Identifying physiological traits associated with improved drought resistance in winter wheat

Field Crops Research 103 (2007) 11–24 www.elsevier.com/locate/fcr

Identifying physiological traits associated with improved drought resistance in winter wheat M.J. Foulkes a,*, R. Sylvester-Bradley b, R. Weightman b, J.W. Snape c a

Division of Agricultural and Environmental Sciences, The University of Nottingham, Sutton Bonington Campus, Loughborough, Leicestershire LE12 5RD, UK b ADAS Centre for Sustainable Crop Management, Boxworth, Cambridgeshire CB3 8NN, UK c John Innes Centre, Norwich Research Park, Colney, Norwich NR4 7RU, UK Received 15 August 2006; received in revised form 30 March 2007; accepted 24 April 2007

Abstract The association of specific target traits for drought resistance (early flowering, high accumulation of stem water soluble carbohydrate (WSC) reserves, presence of awns and high green flag-leaf area persistence) with yield performance under late-season drought was analyzed utilizing two doubled-haploid (DH) populations derived from crosses between Beaver  Soissons and Rialto  Spark in two seasons 2000/2001 and 2001/2002. The aim was to quantify associations between target traits and yield responses to drought, and to prioritize traits for drought resistance. Flowering time variation had a neutral effect on the absolute yield loss under drought, suggesting there may be a trade-off between water-saving behaviour in the shorter pre-flowering period with early flowering and a reduced capacity to access water associated with a smaller rooting system. The presence of awns also had a neutral effect on yield loss under drought amongst lines of the Beaver  Soissons population. The potential advantages of awns for increasing water-use efficiency and sensible heat transfer responsible for a cooler canopy appeared to be of less significance under moderate droughts in the UK than under severe droughts in other regions worldwide. The value of large stem soluble carbohydrate reserves for drought environments alone could not be confirmed in the UK environment. Stem WSC was positively associated with grain yield under both irrigation and drought. The genetic trait which showed the clearest correlation with the ability to maintain yield under drought was green flag-leaf area persistence. Averaged across years, the positive phenotypic correlation of this trait with yield under drought amongst DH lines of the Beaver  Soissons population (r = 0.49; p  0.001) indicated the potential use of this trait as a selection criterion for yield under drought. It is suggested that screens for this trait including marker-assisted selection would have value in future breeding programmes aimed at improving yields in high yielding, rainfed environments, but where drought can also be a problem, such as the UK. # 2007 Elsevier B.V. All rights reserved. Keywords: Drought resistance; Wheat; Traits; Breeding

1. Introduction Winter wheat (Triticum aestivum L.) is the most extensive arable crop in the UK, grown on about 2 million ha per annum. Current national average yield is about 8 t ha1 (DEFRA, 2005) and approximately 30% of the wheat area is on drought-prone soils (Foulkes et al., 2001). The annual yield loss to drought,

Abbreviations: AGDM, above-ground dry matter; ANOVA, analysis of variance; AW, available water; D, drought intensity; DH, doubled haploid; GFLA, green flag-leaf area; GS, growth stage; HI, harvest index; NIL, nearisogenic line; WSC, water soluble carbohydrate; S, drought susceptibility index; SMD, soil moisture deficit * Corresponding author. Tel.: +44 1159 516024; fax: +44 1159 516060. E-mail address: [email protected] (M.J. Foulkes). 0378-4290/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fcr.2007.04.007

which typically occurs post-anthesis, is in the region of 15% (Austin, 1978; Foulkes et al., 2002). With predicted climate change and more frequent summer droughts (Hulme et al., 2002) these losses will be exacerbated. New cultivars may be one way of combating these drought effects. In the present study, the potential usefulness to plant breeders of four traits for maintaining yield under drought was examined: early flowering, high accumulation of stem water soluble carbohydrate (WSC) reserves, presence of awns and high green flag-leaf area persistence. Wheat is a long-day plant, with longer days reducing the time to flowering in photoperiod-sensitive cultivars. Southern European wheats are generally photoperiod insensitive, whereas wheats bred in the UK are sensitive (Worland et al., 1994). This reflects a requirement for wheat crops in southern

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Europe to flower and mature early to avoid regularly hot and desiccating summers. Examining near-isogenic lines (NILs) for the Ppd-D1 gene, which typically advances flowering by about 8 days in the UK, in winter wheat cv. Mercia, Worland (1996) found that summer stress avoidance associated with early flowering promoted a yield advantage of ca. 5%. However, Foulkes et al. (2004) observed neutral responses of yield to Ppd-D1 under post-anthesis drought. The latter authors suggested there may be a trade-off between early flowering and less extensive rooting. Effects of flowering were investigated in the present study by examining the performance of doubled-haploid (DH) lines derived from a cross between Beaver (Ppd-D1b photoperiod sensitive allele)  Soissons (Ppd-D1a photoperiod insensitive allele). By flowering, reserves of water soluble carbohydrate, mostly as fructans, have accumulated in the stems and leaf sheaths of the crop. Maximal amounts are accumulated about 9 days after flowering (Austin et al., 1977; Foulkes et al., 2002). The relative contribution of stem reserves to grain yield varies widely depending on environmental conditions and cultivars, from 6 to 100% (Austin et al., 1980; Blum et al., 1994; Borrell et al., 1993). In general, a reduction in current assimilation under post-anthesis moisture stress will induce greater stem reserve mobilisation to, and utilisation by, the grain (Palta et al., 1994; Yang et al., 2000). Thus, a significant proportion of reserves is usually re-translocated to grains under drought to buffer effects of accelerated senescence (Bidinger et al., 1977; Schynder, 1993). The amount of stem WSC accumulated varies amongst modern UK cultivars in the range 254–447 g m2 (Foulkes et al., 1998), and Foulkes et al. (2002) reported higher reserves to be associated with better maintenance of harvest index (ratio of grain to above-ground biomass at harvest; HI) under drought in six UK-grown cultivars. The genetic association between stem WSC and maintenance of HI and yield under drought was tested more rigorously in the present study by characterizing two DH populations segregating for this trait. Estimates of the relative contribution made by ear photosynthesis to final grain weight vary between 10 and 76% depending on genotype and environment (Biscoe et al., 1975; Duffus et al., 1985; Evans et al., 1975). The greater photosynthetic area of awned ears can be particularly important under source-limited conditions such as drought. The advantage of awns is related to their high ratio between carbon exchange rate and transpiration rate associated with a high water-use efficiency (Blum, 1985) and to the increase in sensible heat transfer responsible for a cooler canopy (Weyhrich et al., 1994). Long awns have been demonstrated to be a sensible selection criterion in wheat for improved production in hot, dry environments (Blum, 1986). In the temperate conditions of the UK, virtually all winter wheat cultivars are unawned. Whereas there is no apparent yield potential advantage associated with awns in UK conditions, it is not clear whether awns may help maintain yields under drought. The genetic association between awns and yield under drought was presently tested by characterizing the Beaver (unawned)  Soissons (awned) DH population.

Flag-leaf photosynthesis in wheat contributes about 30– 50% of the assimilates for grain filling (Sylvester-Bradley et al., 1990), and the onset and rate of senescence are clearly important factors for determining resistance to abiotic stress. ‘Stay-green’ is a reported component of tolerance to terminal drought stress in cereals such as sorghum (Borrell et al., 2000) and maize (Campos et al., 2004). Effects of green flag-leaf area persistence on drought resistance of winter wheat in the UK were previously reported by the present authors (Verma et al., 2004), in the Beaver  Soissons DH population in two seasons (1999/2000 and 2000/2001). The trait exhibited a positive correlation with yield under drought and a major QTL for leaf persistence under drought was reported on the long arm of chromosome 2D. In the present study the effects of leaf persistence on drought resistance were further investigated in the Beaver  Soissons population in the season of 2001/2002. The relationships between traits and drought performance were currently investigated using two DH populations derived from parents contrasting for target traits according to previous field characterisation (Foulkes et al., 1998). The effects of awns and flag-leaf persistence were examined in the Beaver (Ppd-D1b late flowering, intermediate stem WSC, unawned, low leaf persistence)  Soissons (Ppd-D1a early flowering, intermediate stem WSC, awned, high leaf persistence) population. The effects of flowering time and stem WSC were examined in both the Beaver  Soissons and the Rialto (Ppd-D1b intermediate flowering, unawned, high stem WSC, intermediate leaf persistence)  Spark (Ppd-D1b late flowering, low stem WSC, unawned, intermediate leaf persistence) population. This was possible since the Beaver  Soissons population showed transgressive segregation for stem WSC. The parents of each cross also contrasted for the presence/ absence of the 1BL1RS wheat (Triticum aestivum L.)–rye (Secale cereale) translocation, involving the short arm of rye chromosome 1R. Genes located on the fragment of rye chromosome have been reported to determine drought resistance (Villareal et al., 1998; Ehdaie et al., 2003). Therefore, the effect of the translocation on drought performance was quantified in the present study. Winter wheat cultivars incorporating 1BL1RS were first released in the UK in the late 1980s. In this paper, we report on field experiments examining two DH populations over two seasons (2000/2001 and 2001/2002) at Gleadthorpe, Nottinghamshire, UK, where an irrigation treatment was imposed to examine responses to drought. Responses of traits, harvest above-ground biomass, grain yield and yield components to drought were examined. The aims were to: (i) quantify associations between traits and yield responses to drought, (ii) investigate the physiological bases of any associations between traits and yield responses to drought, and (iii) quantify the effects of 1BL1RS on drought resistance in the UK’s temperate climate. It was hoped that information on traits and the 1BL1RS translocation would provide selection criteria for breeding and cultivar choice in drought-prone environments in the UK and worldwide in future years.

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2. Materials and methods 2.1. Plant material Two DH populations were developed using the wheat  maize technique. A population of 46 DH lines was derived from the F1 between Beaver and Soissons. A population of 144 DH lines was derived from the F1 between Rialto and Spark of which a random selection of 26 lines was characterised in the present study. 2.2. Site and experimental treatments Field experiments were carried out at an experimental site at Gleadthorpe, Nottinghamshire (558130 N, 1860 W) in two seasons. In 2000/2001 there were two experiments examining: (i) a random sample of 34 of the 46 Beaver  Soissons DH lines and the two parents and (ii) the 26 Rialto  Spark DH lines and the two parents. In 2001/2002 there was one experiment examining the 46 Beaver  Soissons DH lines and the two parents. In each experiment, the soil was a loamy medium sand to 0.35 m over medium sand (Cuckney Series) with good drainage. The experiments used a randomised block, split–plot design, in which two irrigation treatments (fully irrigated and unirrigated) were randomised on main-plots and the DH lines and the two parents were randomised on sub-plots (1.4 m  6 m). Eight metre discards separated main-plots and there were three replicates. In the irrigated main-plots, water was applied using a linear overhead irrigator to maintain soil moisture deficit (SMD), calculated using the ADAS Irriguide model (Bailey and Spackman, 1996), to <50% available water (AW; 53 mm) up to GS61 + 28 d and <75% AW (77 mm) thereafter. The previous crop was potatoes in both seasons. The experiments were sown on 17 December 2000 and 14 October 2001. Sowing was later in 2000 due to high rainfall during October and November reducing the trafficability of the soil in this season. Seed rate was adjusted by genotype according to 1000-grain weight to achieve a target seed number of 325 m2; rows were 0.12 m apart. In each experiment, 220 kg N ha1 nitrogenous fertilizer as ammonium nitrate was applied in a four-split programme; 40 kg N ha1 was applied in early March, 70 kg N ha1 in late March, 60 kg N ha1 in early May with the remainder in mid May. Prophylactic applications of fungicides were given at GS31, GS39 and GS59 (Tottman, 1987), to keep diseases to very low levels. No plant growth regulators were applied. Pesticides and herbicides were used as necessary to minimise the effects of pests and weeds. 2.3. Crop measurements Date of flowering (GS61; Tottman, 1987) was recorded on all sub-plots in each experiment as the date when 50% of shoots had reached this stage. This was done by observing sub-plots every 2–3 days from the middle of May to the end of June. The amount of stem WSC was measured in the experiments examining the Rialto (R)  Spark (Sp) population in 2000/ 2001 and the Beaver (B)  Soissons (So) population in 2001/

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2002. In each case, the percentage WSC content of stems and attached leaf sheaths was assessed in 10 randomly selected fertile shoots (those with an ear) per sub-plot at GS61 + 75 8C d (base temperature 0 8C), using the anthrone method of Yemm and Willis (1954) as described by Gay et al. (1998). Since GS61 + 75 8C d corresponds approximately to GS61 + 5 d in the UK environment, the amount of WSC in stems and leaf sheaths was close to maximal at this point (Austin et al., 1977). Fertile shoot number was assessed shortly after GS61 by counting all fertile shoots in each of three randomly selected 0.5 m row-lengths per sub-plot. Stem WSC amount was then calculated as the dry weight of WSC in the stem and leaf sheath per fertile-shoot  fertile shoot number per m2. The presence/absence of awns was recorded visually in each sub-plot of the experiments examining the B  So population to confirm classifications previously described during seed multiplication of the DH lines. The percentage green flag-leaf area (GFLA) was measured in the experiments examining the B  So population in 2000/ 2001 and 2001/2002. Percentage GFLA was measured in each sub-plot by a visual assessment of all fertile shoots in situ at 14 (+14 d) and 35 days (+35 d) after flowering (GS61). In each experiment, two to three days prior to harvest approximately 100 shoots were sampled at random per subplot. All above-ground plant material was separated into ears and straw. Ears were counted, threshed and the chaff (rachis, rachilla, glumes, palea, and lemma) and grain weighed separately after drying for 48 h at 80 8C. A 50% subsample of the straw (by fresh weight) was weighed after drying for 48 h at 80 8C. The harvest index was calculated as the fraction of above-ground dry matter (AGDM) present as grain. In each experiment, combine (=machine harvested) grain yields were assessed on a 1.4  5 m2 area in each sub-plot, and values adjusted to 15% moisture. Harvest AGDM was calculated from combine grain yield and the HI. 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 8C. Number of grains per ear was obtained from the individual grain weight and grain weight per ear obtained from the 100-shoot sample pre-harvest. Ears per square metre at harvest were calculated from the combine grain yield, the individual grain weight and the number of grains per ear. 2.4. 1BL1RS classifications The presence of the 1BL1RS translocation in each DH line was determined by evaluating the genotype for simple sequence repeat (SSR) molecular markers specific for loci on chromosome 1BS, namely psr3000, barc149, barc08, gwm11 and gwm413. The presence of a PCR product from DNA of a line indicated the 1BS arm and the absence of a product the 1RS arm. For the Beaver (1BL1RS)  Soissons (1B) population, in 2001, 34 DH lines (16 1BL1RS and 18 1B) were characterized. In 2002, 46 DH lines (24 1BL1RS and 22 1B lines) were characterized, i.e., an additional eight 1BL1RS and four 1B lines. For the Rialto (1BL1RS)  Spark (1B) population, in 2001, 26 (13 1BL1RS and 13 1B) DH lines were characterized.

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2.5. Statistical analysis For individual experiments, analyses of variance (ANOVA) were carried out for physiological traits and grain yield using Genstat version 6.1 (Lawes Agricultural Trust, Rothamsted Experimental Station). Replications were regarded as random effects, while irrigation and genotype were fixed effects. 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. Treatment means were compared using the least significant difference 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. The effect of the presence/absence of 1BL1RS or presence/absence of awns was tested as a contrast between two levels of the main effect of genotype. Treatment means for the 1BL1RS and 1B or for the awned and unawned groups of lines were compared using the variance ratio since there were only two levels for the comparison. Regression analysis using simple linear and multiple linear models (Genstat 6.3) was applied to genotype means in irrigated and unirrigated treatments to examine associations between traits and grain yield. For the simple linear regressions, regression coefficients are presented for the statistically significant regressions. For the multiple linear regressions, the adjusted R2 statistic values are presented calculated as:    residual mean square Adjusted R2 ¼ 100 1  total mean square

(1)

Correlations between traits and between traits and grain yield were calculated using the genotype means. 3. Results 3.1. Growing conditions The available water to 1.2 m at Gleadthorpe is 105 mm. Previous work on this site has shown that the limiting deficit at which onset of drought occurs is 50% AW pre-flowering and 65% AW post-flowering (Foulkes et al., 2001). In 2000/2001, SMD calculated using the irriguide model (Fig. 1) exceeded the limiting deficit from 27 May (ear emergence) to 17 July (late grain fill). Averaging across the two DH populations, grain yield was reduced under drought by 2.62 t ha1 (Table 1). In 2001/2002, onset of drought occurred on 6 May (flag-leaf emergence) and was maintained, with the exceptions of 27–30 May and 6 June, to harvest. Grain yield was reduced on average by 1.65 t ha1. 3.2. Grain yield responses to drought Averaging across DH lines, drought reduced grain yield by 2.82 t ha1 in the B  So population in 2001 ( p  0.001), by 2.41 t ha1 in the R  Sp population in 2001 ( p  0.01) and by 1.65 t ha1 in the B  So population in 2002 ( p  0.05; Table 1). In the R  Sp population in 2001 the lines responded

Fig. 1. Soil moisture deficit from the start of April to the end of July estimated by the irriguide model for winter wheat (Bailey and Spackman, 1996) in the irrigated treatment in 2001 (continuous line), unirrigated treatment in 2001 (broken line), irrigated treatment in 2002 (bold continuous line) and unirrigated treatment in 2002 (bold broken line).

differently to drought; responses (i.e., differences between irrigated- and drought-grown pairs) ranged from 1.33 to 3.26 t ha1 ( p  0.001). Similarly, in the B  So population in 2002, the yield responses of the lines to drought were different in the range 0.48–2.47 t ha1 ( p  0.05). The responses of the B  So lines in 2001 were not significantly different in the range 1.87–4.22 t ha1. In each experiment, there was a positive phenotypic correlation amongst the lines between grain yield potential (as indicated by yield under irrigation) and the absolute grain yield loss to drought ( p  0.05; Table 2). That is, lines with the highest yield potential lost most yield under drought. In general, though, there were few ‘cross-over’ interactions and the lines with highest yields under irrigation still yielded highest under drought. In each population in 2001, drought reduced grains ear1 and individual grain weight ( p  0.05), but not ears m2. In 2002 in the B  So population, when onset of drought was earlier, drought reduced ears m2 and individual grain weight ( p  0.05), but there was no effect on grains ear1. 3.3. Effect of flowering date on yield response to drought Drought had only small effects on flowering date, advancing GS61 on average by 1 day in the three experiments; genotypes responded similarly to drought (Fig. 2). Averaging across irrigation treatments, flowering ranged from 14 to 26 June in the B  So lines in 2001, 20–26 June in the R  Sp lines in 2001, and 28 May to 6 June in the B  So lines in 2002. Correlations between flowering date and grain yield in the R  Sp population were not statistically significant under irrigation or drought (Table 2). Similarly, correlations between flowering date and stem WSC were not significant. In the B  So population, the range in flowering was extended by segregation for the Ppd-D1 gene. Flowering date was positively correlated with yield under drought in 2001 ( p  0.08) and 2002 ( p  0.05; Table 2; Fig. 2), and under irrigation in 2002 ( p  0.05); there was no association under irrigation in 2001. The net result of these associations was that there was a trend for flowering date to be negatively correlated with yield loss under drought in 2001 ( p  0.06), i.e., later flowering lines

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Table 1 Grain yield (85% DM), above-ground dry matter (AGDM, 100% DM), ear number per m2, grains per ear and grain weight (100% DM) for means of doubled-haploid lines and their parents in the irrigated (Irr) and unirrigated (Unirr) treatments for the (a) Beaver  Soissons population in 2001 (34 lines) and 2002 (46 lines) and (b) Rialto  Spark population in 2001 (26 lines) 2001

2002

Grain Yd t ha1

AGDM t ha1

Ears m

Grains ear1

Grains wt mg

Grain Yd t ha1

AGDM t ha1

Ears m2

Grains ear1

Grain wt mg

8.98 10.27 9.41

15.68 17.57 15.27

550.8 569.1 571.7

34.9 36.8 32.1

40.8 41.7 40.0

7.56 8.55 8.34

13.00 14.98 13.92

431.7 462.4 521.1

40.4 43.3 37.1

37.7 36.6 36.8

Unirr Mean Beaver Soissons

6.16 7.14 6.52

12.56 13.51 12.55

535.5 481.4 600.4

28.4 35.5 28.7

35.4 39.6 32.3

5.91 6.81 6.15

10.69 11.18 10.61

378.0 376.5 445.4

40.1 44.3 34.7

34.4 34.4 33.9

SED Irr (DF = 2)

0.118

(a) Beaver  Soissons Irr Mean Beaver Soissons

0.142

2

13.03

1.19

0.65

0.302

0.765

14.06

0.49

0.60

Grain Yd t ha1

AGDM t ha1

Ears m2

Grains ear1

Grain wt mg

9.15 10.08 9.13

16.86 16.92 16.77

585.3 468.2 707.9

35.4 42.3 32.6

38.5 43.5 35.0

Unirr Mean Rialto Spark

6.74 6.97 6.72

13.11 12.91 12.11

538.8 419.5 545.7

30.0 35.0 34.9

36.3 40.2 31.1

SED Irr (DF = 2)

0.086

(b) Rialto  Spark 2001 Irr Mean Rialto Spark

0.316

showed smaller yield losses, whereas there was no association in 2002. Results showed no association between flowering and stem WSC (Table 2). Earlier flowering was associated with greater leaf persistence under irrigation in the B  So population in both years ( p  0.01). The correlations under drought were not significant. 3.4. Effect of stem water soluble carbohydrate on yield response to drought Stem WSC was measured in the R  Sp population in 2001 and the B  So population in 2002. Under irrigation, lines differed in the ranges 1.71–4.26 and 1.52–4.23 t ha1, respectively ( p  0.001; Fig. 3). Drought decreased reserves in the B  So population in 2002 by 0.68 t ha1 ( p  < 0.05). However, the extent of the reduction was similar for the lines. There was no effect of drought in the R  Sp population in 2001. There was a positive linear relationship between stem WSC and grain yield under irrigation ( p  0.001) and under drought ( p  0.001) amongst the B  So lines in 2002 (Fig. 3b). However, there was no association between stem WSC (measured either under irrigation or drought) and the yield loss to drought (Table 2). Parallel regression analysis showed that the slope of stem reserves on yield was not significantly different in the irrigated and unirrigated treatments (0.47 and 0.66 t grain (t stem)1 WSC, respectively). Therefore, the

29.70

1.49

0.39

proportion of reserves remobilised to the grains was apparently similar with or without drought. Similar trends were observed in the R  Sp population in 2001. In this case, however, the positive slope was only weakly significant under irrigation ( p  0.07) and not significant under drought (Table 2; Fig. 3b). The genetic differences in stem WSC accumulation were associated with differences in both stem dry matter and stem %WSC. Thus, in both populations, there was a positive phenotypic correlation amongst the lines between each of stem dry matter and stem %WSC and the amount of stem WSC ( p  0.05; Fig. 4). However, the proportion of variance in stem WSC accounted for by stem dry matter was generally greater than that accounted for by %WSC. When stem WSC was reduced under moisture stress in the B  So population in 2002, this was associated with smaller stem dry matter by 1.49 t ha1 ( p  0.05) but no change in the % WSC. In the R  Sp population in 2001, there was no decrease in stem WSC under drought. In this experiment, stem dry matter was reduced by 1.75 t ha1 under drought ( p  0.05), but this was counteracted by an increase in stem %WSC from 32 to 37. 3.5. Effect of awns on yield response to drought The effect of awns was examined in the Beaver (unawned)  Soissons (awned) population. There was no effect of awns on the grain yield loss of the DH lines under drought in either 2001 or 2002 (Table 3). Averaging across

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Table 2 Phenotypic correlations (r) amongst doubled-haploid lines for combine grain yield (t ha1 85% DM), stem water soluble carbohydrate (WSC) at GS61 + 75 8Cd (base temperature 0 8C), flowering date (GS61) and percentage green flag-leaf area (GFLA) at GS61 + 14 days and GS61 + 35 days in irrigated and unirrigated treatments in (a) Beaver  Soissons population in 2001 (shaded matrix) and 2002 (unshaded matrix) and (b) Rialto  Spark population in 2001 a) Beaver  Soissons in 2001 and 2002

* =p  0.05, ** =p  0.01, *** =p  0.001, y =p  0.10.

years, the yield of awned lines was reduced under drought by 2.25 t ha1 compared to 2.24 t ha1 for unawned lines. Awned lines maintained ears m2 better under drought ( p  0.006), but this was cancelled out by poorer maintenance of individual

grain weight ( p  0.001; data not shown). Averaging across years and irrigation treatments, awns increased harvest biomass by 0.59 t ha1 ( p  0.001). There were small inconsistent effects of awns on grain yield ( p  0.001), with, averaging

Fig. 2. Linear regression of flowering date on grain yield (t ha1 85% DM) in irrigated (~) and unirrigated (~) treatments for (a) 34 Beaver  Soissons DH lines in 2001 ((~) R2 = 0.01; (~) y = 0.07x + 4.40, R2 = 0.10, p  0.10), (b) 46 Beaver  Soissons DH lines in 2002 ((~) y = 0.09x + 6.82, R2 = 0.10, p  0.05; (~) y = 0.09x + 5.82, R2 = 0.13, p  0.001), and (c) 26 DH Rialto  Spark lines in 2001 ((~) R2 = 0.01; (~) R2 = 0.02).

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Fig. 3. Linear regression of stem water soluble carbohydrate at GS61 + 75 8C d (base temperature 0 8C d) on grain yield (t ha1 85% DM) in irrigated (~) and unirrigated (~) treatments for (a) 26 Rialto  Spark DH lines in 2001 ((~) y = 0.32x + 8.10, R2 = 0.13, p  0.10; (~) R2 = 0.05) and (b) 46 Beaver  Soissons DH lines in 2002 ((~) y = 0.47x + 6.03, R2 = 0.22, p  0.001; (~) y = 0.66x + 4.23, R2 = 0.35, p  0.001).

across irrigation treatments, an overall decrease of 0.21 t ha1 in 2001 and an increase of 0.27 t ha1 in 2002 ( p  0.001). Awned lines amassed slightly greater amounts of stem WSC than the unawned lines in 2001 ( p  0.001), but there was no difference in 2002. There was no effect of awns on flag-leaf persistence in either year. 3.6. Effect of green flag-leaf area persistence on yield response to drought The effects of green flag-leaf area persistence were examined in the B  So population. At GS61 + 14 d, drought decreased %GFLA from 88 to 45 in 2001 ( p  0.001) but there was no decrease in 2002. This was consistent with a higher SMD during the first two weeks of grain filling and a larger yield reduction under drought in 2001 than 2002. Averaged across years, %GFLA ranged from 90 to 66 ( p  0.001) amongst lines under irrigation, and from 62 to 33 under drought ( p  0.001). The phenotypic correlation amongst the lines between %GFLA and yield was not

significant either under irrigation or drought in 2001. Nevertheless, there was a negative correlation between %GFLA under drought and yield loss under drought. That is, lines which showed greater leaf persistence under drought at GS61 + 14 d exhibited smaller yield losses ( p  0.01; Table 2). In 2002, there was a positive correlation between %GFLA and yield under irrigation ( p  0.001) and drought ( p  0.05). However, in this case, the correlation between %GFLA under drought and the yield loss under drought was not significant. Later during the grain filling period, at GS61 + 35 d, drought overall decreased %GFLA from 51 to 7 ( p  0.05) in 2001 and from 42 to 11 in 2002 ( p  0.05; Fig. 5). Averaging across years, lines differed in the range 72–26% under irrigation and 17–4% under drought ( p  0.001). The correlation between %GFLA in 2001 and %GFLA in 2002 was significant under both irrigation (r = 0.90; p  0.001) and drought (r = 0.85; p  0.001), indicating consistent genotype rankings across years. Under drought there was a positive correlation between %GFLA and grain yield in 2002

Fig. 4. Linear regressions of stem DM on stem water soluble carbohydrate (WSC) DM at GS61 + 75 8C d (base temperature 0 8C d) in irrigated (~) and unirrigated (~) treatments for (a) 26 Rialto  Spark lines in 2001 ((~) y = 0.34x  0.21, R2 = 0.55, p  0.001; (~) y = 0.35x + 0.16, R2 = 0.81, p  0.001) and (b) 46 Beaver  Soissons lines in 2002 ((~) y = 0.43x  0.44, R2 = 0.89, p  0.001; (~) y = 0.23x + 0.56, R2 = 0.43, p  0.001); and linear regressions of stem percentage WSC on stem WSC DM at GS61 + 75 8C d for (c) 26 Rialto  Spark lines in 2001 ((~) y = 0.102x  0.08, R2 = 0.51, p  0.001; (~) y = 0.066x + 0.60, R2 = 0.14, p  0.10) and (d) 46 Beaver  Soissons lines in 2002 ((~) y = 0.097x  1.12, R2 = 0.24, p  0.01; (~) R2 = 0.10).

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M.J. Foulkes et al. / Field Crops Research 103 (2007) 11–24

Table 3 Above-ground dry matter (AGDM), combine grain yield (85% DM), stem water soluble carbohydrate (WSC) at GS61 + 75 8C d (base temperature 0 8C), flowering date (GS61) and percentage green flag-leaf area (GFLA) at GS61 + 14 days (+14 d) and GS61 + 35 days (+35 d) in irrigated and unirrigated treatments in groups of awned and unawned doubled-haploid lines in the Beaver  Soissons population in 2001 and 2002 2001 Irrigated Awns AGDM 100% DM t ha1 Grain yield t ha1 85% DM Stem WSC t ha1 %GFLA GS61 + 14 d %GFLA GS61 + 35 d a b c

15.65 8.86 – 86.1 49.7

Significancec

2002

a

Unirrigated

Irrigated

Unawned

Awns

Unawned

Awns

15.74 9.16 – 84.7 51.4

12.65 6.12 – 47.5 6.6

12.42 6.24 – 48.9 8.4

13.47 7.73 3.39 81.3 42.1

b

Unirrigated Unawned

Awns

Unawned

12.46 7.36 3.06 72.3 41.5

11.23 5.97 2.67 75.8 11.4

10.03 5.80 2.45 70.2 10.8

Year

Irr

Awns

Year   awns

Irr   awns

* * – *** NS

** *** * *** ***

*** *** *** *** NS

NS y – NS NS

NS NS NS * NS

2001: Awned 20 lines and unawned 13 lines. 2002: Awned 24 lines and unawned 21 lines. * =p  0.05, ** =p  0.01, *** =p  0.001, y =p  0.10.

( p  0.001; Fig. 5b), and lines with greater leaf persistence under drought showed smaller yield losses under drought ( p  0.001; Table 2). For the mean values across the 2 years, there was again a positive correlation between %GFLA and grain yield under drought ( p  0.001; Fig. 5c), and lines with greater leaf persistence under drought showed smaller yield losses under drought (r = 0.33; p  0.09). The relationship between %GFLA and flowering date was not significant under drought. However, as mentioned above, in both years, under irrigation earlier flowering lines exhibited greater flag-leaf persistence ( p  0.01). 3.7. Effect of combinations of traits on yield responses to drought In the B  So population in 2002 each of flowering time, stem WSC and flag-leaf persistence were measured so multiple linear regression analysis was applied to examine whether a combination of traits gave a closer association with yield performance under drought than individual traits (Table 4).

Under drought, a combination of %GFLA and stem WSC (46%) accounted for more of the variance in yield than %GFLA alone (18%; p  0.001; Table 4), and a combination of all three traits accounted for more of the variance in yield (60%) than %GFLA and stem WSC alone ( p  0.001). Similar effects were observed under irrigation, although the percentage of variance in yield accounted for by all three traits was less (31%) than that under drought. With regard to the absolute yield loss to drought, neither a combination of %GFLA and stem WSC or of all three traits measured under drought accounted for significantly more of the variance in yield loss than %GFLA alone measured under drought. In the B  So lines in 2001, in which flowering time and %GFLA only were measured, a combination of the two traits accounted for no more of the variance in yield under drought or in yield loss to drought than individual traits. Similarly, for the R  Sp lines in 2001, in which flowering time and stem WSC only were measured, a combination of the two traits accounted for no more of the variance in yield under drought or in yield loss to drought than individual traits.

Fig. 5. Linear regression of percentage flag-leaf area remaining green at GS61 + 35 days on grain yield (t ha1 85% DM) in irrigated (~) and unirrigated (~) treatments for (a) 34 Beaver  Soissons lines in 2001 ((~) R2 = 0.01; (~) R2 = 0.06), (b) 46 Beaver  Soissons lines in 2002 ((~) R2 = 0.01; (~) y = 0.06x + 5.23, R2 = 0.22, p  0.001), and (c) the mean values for 34 Beaver  Soissons lines in 2001 and 2002 ((~) R2 = 0.01; (~) y = 0.09x + 5.25, R2 = 0.24, p  0.001).

M.J. Foulkes et al. / Field Crops Research 103 (2007) 11–24

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Table 4 Adjusted R2 values for stepwise multiple linear regression of traits (percentage green flag-leaf area at GS61 + 35 days, stem water soluble carbohydrate (WSC) at GS61 + 75 8Cd (base temperature 0 8C) and flowering date (GS61)) on grain yield or on grain yield loss (irrigated–unirrigated yield) for 46 doubled-haploid lines in the Beaver  Soissons population in 2002

Yield Yield Yield Yield

(unirri) vs. trait(s) (unirri) (unirr) vs. trait(s) (irr) (irri) vs. trait(s) (irri) loss vs. trait(s) (unirr)

%GLFA + 35 d

%GLFA + 35 d + Stem WSCb

%GLFA + 35 d + Stem WSC + Floweringc

18.4**

46.3*** 21.2*** 21.3*** 13.7NS

59.7*** 24.1NS 31.1* 13.4NS

a a

13.0**

a

Adjusted R2 not calculated since residual mean square > mean square of response variate. Probability (* =p  0.05, ** =p  0.01, *** =p  0.001) for values in columns indicates whether regression for two traits increased the variance in yield or yield loss accounted for compared to %GFLA alone. c Probability (* =p  0.05, ** =p  0.01, *** =p  0.001) for values in columns indicates whether regression for three traits increased the variance in yield or yield loss accounted for compared to two traits. b

individual grain weight, but these were inconsistent with an increase of 1.4 mg in 2001 and decrease of 0.7 mg in 2002 ( p  0.001, data not shown). The effect of 1BL1RS to increase harvest biomass in both irrigation treatments was also observed in the R  Sp population ( p  0.01; Table 5). There was again, however, a neutral effect of 1BL1RS on the responses of each of harvest biomass and grain yield to drought.

3.8. Effect of 1BL1RS translocation on yield response to drought In the B  So population, averaging across irrigation treatments, 1BL1RS increased harvest biomass by 0.47 t ha1 in 2001 and 0.42 t ha1 in 2002 ( p  0.05; Table 5). This resulted in a positive effect of 1BL1RS on grain yield in 2002 ( p  0.01); there was a neutral effect of 1BL1RS on yield in 2002 due to a counteracting decrease in HI. There was a neutral effect of 1BL1RS on the responses of each of harvest biomass and grain yield to drought in both years. 1BL1RS reduced %GFLA by 6–9 in both years ( p  0.001) under irrigation, whereas the effect of 1BL1RS on %GFLA under drought was not statistically significant. There was a small delay in flowering with 1BL1RS by 2 days in 2001 and 1 day in 2002. 1BL1RS had small effects on

4. Discussion The data collected allowed us to consider firstly the relationship between yield potential (as indicated by yield under irrigation) and drought resistance and secondly the potential value of the physiological traits for drought resistance in winter wheat.

Table 5 Above-ground dry matter (AGDM), combine grain yield (85% DM), ears per m2, grains per ear, individual grain weight, stem water soluble carbohydrate (WSC) at GS61 + 75 8Cd (base temperature 0 8C), flowering date (GS61) and percentage green flag-leaf area (GFLA) at GS61 + 14 days and GS61 + 35 days in irrigated and unirrigated treatments for groups of 1BL1RS and 1B doubled-haploid lines in the (a) Beaver  Soissons population in 2001 and 2002 and (b) Rialto  Spark population in 2001 2001 Irrigated 1BL1RS (a) Beaver  Soissons population 16.02 AGDM t ha1 Grain yield t ha1 85% DM 9.10 Stem WSC t ha1 – %GFLA GS61 + 14 d 86.1 %GFLA GS61 + 35 d 45.5

Unirrigated a

Irrigated

1B

1BL1RS

1B

1BL1RS

15.36 8.87 – 85.0 54.8

12.70 6.24 – 48.3 7.1

12.42 6.09 – 47.8 7.5

12.91 7.48 3.22 76.5 39.3

Irrigated

(b) Rialto  Spark population in 2001 AGDM 100% DM t ha1 Grain yield t ha1 85% DM Stem WSC t ha1 a b c d

Significance d

2002 Unirrigated b

1B

1BL1RS

1B

13.04 7.63 3.27 77.3 45.1

11.15 5.90 2.57 71.9 10.7

10.18 5.92 2.58 74.8 11.9

Year

Irr

1B1R

Year  1B1R

Irr  1B1R

* * – *** NS

*** ** * *** ***

* NS NS * ***

NS ** NS NS

NS NS NS NS ***

Significance d

Unirrigated

1BL1RSc

1B

1BL1RS

1B

Irr

1B1R

Irr  1B1R

17.10 9.16 3.49

16.80 9.14 2.98

13.02 6.67 3.27

13.19 6.79 2.82

** *** NS

** NS NS

NS NS NS

2001: 16 1BL1RS and 18 1B lines. 2002: 24 1BL1RS and 22 1B lines. 13 1BL1RS and 13 1B lines. * =p  0.05, ** =p  0.01, *** =p  0.001.

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M.J. Foulkes et al. / Field Crops Research 103 (2007) 11–24

4.1. Relationship between yield potential and drought resistance Overall yield was reduced by 2.19 t ha1 (26%) in the B  So population and by 2.41 t ha1 (26%) in the R  Sp population under drought. These yield losses were broadly typical of late-season droughts in winter wheat in the UK (Innes et al., 1985; Foulkes et al., 2002). In both populations, the highest yielding lines under irrigation tended to lose most yield under drought but still had the highest yields under the moderate water stress conditions applied in this study. This is consistent with other investigations into genotypic responses to applied moderate moisture stress under relatively controlled conditions, e.g., in Israel (Blum, 1986) and Mexico (Fischer and Maurer, 1978). The ‘crossover point’ at which a negative correlation is observed between yield potential and yields under drought usually occurs when yields under drought are in the region of 2 t ha1, significantly below the level of droughted yields in this study. From the physiological standpoint, it is not surprising that absolute reduction in yield for a given reduction in water resource is strongly influenced by yield potential. Fischer and Maurer (1978) previously showed that higher yield potential is associated with greater yield loss per unit increase in drought intensity. Those authors proposed an equation to express the separate effects of yield potential (Yp) and drought susceptibility on yields under drought (Y), as Y ¼ Yp ð1  ðS  DÞÞ

(2)

where D (drought intensity) = 1  X/Xp; X is the mean yield of genotypes under drought; Xp the mean yield of genotypes in non-limiting conditions; and S (drought susceptibility index) = D(Y/Yp)/D(X/Xp). The drought susceptibility index is independent of yield potential and drought intensity, and is potentially useful for comparisons of drought susceptibility of genotypes between drought levels and experiments. For the B  So lines, we have calculated S in addition to the absolute yield loss and the relative yield (Y/Yp) under drought. Amongst the 34 DH lines

grown in both experimental seasons, S ranged from 0.74 to 1.22. For the R  Sp population, S could not be calculated since the population was only examined in one season. In the following sections of this discussion the extent to which the differences in yield responses to drought amongst the genotypes can be associated with differences in physiological traits will be analyzed. In doing so, the phenotypic correlations amongst the lines between traits and the absolute yield reduction under drought, the relative yield under drought and S will be considered (Table 6). 4.2. Relationship between flowering date and drought resistance Early flowering has been associated with drought escape in spring wheat in environments subjected to severe early season drought stress, e.g., in northern Mexico (Fischer and Maurer, 1978). In the UK, the evidence for drought escape in winter wheat is less certain. Innes et al. (1985) showed heading date amongst F5 selections of a Norman  Talent cross to correlate with yield response to late drought, and Worland et al. (1998) reported increased yield for Ppd-D1a early flowering NILs by ca. 5% compared to Ppd-D1b controls in dry years. However, no associations were found by Innes et al. (1985) amongst F5 selections under early drought, by Foulkes et al. (2002) amongst six cultivars under either early or late drought and by Foulkes et al. (2004) between Ppd-D1 NILs in Mercia and Cappelle-Desprez backgrounds under late drought. In the present study, the best test of the value of early flowering for drought resistance was provided by the B  So DH lines, which differed in flowering on average from 6 to 15 June. Most, although not all, of this variation is associated with the Ppd-D1 alleles on chromosome 2D. Phenotypic correlations amongst the lines between flowering date and: (i) yield loss under drought, (ii) relative yield under drought and (iii) S were all non-significant (Table 6). The conclusion is that early flowering did not lead to improved drought resistance amongst the lines of the B  So population. In fact, in 1 year there was a trend for later flowering lines to maintain yield better under

Table 6 Phenotypic correlations (r) amongst doubled-haploid lines between flowering date (GS61), stem water soluble carbohydrate (WSC) at GS61 + 75 8Cd (base temperature 0 8C), percentage green flag-leaf area (GFLA) at GS61 + 35 d in unirrigated treatment and each of absolute yield loss under drought, relative yield under drought (unirrigated yield/irrigated yield) and drought susceptibility index in (a) Beaver  Soissons population (mean values across 2001 and 2002) and (b) Rialto  Spark population (values in 2001)

(a) Beaver  Soissons Flowering date Stem WSC %GFLA

(b) Rialto  Spark Flowering date Stem WSC

Yield loss in unirrigated treatment

Relative yield in unirrigated treatment

Drought susceptibility index

0.21 0.07 0.33y

0.29 0.27 0.69***

0.22 0.05 0.09

Yield loss in unirrigated treatment

Relative yield in unirrigated treatment

0.11 0.21

0.15 0.24

* =p  0.05, ** =p  0.01, *** =p  0.001, y =p  0.10.

M.J. Foulkes et al. / Field Crops Research 103 (2007) 11–24

drought. In both years, yield was increased with delayed flowering under drought, each day’s delay raising yield by 0.07–0.09 t ha1. Since yield also tended to increase with delayed flowering in the absence of water stress, there was, averaging across years, no significant association between flowering and yield losses. A similar non-significant correlation between flowering date and maintenance of yield under drought was observed in the R  Sp population. Present results supported previous findings (Foulkes et al., 2002; Foulkes et al., 2004) indicating a neutral effect of flowering time on yield losses under mild droughts in the UK. It may be that there is a trade-off between early flowering and the development of a smaller root system, as suggested by Foulkes et al. (2004). Foulkes et al. (2001) proposed that an extended stemelongation period may favour a larger rooting system, and this could perhaps be a developmental pattern associated with drought resistance in the UK rather than early flowering per se. Observations regarding flowering time and drought resistance are very much dependent on the exact timing of drought stress and we recognise that the present experiments would need to be grown over more years before we could conclude with certainty that flowering date overall has a neutral effect on yield losses under mild UK droughts. 4.3. Relationship between stem carbohydrate reserves and drought resistance By flowering, reserves of water soluble carbohydrate have accumulated in the stems and leaf sheaths of the crop. A significant proportion of these reserves can be subsequently retranslocated to grains under water-stressed conditions (Bidinger et al., 1977). In previous investigations worldwide, there is evidence that the utilization efficiency (the proportion of the maximal reserves accumulated subsequently re-translocated to the grains) is increased under drought. Palta et al. (1994) in pot experiments on spring wheat in Australia found post-anthesis assimilation was reduced by 57% by drought, while remobilisation of reserves was increased by 36%. Yang et al. (2000) in a field experiment in northern China reported that senescence of winter wheat induced by drought during grain filling increased the remobilization of pre-stored carbohydrate assimilates to the grains from 57 to 79%. In the present study, both DH populations provided a large genetic range for stem WSC under irrigation, in the region of 1.5–4.5 t ha1. In all cases, the phenotypic correlations amongst the lines between stem WSC (measured either under irrigation or drought) and: (i) yield loss under drought, (ii) relative yield under drought and (iii) S were not significant. A positive slope of the regression of stem WSC on grain yield was observed under irrigation and under drought for the B  So lines, but these slopes were not significantly different. Similar trends were observed in the R  Sp population. This indicated that the utilization of stem WSC was apparently similar with and without drought in these experiments. Thus, it appears that utilization of stem reserves is high in the UK even under favourable post-anthesis conditions. Therefore, when the postanthesis duration of the canopy (and hence photosynthesis) is

21

constrained by drought, there may be little scope for further increasing soluble carbohydrate utilization. Such a conclusion is consistent with the investigation of Shearman et al. (2005) which showed stem WSC was positively correlated with yield potential amongst eight UK cultivars released from 1972 to 1996. Physiological bench-marks reported for the UK cultivar Consort (released in 1996) averaged across six site-seasons under optimal conditions also point to the value of stem sugars for yield potential (Spink et al., 2000). Grain yield equated to the increment in AGDM between flowering and harvest plus 70% of the maximal stem WSC. Hence, unlike in other regions of the world subjected to more severe droughts, high stem WSC appears to be a trait of value in optimal and drought environments in the UK. Present results are consistent with evidence of no association between stem WSC and disease tolerance of Septoria leaf blotch or stripe rust in modern wheat cultivars in the UK (Foulkes et al., 2006). In summary, it seems that stem reserves is a trait which favours yield in all situations in the UK rather than specifically maintenance of yield under late-season drought stress. 4.4. Effect of awns on drought resistance Long awns have been demonstrated to be a useful selection indicator in wheat for improved production in hot, dry environments (Blum, 1986) where impaired leaf function due to abiotic stress is common. In the UK the presence of awns was selected against in the second half of the 20th century (through selection for the awn suppressor at the locus B1 on chromosome 5A) and virtually all UK modern cultivars are unawned (Snape et al., 1985). It is not certain why awnless phenotypes predominate in the UK. It can be hypothesised this was because of association with desirable genes linked to the awn inhibitor, such as flowering time and yield, and awnlessness. Alternatively, awns may have an unknown physiological effect on disease susceptibility which was selected against. Soissons which is an awned cultivar was bred in France and has maintained a small UK market share since the mid 1990s mostly in southern England. Under irrigated conditions, there was a small increase or decrease in yield associated with the presence of awns in the region of 0.3 t ha1 depending on season. However, present results showed no association between the presence or absence of awns and yield loss under drought. It appears, therefore, that the potential advantages of awns under drought – i.e., high wateruse efficiency (Blum, 1985) and high sensible heat transfer responsible for a cooler canopy (Weyhrich et al., 1994) – are of less significance under moderate droughts in the UK than under more severe droughts in other regions worldwide. 4.5. The relationship between flag-leaf persistence and drought resistance The genetic trait which showed the clearest correlation with maintenance of yield under drought was flag-leaf persistence. Averaging across years, there was a phenotypic correlation amongst the lines between %GFLA at GS61 + 35 d and each of

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M.J. Foulkes et al. / Field Crops Research 103 (2007) 11–24

yield loss (r = 0.33; p  0.09) and relative yield (Y/Yp) under drought amongst the B  So lines (r = 0.69; p  0.001; Table 5). The correlation with S was not statistically significant. Genetic differences in leaf area duration have been previously reported in wheat (Simon, 1999; Verma et al., 2004), sorghum (Borrell et al., 2000), maize (Banziger et al., 1999) and durum wheat (Hafsi et al., 2000) and related to yield increases under drought in wheat (Verma et al., 2004) and sorghum (Borrell et al., 2000). In sorghum ‘stay-green’ lines were associated with greater leaf N concentration at anthesis and greater N uptake during grain filling. Present results confirmed the importance of flag-leaf area duration for yield under drought. The mechanisms underlying the genetic differences in leaf persistence cannot be certain from present measurements. Early flowering prolonged leaf persistence under irrigation, but there was no association under drought. In previous work, we observed the Ppd-D1a allele conferring early flowering to be associated with a smaller green canopy area at anthesis and prolonged canopy survival during the post-anthesis period under irrigation in Mercia NILs (Foulkes et al., 2004). It may be that green canopy area is reduced by early flowering relatively more than N uptake at anthesis with Ppd-D1a, resulting in delayed senescence. In the present study, grain N offtake (kg N ha1) was measured in the B  So DH lines in 2002. Grain N offtake was negatively correlated with leaf persistence at GS61 + 14 d under irrigation (r = 0.49; p  0.001) and under drought (r = 0.42; p  0.001), and at GS61 + 35 d under drought (r = 0.28; p  0.08); there was no association under irrigation (data not shown). Present results therefore implied that senescence was linked to grain N relocation and hence that either N uptake at anthesis and/or N uptake post-anthesis may be positively associated with flag-leaf persistence in winter wheat in the UK. 4.6. Effects of combinations of traits on drought resistance The present study indicated that flowering time and stem WSC had neutral effects on maintenance of yield under drought. The trait which showed the clearest correlation with yield loss under drought was leaf persistence. Lines with greater leaf persistence showed smaller yield losses under drought. Multiple linear regression analysis indicated that combinations of traits accounted for no more of the variance in yield loss under drought amongst the B  So lines than %GFLA alone. It is therefore suggested that screens for leaf persistence including marker-assisted selection may be of value in breeding programmes aimed at improving yields in high yielding, rainfed environments, but where drought can also be a problem, such as the UK. Under drought, a combination of flowering time, stem WSC and %GFLA accounted for more variance in yield than stem WSC and %GFLA; and these two traits, in turn, accounted for more variance in yield than %GFLA alone. A combination of these three traits may therefore be useful as indirect selection criteria for yield in breeding programmes aimed at raising yields in environments routinely subjected to droughts. The traits could potentially be used to identify new and better parental sources

of variation for drought resistance and/or for screening early generation material to prioritise lines for yield trials in F5–7 generations. For a trait to be most useful in early generation selection it must show a good association with performance and high heritability. Flowering date is of high heritability since it depends on a small number of major genes controlling sensitivity to photoperiod (daylength), vernalization (exposure to low temperatures) and intrinsic earliness (Snape et al., 2001). A recent study on genetic variation in stem WSC by Ruuska et al. (2006) reported significant and repeatable differences in stem WSC accumulation among 22 wheat genotypes grown in Australia (means ranging from 112 to 213 mg g1 dry weight averaged across environments), associated with large broad-sense heritability (H = 0.90  0.12). Therefore breeding for high stem WSC should also be possible. Present results showed the correlation between %GFLA in 2001 and %GFLA in 2002 was significant under both irrigation (r = 0.90; p  0.001) and drought (r = 0.85; p  0.001), indicative of at least moderate heritability for this trait. Therefore present results indicated there is scope for using a combination of these traits, and/or associated molecular markers, as screens for early generation selection for yield in environments routinely subjected to droughts in future breeding programmes worldwide. 4.7. Effect of the 1BL1RS translocation on drought resistance The 1BL1RS wheat translocation has been extensively used in wheat breeding programmes across the world in recent decades (Lukaszewski, 1990; Rajaram et al., 1990). In spring wheat, genes located on the fragment of rye chromosome have been reported to determine wide adaptation (Rabinovich, 1998), high yield potential (Rajaram et al., 1990; Villareal et al., 1995,1998) and drought resistance (Villareal et al., 1998; Ehdaie et al., 2003). In winter wheat, 1BL1RS has also been associated with high yield potential (Rajaram et al., 1983; Carver and Rayburn, 1994; Moreno-Sevilla et al., 1995a) and drought resistance (Moreno-Sevilla et al., 1995b). Winter wheat cultivars incorporating 1BL1RS were first released in the UK in the late 1980s to introgress race-specific disease resistance genes on the rye fragment into UK material. Since then most of the disease resistances have been overcome, but prior to the present study it was not known whether there was a residual benefit for drought resistance. Present findings were unable to confirm previous reports of the yield advantage of 1BL1RS under moisture stress. This may relate to differences in drought stress intensity; stress intensity was lower in the present study than in most previous investigations. Higher biomass with 1BL1RS was detected in two experiments and grain yield in one experiment in both irrigated and unirrigated treatments. Shearman et al. (2005) observed higher biomass for modern 1BL1RS cultivars compared to their predecessors and suggested this was associated with higher pre-anthesis radiation-use efficiency. Further work seems justified to test whether this is the case. However, the present study indicated 1BL1RS offered no

M.J. Foulkes et al. / Field Crops Research 103 (2007) 11–24

greater yield advantage under drought than under optimal conditions in the UK environment. 5. Conclusion The present study indicated that early flowering and high stem WSC had neutral effects on maintenance of yield under drought resistance. The trait which showed the clearest correlation with maintenance of yield under drought was leaf persistence. It is suggested that screens for leaf persistence including molecular markers may therefore have value in breeding programmes aimed at improving yields in high yielding, rainfed environments, but where drought can also be a problem, such as the UK. Droughts within the UK cannot be predicted with certainty even on soils of low available water due to unpredictability of rainfall. Traits for maintaining yield must therefore carry no yield penalty in the absence of drought. Present results suggested that leaf persistence may be such a trait. The presence of awns or the 1BLIRS translocation was not useful for drought resistance, although 1BL1RS did increase biomass and yield in some experiments in both irrigated and unirrigated conditions. A combination of flowering time, stem WSC and leaf persistence explained up to 60% of the variance in yield in the unirrigated treatment. It is suggested that screens for these traits including molecular markers may have value in breeding programmes aimed at improving yields in environments routinely subjected to drought worldwide. Acknowledgements The authors thank UK government Department of Environment, Food and Rural Affairs for funding projects CC0370 and AR0908. References Austin, R.B., 1978. Actual and potential yields of wheat and barley in the United Kingdom. ADAS Q. Rev. 29, 277–294. Austin, R.B., Edrich, J.A., Ford, M.A., Blackwell, R.D., 1977. The fate of the dry matter, carbohydrates and 14C lost from leaves and stems of wheat during grain filling. Ann. Bot. 41, 1309–1321. Austin, R.B., Morgan, C., Ford, M.A., Blackwell, R.D., 1980. Contributions to grain yield from pre-anthesis assimilation in tall and dwarf barley genotypes in two contrasting seasons. Ann. Bot. 45, 309–316. Bailey, R.J., Spackman, E., 1996. A model for estimating soil moisture changes as an aid to irrigation scheduling and crop water-use studies. I. Operational details and description. Soil Use Manage. 12, 12–128. Banziger, M., Edmeades, G.O., Lafitte, H.R., 1999. Selection for drought tolerance increases maize yields across a range of nitrogen levels. Crop Sci. 39, 1035–1040. Bidinger, F., Musgrave, R.B., Fischer, R.A., 1977. Contributions of stored preanthesis assimilate to grain yield in winter wheat and barley. Nature 270, 431–433. Biscoe, P.V., Gallagher, J.N., Littlejohn, E.J., Monteith, J.L., Scott, R.K., 1975. Barley and its environment IV. Sources of assimilate for the grain. J. Appl. Ecol. 12, 295–318. Blum, A., 1985. Photosynthesis and transpiration in leaves and ears of wheat and barley varieties. J. Exp. Bot. 36, 432–440. Blum, A., 1986. The effect of heat stress on wheat leaf and ear photosynthesis. J. Exp. Bot. 37, 111–118.

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