosynthetically active radiation • PGR, plant growth regulator • RUE, radiation-use efficiency • WSC, water soluble carbohydrate


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
AVERAGE YIELDS of winter wheat in the UK have increased progressively from the 1950s to the present at a rate of approximately 100 kg ha–1 yr–1 (Department of Environment, Food, and Rural Affairs, 2002). Until the late 1980s, the increase was attributed about half to plant breeding and half to husbandry (Silvey, 1986; Austin et al., 1989). Yields in high potential systems have shown similar trends in France (Brancourt-Hulmel et al., 2003), Mexico (Sayre et al., 1997), and Italy (Canevara et al., 1994).

The study of Austin et al. (1989) on UK cultivars introduced from 1908 to 1986 showed HI to be positively associated with yield progress, with only a slight, nonsignificant increase in aboveground dry matter (AGDM) at harvest. Improved HI was associated with altered partitioning in favor of the ear with the introduction of the Rht-D1b semidwarf allele leading to more grains per square meter in the modern group of cultivars (1981–1986). Higher grain number, in turn, resulted mainly from more grains per 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 preflowering period (Fischer, 1983; Calderini et al., 1995; Miralles et al., 1998).

Studies of the physiological basis of genetic gains in grain yield in sets of historic wheat cultivars worldwide have generally shown number of grains per square meter (usually because of number of grains per ear) and HI to be positively correlated with grain yield, but no similar positive correlation between AGDM 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, however, have shown biomass to be positively associated with yield progress (e.g., Siddique et al., 1989; Donmez et al., 2001). Furthermore, during the 1990s there have been several reports of biomass increases attributed to introductions of alien chromatin into wheat germplasm: namely, the 1BL.1RS wheat–rye (Secale cereale L.) translocation in spring wheat in Mexico (Villareal et al., 1994, 1995) and winter wheat in the Great Plains (Carver and Rayburn, 1994), and the 7DL.7Ag wheat–tall wheatgrass [Agropyron elongatum (Host) P. Beauv. = Elytrigia elongata (Host) Nevski] translocation in spring wheat in Mexico (Reynolds et al., 2001). Although the 1BL.1RS translocation was introduced into UK-bred wheat cultivars in the late 1980s, there have been no studies on the effects of UK breeding on harvest AGDM since this time.

It has been suggested that, in temperate North Western Europe, an upper limit for HI may soon be reached, estimated to be about 0.62 for winter wheat (Austin, 1980). There are already reports of UK winter wheats achieving close to this, 0.61 for ‘Consort’ for example (Spink et al., 2000), introduced in 1995. Thus, future gains in yield may increasingly depend on achieving greater harvest AGDM while maintaining HI. Improved knowledge about the physiological basis of recent increases in number of grains per square meter, HI, and possibly harvest AGDM in UK wheats may be important for indicating future strategies for progress. In particular, it would be instructive to know whether there have been changes in traits influencing potential assimilate production (source), in addition to those influencing the capacity of grains to store assimilate (sink) in recent decades. Thus, the present paper aims to (i) quantify changes in number of grains per square meter, HI, and harvest AGDM associated with grain yield progress for a set of eight representative UK winter wheat cultivars introduced between 1972 and 1995, and (ii) identify the physiological basis of these changes by examining developmental rates, green area production, radiation capture, RUE, and partitioning of dry matter through the season.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Site and Experimental Treatments
The experiments were located at University of Nottingham Farm, Sutton Bonington, Leicestershire, UK (52°50' N, 1°15' W) on a medium stony loam to 0.80 m over clay (Dunington Heath series) with good drainage. A randomized block experiment was completed in each of 1996–1997, 1997–1998, and 1998–1999, examining a set of eight representative UK-bred winter wheat cultivars randomized on plots in three replicates. Plot size was 24 by 1.6 m2, with a gap width of 0.5 m between adjacent plots. One half (12 by 1.6 m2) of each plot area was allocated for growth-analysis sampling and for assessment of crop development, and the remaining half for measuring combine grain yield and radiation interception, using in situ solarimeters and hand-held ceptometers. Cultivars ranged in their year of release (taken as their year first included on the UK Recommended List) from 1972 to 1995, and were chosen because they were the most widely grown through the 1970s to 1990s (Table 1). The cultivars were known to contrast for possession of major gene(s) introduced into UK germplasm in recent decades, including the semidwarf Rht-D1b allele and the 1BL.1RS wheat-rye translocation. One cultivar (Maris Huntsman) was a nonsemidwarf cultivar (rht-D1b) and seven were semidwarfs (Rht-D1b). The three most recently introduced cultivars (Haven, Brigadier, and Rialto) possessed the 1BL.1RS translocation whereas others did not. All cultivars were feed wheats, except for Avalon (bread-making), Riband (biscuit-making), and Rialto (feed, but with some potential for bread-making). Seven cultivars were bred at the Plant Breeding Institute, Cambridge, consistent with the large market share of this breeding program during the period. The remaining cultivar, Brigadier, was bred at Zeneca Seeds, UK.


View this table:
[in this window]
[in a new window]

  		Table 1. Dates of GS31 (Growth Stage 31) and GS61, crop height at harvest and percentage plot area lodged at harvest for eight cultivars in 1997 and 1998.

 
Plot Management
The previous crop was winter oat (Avena sativa L.) in each season. The experiments were sown on 14 Oct. 1996, 10 Oct. 1997, and 11 Oct. 1998 with 320 seeds m–2 and rows 0.13 m apart. In each season, 180 to 205 kg ha–1 nitrogen fertilizer as ammonium nitrate was applied in a two-split program. Fungicides {prochloraz [N-propyl-N-(2-(2,4,6-trichlorophenoxy)ethyl)-1H-imidazole-1-carboxamide], a.i. 405 g ha–1; and fenpropidin [1-(3-(4-(1,1-dimethylethyl)phenyl)-2-methylpropyl)piperidine], a.i. 563 g ha–1 at GS31 (Growth Stage 31); epoxiconazole [(2RS,3SR)-1-[3-(2-chlorophenyl)-2,3-epoxy-2-(4-fluorophenyl)-propyl]-1H-1,2,4-triazole] + fenpropimorph [4-(3-(4-(1,1-dimethylethyl)phenyl)-2-methylpropyl)-2,6-dimethylmorpholine], a.i. 125 + 375 g ha–1, respectively at GS39; and tebuconazole [1,2,4-Triazole-1-ethanol, alpha-(2-(4-chlorophenyl)ethyl)-alpha-(1,1-dimethylethyl)-], a.i. 250 g ha–1 at GS61} and herbicides [Isoproturon (3-(4-isopropylphenyl)-1,1-dimethylurea), a.i. 750 g ha–1 applied during autumn tillering and again in early spring before GS30] were applied to minimize diseases and weeds. Plant growth regulator {PGR; chlormequat [(2-Chloroethyl)trimethylammonium chloride], a.i. 1610 g ha–1 was applied at GS31 to reduce the risk of lodging and to represent the way in which UK cultivars have been grown in recent decades. The percentage of UK wheat fields treated with PGRs was 25% in 1980, 56% in 1988, and 79% in 1996 (Ministry of Agriculture, Fisheries, and Food, 1997; and preceding publications). It is presumed that PGR did not mask differences in height or other traits studied here because Gale and Youseffian (1985) reported only a small differential height response due to Rht-D1b (about 5 cm) and no differences in response due to Rht-D1b have been detectable in other traits, for example, canopy structure affecting light attenuation deeper in the canopy in the postanthesis period (Green et al., 1985) or harvest biomass (Cox and Otis, 1989), or have been consistently found for grain yield (Green, 1986). Despite PGR use, the rht-D1b cultivar Maris Huntsman was on average 19 cm taller than the mean of Rht-D1b cultivars studied here (Table 1).

Crop Measurements
Unless stated otherwise, the methods described below refer only to assessments performed in the two seasons, 1996–1997 and 1997–1998. The third field experiment in 1998–1999 was performed just to obtain data on flag leaf size and culm leaf number per mainstem at GS39, and no measurements other than these were taken.

Developmental Stages and Growth Analysis
Dates of onset of stem extension (GS31), flag leaf emergence (GS39), flowering (GS61) (Tottman, 1987) and complete canopy senescence (CCS; taken as the date when all green lamina area had senesced, and there was <10% stem green area remaining) were recorded in all plots. Final plant populations were determined at GS31 by digging up and counting all plants contained within a 1.2- by 0.6-m2 quadrat per plot.

In each experiment, growth of the aboveground plant material was analyzed in one 1.2- by 0.6-m2 quadrat per plot at five stages: GS31, GS39, GS61, CCS, and harvest. Two border rows were left unsampled on each side of the plot. In the samplings up to and including CCS, cultivars were sampled on the actual calendar dates that they reached the specific stage, that is, cultivars were sampled on different dates. At harvest, all cultivars were sampled on the same calendar date. In the samplings up to and including CCS, the number of fertile, dying, and dead shoots in a 10% subsample (by fresh weight) was counted. For the fertile shoots (and the dying shoots if they constituted >5% of the subsample), projected green areas were measured for (i) leaf lamina, (ii) stem and attached leaf sheath, and (iii) ear, using a Li-Cor 3100 leaf area meter (LI-COR Inc., Lincoln, NE), and the components summed to give the green area index (GAI). Aboveground dry matter was measured on a 20% subsample (by fresh weight) of the sampled material by weighing after drying for 48 h at 80°C. Dry matter of crop components (e.g., leaf lamina, stem, and leaf sheath, etc.) was obtained by weighing components of the 10% subsample after drying for 48 h at 80°C. At harvest, all plant material was separated into ears and straw. Ears were counted and threshed, and the chaff (rachis, rachilla, glume, palea, and lemma) and grain were weighed separately after drying for 48 h at 80°C. A 25% subsample of the straw (by fresh weight) was weighed after drying for 48 h at 80°C. The HI was calculated as the fraction of AGDM present as grain.

In January 1999, the number of culm leaves (leaves associated with a node on the extended stem) per mainstem was recorded on 10 mainstems per plot at GS39. The 10 mainstems were sampled shortly after GS39 and the green areas of the flag leaf, the penultimate leaf, and the two leaves below this were recorded separately using a Li-Cor 3100 leaf area meter. At GS61 + 75°Cd (base temperature 0°C), percentage WSC content of stems and attached leaf sheaths was assessed in all plots, using the anthrone method of Yemm and Willis (1954) as described by Gay et al. (1998). Since GS61 + 75°Cd equates approximately to GS61 + 5d in the UK environment, the amount of WSC in stems and leaf sheaths is close to maximal at this point (Austin et al., 1977). An estimate of the maximal amount of stem WSC accumulated was then obtained by multiplying the stem-and-leaf-sheath DM measured at GS61 by the percentage WSC measured at GS61 + 75°Cd. This probably underestimated the maximal amount of WSC DM by about 5 to 10%, since stem-and-leaf sheath DM usually increases by about 5 to 10% in UK conditions from GS61 to GS61 + 75°Cd (Sylvester-Bradley et al., 1998). Nonetheless, given that differences in stem WSC amongst UK modern cultivars and their old counterparts could be anticipated in the range of about 200 to 400 g m–2 (Foulkes et al., 1998, 2002), the present methodology would be expected to quantify any meaningful change across time with breeding in stem WSC.

Shortly before harvest, the height from ground level to the tip of the uppermost ear was recorded at 10 randomly located positions in each plot, avoiding any lodged areas of the plot. Dates of lodging incidents were observed, and at harvest a visual assessment of the percentage area of the crop lodged (lodging defined as when crop lodges between 45° and 90° from the vertical) was made in each plot.

Combine Grain Yield and Yield Components
In each experiment, combine (i.e., machine harvested) grain yields were assessed on a 1.6 x 10 m2 area in each plot, and values adjusted to 15% moisture. Individual grain weight was assessed on a 75-g sample; after hand-cleaning, the number of grains was counted and the sample weighed after drying for 48 h at 80°C. Number of grains per square meter was calculated from the combine grain yield and the individual grain weight. Number of grains per ear was calculated from the combine grain yield, the individual grain weight and number of ears per square meter (obtained from growth analysis of plant material sampled in the 1.2- by 0.6-m2 quadrat at harvest).

Interception of Radiation
Interception of photosynthetically active radiation (PAR, 400–700 nm) was measured using a Sunfleck Ceptometer (Delta-T Devices, Burwell, Cambridge, UK) in all plots at GS39 in both years and again shortly after GS61 in 1998. Measurements were taken above the crop and at ground level diagonally across the rows, and of the reflected PAR by inverting the ceptometer about 5 cm above the crop. In addition, in 1997 the hourly averages of incident solar radiation above the crop were recorded using two tube solarimeters from mid-April to mid-July. Solar radiation below the canopy was measured using one tube solarimeter per plot placed at ground level at right angles to the rows. For both PAR and solar radiation, the extinction coefficient (K) was calculated from GAI and fractional interception using a modified version of Beer's Law (Eq. [1]), where Io is the incident radiation and I is the amount of radiation transmitted below a GAI value of L (Monsi and Saeki, 1953).


Formula 1[1]
Radiation-use efficiency (g MJ–1) was estimated in the preflowering period from the slope of the linear regression of cumulative PAR interception on AGDM from the sequential samplings at GS31, GS39, and GS61. Slopes were calculated individually for each plot, and the plot values subjected to ANOVA.

Statistical Analysis
For individual experiments, ANOVA was performed for traits and grain yield using Genstat v. 6.1 (Lawes Agricultural Trust, Rothamsted Experimental Station, Harpenden, Hertfordshire, UK). Replications were regarded as random effects, while cultivar was a fixed effect. For ANOVAs across years, Bartlett's test (P = 0.05) was used to test for the homogenity of variances, and years were regarded as random effects. The mean square for the year effect was tested against an error A mean square representing the variation between blocks within years. The mean square of the cultivar and year x cultivar effects was tested against an error B mean square representing residual variation. Treatment means were compared using the LSD of the means of Fisher, calculated from standard errors of the difference of the means using appropriate degrees of freedom, when the ANOVA indicated significant differences.

Regression analysis with standard linear and curvilinear models (quadratic and hyperbolic) (Genstat 6.1) applied to 2-yr cultivar means was used to calculate rates of change for traits and grain yield with year of release. Regression coefficients are presented for all variables for the linear regressions. Where quadratic functions increased the proportion of the variance in the variable attributable to year of release compared with the linear function, regression coefficients for the quadratic function are also presented. Correlations between traits and between respective traits and grain yield were calculated using the 2-yr cultivar means.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Growing Conditions
In summary, the 1996–1997 season was characterized by a cold December and January, followed by a warm spring, and then average temperatures until harvest. Approximately average rainfall was experienced from sowing until a dry January. March and April were also drier than the long-term mean (LTM), but June was much wetter, with 114-mm rainfall (cf, LTM of 55 mm). Radiation levels were generally close to the LTM, except for a bright May and dull June.

In the 1997–1998 season, temperatures during the autumn and winter were generally above the LTM. Thereafter, from March to August, temperatures were close the LTM. Approximately average rainfall was experienced from sowing until a very dry February. April and June, however, were much wetter than average, with 105- and 106-mm rainfall (cf, LTMs of 46 and 55 mm, respectively). June was duller than average, but otherwise radiation levels in this season were generally similar to the LTM.

There was no lodging in either year before flowering. After flowering in 1997, lodging occurred on 24 June in some plots, and two cultivars (Galahad and Riband) had >20% lodging at harvest; in 1998 there was a lodging event on 27 June, and one cultivar (Maris Huntsman) had >20% lodging at harvest (Table 1). Averaging across cultivars, the percentage lodging was 19.6% in 1997 and 4.8% in 1998. For the 2-yr cultivar means, the regression of lodging (linear or curvilinear) on year of release was not statistically significant. The statistical analyses presented therefore include values for lodged cultivars.

With one exception, mean incidence of foliar diseases on culm leaves of any cultivar was <3%. The exception was Brigadier (introduced 1993) in 1998, which suffered a breakdown of a race-specific resistance to yellow rust (Puccinia striiformis Westend.). There was a GAI loss of about 0.5 due to yellow rust at GS61 for Brigadier, based on growth-analysis measurements of GAI and concurrent disease observations at GS61. The statistical analyses presented include data for Brigadier in 1998.

Crop Development
There were no statistically significant cultivar differences in plant population density in either season, with a mean of 201 plants m–2 established in the spring in 1997 and 207 m–2 in 1998. Averaging across years, cultivars reached GS31 from 27 March to 16 April and GS61 from 7 to 15 June (Table 1), but in each case there was no apparent trend with breeding.

Cultivars differed in plant height at harvest (P < 0.001), with the nonsemidwarf Maris Huntsman as expected about 20 cm taller than the semidwarfs (Table 1). No trend for a further decrease with breeding was apparent within the group of seven semidwarfs.

Grain Yield and Yield Components
Combine grain yield increased linearly with year of release by 119 kg ha–1 yr–1 (P < 0.001; Fig. 1 , Tables 2 and 3). Although in both years yield progress appeared to level off amongst the three most recently introduced cultivars, curvilinear functions (quadratic or hyperbolic) fitted to data did not increase the proportion of the variance in grain yield attributable to year of release compared with the linear function. Present results are broadly consistent with UK genetic yield progress, estimated by Ingram et al. (1997) at 12% (or 93 kg ha–1 yr–1) between the release of Norman (1981) and Brigadier (1993). They are also comparable with yield progress of 102 kg ha–1 yr–1 from 1970 to 1983 reported for UK winter wheat by Austin et al. (1989).


Figure 1
View larger version (32K):
[in this window]
[in a new window]

  		Fig. 1. Fitted linear (y = a + bx) or quadratic (y = a + bx + cx2) curves on year of release for (a) combine grain yield (15% moisture content), (b) grains per unit area, (c) ears per unit area, (d) grains per ear (no significant curve; SE of the difference of the means = 2.73, df = 28), (e) harvest aboveground dry matter (DM), and (f) harvest index. Fitted data were 2-yr means for eight cultivars released from 1972 to 1995.

 

View this table:
[in this window]
[in a new window]

  		Table 2. Fitted parameter estimates for changes in crop traits with year of release for eight wheat cultivars released between 1972 and 1995. Linear and quadratic functions were fitted to 2-yr cultivar means (1997 and 1998).

 

View this table:
[in this window]
[in a new window]

  		Table 3. Combine grain yield, harvest aboveground dry matter (AGDM), harvest index, and individual grain weight for eight wheat cultivars in 1997 and 1998.

 
Number of grains per square meter increased linearly with year of release by 217 m–2 yr–1 (P < 0.001; Fig. 1, Table 2). This was associated with a linear increase in number of ears per square meter (P < 0.05; Fig. 1, Table 2) and an apparent increase in number of grains per ear (P < 0.07) for the majority of the group of seven Rht-D1b cultivars (e.g., Riband 41.8, Norman 39.1, Rialto, 38.2, Brigadier 38.2, Avalon, 37.3) compared with the rht-D1b cultivar Maris Huntsman (32.9); there was no evidence for a further increase in number of grains per ear amongst the seven Rht-D1b cultivars (Fig. 1). There was no change with modern cultivars in grain weight (Tables 2 and 3). Although there were differences in the ratio of grains-to-ear DM at flowering (P < 0.001), with Brigadier (128.5 grains g–1) exhibiting a higher ratio than all other cultivars (mean, 89.5 grains g–1), there was no change in this ratio with date of introduction (Table 4).


View this table:
[in this window]
[in a new window]

  		Table 4. Photosynthetically active radiation (PAR) interception, accumulated aboveground dry matter (AGDM) and radiation-use efficiencyPAR (RUEPAR) from Growth Stage 31 (GS31) to GS61; AGDM, stem and leaf sheath dry matter (St + Sh DM) and ear dry matter (ear DM) at GS61; ratio of grains-to-ear DM at GS61; stem water soluble carbohydrate (WSC) at GS61 + 75°Cd{dagger}; and accumulated AGDM from GS61 to complete canopy senescence (CCS) for eight wheat cultivars. Values represent means across 1997 and 1998.

 
Crop Dry Matter Growth and Partitioning
The linear increase in grain yield with year of release was due to counteracting curvilinear (quadratic function) increases in harvest AGDM (P < 0.001; Fig. 1, Tables 2 and 3) and HI (P < 0.001; Fig. 1, Tables 2 and 3). For harvest AGDM, the rate of increase accelerated after about 1983; indeed, little improvement was discernible between the introduction of Maris Huntsman (1972) and Galahad (1983). Whereas for HI, the rate of increase decelerated after about 1980, HI having increased significantly in the earlier period presumably in large part associated with introduction of the Rht-D1b allele (Maris Hunstman, rhtD1b, 0.39; cf, Avalon, Norman, Galahad, all Rht-D1b, mean 0.48). Trends for combine grain yield, harvest AGDM, and HI across time were consistently observed in the individual years, with the year x cultivar interaction in each case not reaching statistical significance at the 5% significance level (Table 3).

The present results suggested that the greater harvest AGDM observed with modern cultivars released since about the mid-1980s was associated with more dry matter accumulation in the preanthesis phase. There was a tendency for greater anthesis AGDM in modern cultivars compared with their older counterparts; for example, Rialto (1995) and Haven (1990) each produced greater biomass than the three oldest cultivars (P = 0.07; Table 4). Furthermore, there was a linear increase in stem-and-leaf sheath DM at GS61 with year of release (P = 0.08; Tables 2 and 4), consistent with a linear increase across time in the amount of WSC accumulated in the stems and leaf sheaths (P = 0.07; Tables 2 and 4). Ear DM at GS61 was lower for Maris Huntsman compared with each of the group of seven semidwarfs (P < 0.05). Amongst the seven semidwarfs, however, there was no consistent pattern in ear DM with year of release (Table 2). Biomass growth in the postflowering period was calculated as the difference in AGDM between GS61 and CCS. Although cultivars differed in the range 432 to 634 g m–2 (Table 4), there was no trend with year of release.

Green Area Production, Radiation Interception, and Radiation-Use Efficiency
There was no change with year of release in GAI at GS31, GS39, or GS61 (Tables 2 and 5). Green area per fertile shoot at both GS39 and GS61 decreased with breeding by about 1 cm2 shoot–1 yr–1 (P < 0.001; Table 2). In 1999, a detailed examination of flag leaf green area at GS39 showed a decrease of 0.8 cm2 yr–1 (P < 0.001, R2 = 0.85, df = 6; Table 5). Similar effects were observed for Leaves 2, 3, and 4 (e.g., where Leaf 2 = penultimate leaf to emerge; data not shown). However, there was no change in the number of culm leaves on mainstems with modern cultivars (Table 5). Thus, the decrease in green area per shoot was associated with smaller leaves rather than fewer leaves per shoot. As GAI showed no trends with breeding, a change in fertile shoot number is indicated, consistent with the trend for an increase in number of ears per square meter (= number of fertile shoots per square meter) detected at harvest (Table 2 and Fig. 1).


View this table:
[in this window]
[in a new window]

  		Table 5. Green area index (GAI) at GS31 (Growth Stage 31), GS39, and GS61, radiation extinction coefficients at GS39 and GS61 measured (KPAR) and calculated from total radiation (KPAR-CALC), flag leaf area at GS39, culm leaves (lvs) per mainstem at GS39, specific leaf dry weight (SLDW) of lamina at GS61, and flag leaf attitude at GS39 for eight wheat cultivars. Values represent means across 1997 and 1998, except for KPAR-CALC at GS39 for which values are 1997, KPAR at GS61 for which values are for 1998, and flag leaf area and culm leaves per mainstem for which values are for 1999.

 
Assessing effects across both years, there was no change with year of release in KPAR at GS39 (Table 5). In 1997, it was possible to calculate the equivalent KPAR-CALC values at GS39 from values of fractional total solar radiation interception at GS39 measured using solarimeters, using the relationship between fractional interception of PAR (FPAR) and total solar radiation (FTOTAL-SOLAR) (Squire, 1990).


Formula 2[2]
There was no statistically significant difference between KPAR and KPAR-CALC values derived by the two methods. In 1998, when KPAR was measured at both GS39 and GS61, ear emergence increased the value of KPAR, averaged across the eight cultivars, by 0.06, although not statistically significantly (P = 0.480; Table 5). This is in agreement with the findings of Thorne et al. (1988), who observed that K did not vary with growth stage from the unfolding of the first leaf to flag leaf emergence, and that after ears emerged K appeared to increase slightly, although these authors were not able to quantify the difference precisely. However, the difference in post-ear-emergence K will only be of consequence in thin crops that do not intercept much light, or in crops toward the end of grain growth when interception by green tissues at the top of the canopy becomes small. Therefore, in the present study, PAR interception accumulated during the GS31 to GS61 period was calculated by applying KPAR at GS39 to incident PAR throughout this period, and assuming that GAI changed linearly with calendar time between sequential samplings. There was no change with modern cultivars in PAR interception during the GS31 to GS61 period (Table 5).

There was a linear increase in preflowering RUE with breeding of 0.012 g MJ–1 yr–1 (P < 0.05; Tables 2 and 5). This was consistent with a tendency for an increase in AGDM at GS61 with breeding, which in turn appeared to be associated with greater accumulation of stem-and-leaf-sheath DM. Our results showed a linear increase in specific leaf dry weight at GS61 of 0.40 g m–2 yr–1 with breeding (P < 0.05, Tables 2 and 5), implying that more modern cultivars may have a greater amount of photosynthetic apparatus per square centimeter of leaf area.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Physiological Basis of Grain Yield Progress
Harvest Biomass and Grain Partitioning
Our results showed a relative genetic gain (the slope of the linear regression of grain yield against year of release, as a percentage of the mean yield) for combine yield of 1.2% yr–1. This figure is greater than rates of approximately 0.45% yr–1 in most regions in the world (Slafer et al., 1994; Donmez et al., 2001), but is comparable with rates of about 0.9% yr–1 reported in other high-output environments, including the CIMMYT wheat program in Mexico (Calderini et al., 1999). The UK Recommended List trials indicate continued yield potential improvement for cultivars released from 1996 to 2004 (Fig. 2) , so yield potential still appears to be a principal achievement of UK wheat breeders, alongside the maintenance of disease and pest resistances and the improvement of grain quality.


Figure 2
View larger version (14K):
[in this window]
[in a new window]

  		Fig. 2. Fungicide-treated grain yield against year of release for Group 3 (biscuit-making and export market) and Group 4 (feed market) cultivars (provisionally recommended or recommended for general use) on HGCA UK Recommended List (RL) 2004/05 (HGCA, 2004).

 
Yield progress from 1972 to 1995 occurred due to increases in both harvest AGDM and HI. Progress in HI was most apparent from about 1972 to 1980, covering the period of introduction of the Rht-D1b cultivars in the UK, whereas progress in AGDM contributed most in cultivars introduced since about 1983. Thus, present results show some correspondence with those of Austin et al. (1989), who attributed UK yield progress from 1953 to 1986 principally to HI, but also reported a nonsignificant trend for increased biomass for the 1981 to 1986 group of cultivars. In the present study, biomass progress explained most of the yield progress within the group of cultivars introduced after 1983. Similar findings for genetic progress in grain yield associated with harvest AGDM are reported for recent winter wheat releases in the U.S. Great Plains (Donmez et al., 2001) and for recent CIMMYT spring wheat releases in Mexico (Singh et al., 1998; Reynolds et al., 1999). Present findings indicate harvest AGDM progress resulted from greater preflowering growth. Preanthesis RUE was positively associated with progress in anthesis AGDM (r = 0.92, P < 0.01), whereas neither canopy size nor KPAR were effective. Calderini et al. (1999) similarly reported leaf area index and light extinction coefficient to be poorly correlated with yield progress in wheat.

Preflowering RUE showed significant genetic change across time and was positively correlated with grain yield (r = 0.78, P < 0.05) in the present study. Most research on the relationship between genetic gains in yield and crop photosynthesis has focused on light-saturated net CO2 exchange rate (Amax) of individual leaves. Some caution should be exercised when extrapolating from leaf Amax to whole-canopy photosynthesis through the crop cycle, because of the need to account for physiological processes such as dark respiration and because individual leaves may operate well below light saturation in the canopy (Reynolds et al., 2000). For spring wheat cultivars, a positive association between Amax and yield improvement has been reported in Mexico (Fischer et al., 1998; Reynolds et al., 2001) and Australia (Watanabe et al., 1994). In these cases, though, the genetic gain in Amax was only detected postflowering and greater values were attributed to an indirect effect of increased sink strength. The apparent improvement in preanthesis RUE for modern cultivars reported in the present study, on the other hand, could imply an increase in intrinsic photosynthetic rate. Since RUE is modified by crop development, pre- and postflowering in wheat (Sinclair and Muchow, 1999), it is not possible to infer from the present results that RUE postflowering was increased in modern cultivars.

The mechanisms responsible for the apparent improvement of RUE in the present study cannot be identified with certainty. Altered photosynthetic metabolism or respiratory costs cannot be discounted (Reynolds et al., 2000), but these processes were not measured in our study. 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 and Blackwell, 1983) and stomatal conductance (Araus et al., 1993) in wheat. Present results showed flag leaves were generally more erect in the modern cultivars (Table 5), but there was no trend in KPAR. Additionally, RUE continued to increase amongst the four most recently introduced cultivars, amongst which there was no change in flag leaf attitude, suggesting it was not a primary determinant of improved RUE. Smaller flag leaf area in modern cultivars may have reduced suprasaturating light intensities at the top of the canopy and improved distribution of light to lower leaves. Additionally, greater flag leaf specific dry weight may have been indicative of an increase in photosynthetic tissues per unit leaf area and hence improved RUE. In summary, present results suggest preflowering RUE as a candidate target in UK wheat breeding programs. Estimating RUE in breeders' plots is infeasible, so further work seems justified to identify surrogates or smart-screens for RUE, including genetic markers, and to develop appropriate protocols for their use in breeding programs.

The 1BL.1RS translocation has been associated with improved harvest AGDM in previous studies (Carver and Rayburn, 1994; Villareal et al., 1994, 1998) and may have contributed at least a part of the biomass progress presently reported. It can be speculated that 1BL.1RS may be associated with improved RUE, and further work seems justified to test whether this is the case. The AGDM trends presently reported seem unlikely to be due to a decrease in root-to-shoot partitioning in modern cultivars. Root reduction of sufficient magnitude to account for the biomass increase of 2.5 Mg ha–1 across 23 yr seems improbable, and there is no evidence of changes in rooting depth with breeding from other UK field investigations (Foulkes et al., 2001a).

Progress in HI occurred mainly from about 1972 to 1980, covering the period of introduction of the Rht-D1b cultivars. It is well established that the Norin 10 semidwarf alleles increase assimilate partitioning to the ear, hence HI (Fischer, 1983; Slafer et al., 1994). No further progress in HI was observed amongst the seven Rht-D1b cultivars in the present study. There was no trend for a further decrease in height amongst the seven Rht-D1b cultivars, which might indirectly support the suggestion that crop height is close to optimum in most modern wheat breeding programs worldwide at about 70 to 80 cm (Richards, 1992; Flintham et al., 1997; Reynolds et al., 1999) and that little yield progress may be available by further reduction of crop height in UK conditions. Austin (1980) suggested a theoretical upper limit to HI of 0.62 for winter wheat grown in UK conditions. This calculation used the dry weight of component organs from the average of four high-yielding cultivars studied by Austin et al. (1980a), and assumed: (i) constant harvest AGDM; (ii) lamina and chaff dry weight could not be reduced because of physiological and mechanical considerations, respectively; (iii) stem and sheath dry weight could be reduced by 50%; and (iv) chaff weight could be increased pro rata to accommodate the extra grain. If this analysis is modified to allow harvest AGDM to increase by 20% (and other assumptions maintained), we calculate the theoretical maximum HI in the UK in future years could be as high as 0.66. In the present study, a maximum HI of 0.53 was observed for Riband in 1997, although another recent UK investigation has shown a HI of 0.61 for Consort, released in 1995 (Spink et al., 2000). In summary, it seems that further improvement of HI in the UK environment may be possible but that it is likely to be only moderate in magnitude.

Ear Fertility and Number of Grains per Square Meter
Present results showed number of grains per square meter was phenotypically correlated with grain yield (r = 0.83, P < 0.001) and contributed to its genetic gain, as was the case in other recent investigations of the physiological basis of genetic gains in winter wheat yields in France (Brancourt-Hulmel et al., 2003) and the Great Plains (Donmez et al., 2001). In the present study, genetic gains in number of grains per square meter were associated with changes across time in both number of ears per square meter and number of grains per ear. The change in number of grains per ear was mainly accounted for by the introduction of the Rht-D1b allele. It is well established that Rht-D1b is associated with altered partitioning in favor of the ear leading to improved spikelet fertility (Fischer, 1983). Number of ears per square meter increased linearly during the 23-yr period, with more ears per square meter in modern cultivars possibly associated with a tendency for greater dry matter accumulation in the GS31 to GS61 period promoting greater tiller survival. Although the modern cultivar Brigadier exhibited a greater grains-to-ear DM ratio at flowering than all other cultivars, there was no trend in this ratio with breeding. Therefore, whereas yield progress has been associated with greater grains-to-ear DM ratio in other countries such as Argentina (Abbate et al., 1998), this appeared not to be the case in modern UK cultivars. There was no genetic change in grain weight across time, consistent with other reports of little change in grain weight with breeding in recent decades worldwide (e.g., Calderini et al., 1999; Brancourt-Hulmel et al., 2003).

The Contribution of Stem Soluble Carbohydrate
The amount of stem WSC accumulated showed significant genetic change across time and was positively correlated with grain yield (r = 0.74, P < 0.05), confirming previous reports of greater stem WSC accumulation in modern UK cultivars (Foulkes et al., 2002). Evidence for significant deposition of stem WSC reserves in grains in the absence of postanthesis stress in wheat was provided by Gebbing et al. (1999), and our results bear out the potential importance of stem WSC for grain yield potential even in the absence of postanthesis stress. In contrast to a previous report of a smaller amount of stem WSC with Rht-Dlb in spring wheat in Australia (Borrell et al., 1993), present results indicate the smallest amount of stem WSC occurred in the rht-Dlb cultivar Maris Huntsman, implying that Rht-Dlb probably did not have a negative effect on stem WSC. There is some evidence that the effect of Rht-Dlb is largely neutral under UK conditions, with greater percentage stem WSC with Rht-Dlb being cancelled out by smaller stem DM, e.g., in spring barley (Hordeum vulgare L.) (Austin et al., 1980b) and in winter wheat (Faulkes et al., 2001b). In summary, our results indicate that stem WSC may have value as in indirect selection criterion in UK conditions. Phenotyping lines in the field for this trait using the established anthrone method (Yemm and 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 (Foulkes et al., 2001b).

Implications for Management of Modern UK Cultivars
At its outset, one of the justifications for this work was the thought that, with changes in preflowering growth favoring partitioning to the ear rather than leaf and stem, modern UK cultivars might be closer to source limitation than their predecessors; and, if so, canopy maintenance with fertilizers and fungicides might be coming more crucial during grain filling (Sylvester-Bradley et al., 1990). In the event, it appears that there have been improvements in source-type traits such as stem WSC, alongside improvements in number of grains per square meter. Although it is not possible to quantify the source-to-sink balances of the cultivars from the measurements reported in this paper, present results suggest modern cultivars might conceivably be no nearer to source limitation than their predecessors. Hence the changes in fertilizer management indicated by these results might be largely associated with the need to maintain protein concentrations (for bread-making wheat) in the increased output of grain. Turning to the need to control lodging, it is clear that the two counteracting effects of decreased height and increased yield have resulted in a decreased risk. However, given that there is no evidence of a further decrease in height since the first cultivar was introduced with Rht-D1b in 1980, it would appear that lodging risk might now be increasing. Clearly, if lodging is not to become a major constraint on yield again, crop managers must pay ever-greater attention to factors which can significantly reduce the risks of root and stem failure, such as use of low plant population densities and PGRs (Berry et al., 2000). Lastly, considering the need for fungicides, improved preanthesis RUE might be expected to lead to a tendency for modern cultivars to be less tolerant of green area loss due to foliar diseases during the preanthesis period. Indeed, there is evidence to suggest that modern UK cultivars have become less tolerant of Septoria tritici Roberge in Desmaz (Parker et al., 2004). This implies there may be an increased need for disease control to minimize green area loss to foliar pathogens, in order that the newer cultivars reach their full yield potential.


   ACKNOWLEDGMENTS
 
We thank the UK government (Department of Environment, Food, and Rural Affairs) for funding the Ph.D. studentship of V.J.S., and Dr. Sayed Azam-Ali of the Division of Agricultural and Environmental Sciences, University of Nottingham, UK, for supervision of that studentship.

Received for publication October 8, 2003.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 


Related articles in Crop Science:

THIS ISSUE IN CROP SCIENCE

Crop Science 2005 45: xi. [Full Text]  



This article has been cited by other articles:


Home page
 J Exp BotHome page
M. A. Semenov and N. G. Halford
Identifying target traits and molecular mechanisms for wheat breeding under a changing climate
J. Exp. Bot., July 1, 2009; 60(10): 2791 - 2804.
[Abstract] [Full Text] [PDF]

Home page
 Crop Sci.Home page
O. Gaju, M.P. Reynolds, D.L. Sparkes, and M.J. Foulkes
Relationships between Large-Spike Phenotype, Grain Number, and Yield Potential in Spring Wheat
Crop Sci., May 11, 2009; 49(3): 961 - 973.
[Abstract] [Full Text] [PDF]

Home page
 J Exp BotHome page
M. Reynolds, M. J. Foulkes, G. A. Slafer, P. Berry, M. A. J. Parry, J. W. Snape, and W. J. Angus
Raising yield potential in wheat
J. Exp. Bot., May 1, 2009; 60(7): 1899 - 1918.
[Abstract] [Full Text] [PDF]

Home page
 J Exp BotHome page
R. Sylvester-Bradley and D. R. Kindred
Analysing nitrogen responses of cereals to prioritize routes to the improvement of nitrogen use efficiency
J. Exp. Bot., May 1, 2009; 60(7): 1939 - 1951.
[Abstract] [Full Text] [PDF]

Home page
 Plant Physiol.Home page
J. Bertheloot, P. Martre, and B. Andrieu
Dynamics of Light and Nitrogen Distribution during Grain Filling within Wheat Canopy
Plant Physiology, November 1, 2008; 148(3): 1707 - 1720.
[Abstract] [Full Text] [PDF]

Home page
 Agron. J.Home page
C. Royo, V. Martos, A. Ramdani, D. Villegas, Y. Rharrabti, and L. F. Garcia del Moral
Changes in Yield and Carbon Isotope Discrimination of Italian and Spanish Durum Wheat during the 20th Century
Agron. J., February 26, 2008; 100(2): 352 - 360.
[Abstract] [Full Text] [PDF]

Home page
 Plant Physiol.Home page
G.-P. Xue, C. L. McIntyre, C. L.D. Jenkins, D. Glassop, A. F. van Herwaarden, and R. Shorter
Molecular Dissection of Variation in Carbohydrate Metabolism Related to Water-Soluble Carbohydrate Accumulation in Stems of Wheat
Plant Physiology, February 1, 2008; 146(2): 441 - 454.
[Abstract] [Full Text] [PDF]

Home page
 ANN BOT (LOND)Home page
J. G. Waines and B. Ehdaie
Domestication and Crop Physiology: Roots of Green-Revolution Wheat
Ann. Bot., October 1, 2007; 100(5): 991 - 998.
[Abstract] [Full Text] [PDF]

Home page
 Crop Sci.Home page
B. Ehdaie, G. A. Alloush, M. A. Madore, and J. G. Waines
Genotypic Variation for Stem Reserves and Mobilization in Wheat: II. Postanthesis Changes in Internode Water-Soluble Carbohydrates
Crop Sci., September 8, 2006; 46(5): 2093 - 2103.
[Abstract] [Full Text] [PDF]

Home page
 Crop Sci.Home page
M. A. Babar, M. P. Reynolds, M. van Ginkel, A. R. Klatt, W. R. Raun, and M. L. Stone
Spectral Reflectance to Estimate Genetic Variation for In-Season Biomass, Leaf Chlorophyll, and Canopy Temperature in Wheat
Crop Sci., March 27, 2006; 46(3): 1046 - 1057.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Related articles in Crop Science
Right arrow Similar articles in this journal
Right arrow Similar articles in Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Web of Science (39)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Shearman, V. J.
Right arrow Articles by Foulkes, M. J.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Shearman, V. J.
Right arrow Articles by Foulkes, M. J.
Agricola
Right arrow Articles by Shearman, V. J.
Right arrow Articles by Foulkes, M. J.
Related Collections
Right arrow Crop Physiology & Metabolism
Right arrow Wheat

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Agronomy Journal Vadose Zone Journal
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome