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Field Crops Research 121 (2011) 168–174

Contents lists available at ScienceDirect

Field Crops Research

journa l homepage: www.e lsev ier .com/ locate / fc r

gronomic performance of rice breeding lines selected based on plant traits orrain yield

eiling Yuana, Shaobing Pengb,∗, Cougui Caoa, Parminder Virkb, Danying Xingc,unbo Zhangd, Romeo M. Visperasb, Rebecca C. Lazab

MOA Key Laboratory of Huazhong Crop Physiology, Ecology and Production, Huazhong Agricultural University, Wuhan, Hubei 430070, ChinaInternational Rice Research Institute (IRRI), DAPO Box 7777, Metro Manila, PhilippinesAgricultural College, Yangtze University, Jingzhou, Hubei 434025, ChinaCrop Physiology, Ecology, and Production Center, Hunan Agricultural University, Changsha, Hunan 410128, China

r t i c l e i n f o

rticle history:eceived 22 September 2010eceived in revised form 9 December 2010ccepted 9 December 2010

eywords:rain yield

deotype breedinglant traitsrait-based selectionield-based selection

a b s t r a c t

Selection for yield per se has greatly contributed to yield improvement in many crops. It is expectedthat selection based on plant traits is more effective in increasing crop yield potential. This study wasconducted to compare the effectiveness of trait-based and yield-based selection in increasing rice yieldand to determine whether lines with ideotype traits have the potential to express higher yield underoptimal crop management conditions. Lines were selected based on plant traits or on grain yield measuredin a breeder’s replicated yield trial. The main target traits for selection were plant height, leaf and paniclemorphology, grain size, total dry weight, and grain-filling percentage. Yield performance of trait-basedselection was compared with that of yield-based selection in an agronomic trial with optimum cropmanagement for three seasons. Trait-based selection increased leaf area index and total dry weight butreduced spikelet number per m2 and harvest index compared with yield-based selection. Consequently,

selection based on plant traits did not increase grain yield compared with selection based on yield per se.In one of the three seasons, yield of trait-based selection was significantly lower than that of yield-basedselection. Among all tested breeding lines, maximum yield was produced by yield-based selection andminimum yield came from trait-based selection. These results suggest that lines with ideotype traits didnot express higher grain yield than lines selected based on yield per se under optimal crop managementconditions, and yield-based selection was as effective in increasing rice grain yield as trait-based selection

the b

in the late generations of

. Introduction

Improvement in the yield potential of cereal crops has proveno be the major strategy to increase world grain production to

eet the demand of the growing population because average farmield is largely influenced by the yield potential of crop varietiesCassman et al., 2003; Otegui and Slafer, 2004). Therefore, yieldmprovement remains the main target in most rice breeding pro-rams. Yield potential is defined as the yield of a variety whenrown in environments to which it is adapted, with ample nutri-nts and water, and with pests, diseases, weeds, lodging, and other

tresses effectively controlled (Evans, 1993). An increase in yieldotential was achieved in the late 1950s and early 1960s by theevelopment of semi-dwarf varieties in China and at the Inter-ational Rice Research Institute (IRRI) (Huang, 2001; Khush et al.,

∗ Corresponding author. Tel.: +63 2 580 5600; fax: +63 2 891 1292.E-mail address: s.peng@cgiar.org (S. Peng).

378-4290/$ – see front matter © 2011 Published by Elsevier B.V.oi:10.1016/j.fcr.2010.12.014

reeding cycle.© 2011 Published by Elsevier B.V.

2001). The success of F1 hybrid development in China in 1976 fur-ther increased rice yield potential (Yuan et al., 1994).

Yield stagnation of newly developed rice varieties has beenobserved in the tropics since the release of the first semi-dwarfvariety, IR8, in 1966 (Peng et al., 1999). Stimulated by the ideotypebreeding approach of Donald (1968), IRRI scientists proposed a newplant type (NPT) concept based on the results of simulation model-ing with the goal of breaking the yield potential barrier (Khush,1995). The key morphological traits of NPT are large and heavypanicles, reduced tillering capacity, thick and sturdy stems, erectand long leaves, and slightly increased plant height (Peng et al.,1994). However, NPT lines did not increase grain yield as expectedbecause of poor grain filling, limited biomass production, and inef-ficient translocation (Peng et al., 2008). These physiological defects

could be associated with the tropical japonica background of theNPT lines. “Super” rice varieties with most of the plant type traitsof IRRI’s NPT were developed in China in recent years (Cheng et al.,2007). In the plant type of “super” rice, panicles are kept inside thecanopy so that more leaves can intercept solar radiation. This trait

W. Yuan et al. / Field Crops Research 121 (2011) 168–174 169

Table 1Grain yield of rice breeding lines and check varieties grown in a replicated yield trial in a breeder’s field at the IRRI farm in the dry season of 2008.

Genotypic group No. of entries Minimum yield (t ha−1) Maximum yield (t ha−1) Average yield (t ha−1) CV (%)

All lines 196 4.35 7.68 6.07 9.9Plant traitsa 52 4.66 7.47 6.12 9.6Top yielderb 52 6.45 7.68 6.77 4.2Overlapc 18 6.46 7.47 6.74 3.9Checkd 4 5.49 7.19 6.25 11.6

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a These lines were selected visually based on plant morphological traits.b The 52 lines produced the highest yield among 196 lines.c The 18 lines selected based on plant morphological traits were among the 52 tod Check varieties were IR36, IR64, PSBRc68, and PSBRc82.

as not included in IRRI’s original NPT design (Peng et al., 2008).n increase in yield potential by “super” rice has been reported

n several studies (Katsura et al., 2007, 2008; Zhang et al., 2009).he success of “super” rice breeding suggests that the ideal plantype traits have contributed individually or collectively to rice yieldmprovement.

Improvement in yield potential of cereal crops has mainly beenhe result of empirical selection for yield per se (Austin et al., 1980;vans, 1993; Khush, 1987; Richards, 2000). Because genetic gainn yield potential is becoming more difficult to achieve in a cropreeding program and there is a sign of leveling-off in both potentialnd actual yields (Cassman et al., 2003; Sharma-Natu and Ghildiyal,005), many researchers have attempted indirect selection for yieldased on plant traits (Gravois and McNew, 1993; Otegui and Slafer,004; Singh and Balyan, 2003). Indirect selection could be more effi-ient than direct selection if the target trait is highly correlated withield, has higher heritability, and can be easily measured (Falconer,981; Kumar and Bahl, 1992; McNeal et al., 1978). Reynolds et al.1998) reported a positive response in wheat yield to selection forost-anthesis canopy temperature depression in irrigated warmnvironments. Studies on several crop species found that indirectelection for yield based on yield components was more efficienthan direct selection for yield, especially in the early generationsKumar and Bahl, 1992; Saadalla, 1994; Takeda and Frey, 1976;otok et al., 1998). However, Gravois and McNew (1993) reportedhat selection for either panicle weight or panicle number aloneas ineffective in increasing rice grain yield. They also found that

election for both increased panicle weight and panicle number wasess effective in increasing grain yield compared with selection forield per se.

In this study, we selected 52 out of 196 F6–F7 breeding lines visu-lly based on several ideotype traits in a breeder’s replicated yieldrial. Sixteen lines were further selected from the 52 lines based onotal dry weight and grain-filling percentage. Yield performance ofhe lines selected based on plant traits was compared with that ofhe lines selected based on yield per se in an agronomic trial for

hree seasons. The objectives of this study were to (1) compare theffectiveness of trait-based and yield-based selection in increas-ng rice grain yield, and (2) determine whether lines with ideotyperaits have the potential to express higher yield under optimal crop

anagement conditions.

able 2rain yield of the lines selected based on plant traits and the highest yielding lines grow008.

Genotypic group No. of entries Minimum yield (t ha−1)

Plant traitsa 16 5.53Top yielderb 16 6.87Overlapc 3 6.87All lines 29 5.53

a Out of the 52 lines visually selected based on plant morphological traits, 16 lines werb The 16 lines produced the highest yield among 196 lines.c The 3 lines selected based on plant traits were among the 16 top yielders.

ders.

2. Materials and methods

2.1. Line selection in a breeder’s replicated yield trial

In the dry season (DS) of 2008, 196 breeding lines (F6 to F7 gen-erations) and 4 check varieties were grown in a replicated yield trialin a breeder’s field at the IRRI farm at Los Banos (14◦11′N, 121◦15′E,altitude 21 m) in the Philippines. These 196 lines were bred usingthe pedigree method of breeding and were selected based on visualevaluation for yield component traits, screening for resistance tosome diseases and insects, and grain quality parameters. Totalnutrient input was 180 kg N ha−1, 30 kg P ha−1, and 30 kg K ha−1.The experiment was replicated three times and plot size was 12 m2.Twenty-two-day-old seedlings were transplanted on 11 January ata hill spacing of 20 cm × 20 cm with 2–3 seedlings per hill. Withinone week before final harvest, 52 lines were selected visually basedon plant morphological traits. These traits included moderatelytall plants (approximately 120 cm), erect top three leaves, panicleslocated inside the canopy, large and compact panicles (i.e. morespikelets per unit panicle length), and large grain size. Abovegroundtotal dry weight and yield components were measured at matu-rity from a 0.48-m2 area only for the 52 lines. Sixteen lines werefurther selected from the 52 lines based on total dry weight andgrain-filling percentage. Grain yield of all 196 entries was mea-sured from a 6-m2 area and was expressed at moisture content of0.14 g H2O g−1 fresh weight. Three out of the 16 lines selected basedon plant traits were also ranked in the top 16 based on grain yield.Three groups of selected genotypes were formed: 13 lines basedon plant traits (trait-based selection), 13 lines based on grain yield(yield-based selection), and 3 lines based on both plant traits andgrain yield (overlapped lines). Grain yield of the 52 lines selectedbased on morphological traits and of the 16 lines selected based ontotal dry weight and grain-filling percentage was compared withthat of other genotypic groups in Tables 1 and 2, respectively.

2.2. Agronomic trial on selected lines

Yield and yield attributes of the three selection groups werecompared in an agronomic trial in the same field at the IRRI farmfor three consecutive seasons (2009 DS, 2009 wet season (WS), and2010 DS). In addition to the three groups, a group of three check

n in a replicated yield trial in a breeder’s field at the IRRI farm in the dry season of

Maximum yield (t ha−1) Average yield (t ha−1) CV (%)

7.08 6.25 8.47.68 7.07 3.97.08 6.94 1.87.68 6.63 9.2

e further selected based on total dry weight and grain-filling percentage.

1 s Research 121 (2011) 168–174

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arieties (IR72, NSICRc158, and a hybrid) was also included in alleld experiments. The hybrid variety was SL-8H in the DS and Mes-izo7 in the WS because SL-8H was susceptible to diseases in the

S. The soil was an Aquandic Epiaquoll with pH 6.2, 19.4 g kg−1

rganic C, 2.0 g kg−1 total N, 6.8 mg kg−1 Olsen P, 0.98 cmol kg−1

xchangeable K, 39.9 cmol kg−1 cation exchange capacity, 63.0%lay, 30.3% silt, and 6.7% sand. The soil test was based on samplesaken from the upper 20 cm of the soil before transplanting in 2009

S.The experiments were laid out in a randomized complete block

esign with four replications. Pregerminated seeds were sownn seedling trays to produce uniform seedlings. Fourteen-day-oldeedlings were manually transplanted on 23 January, 25 June,nd 30 December 2009 for 2009 DS, 2009 WS, and 2010 DS,espectively. Four seedlings per hill were transplanted at a hillpacing of 20 cm × 20 cm. Fertilizers were manually broadcast andncorporated during basal application: 30 kg P ha−1, 40 kg K ha−1,nd 5 kg Zn ha−1 in the DS and 15 kg P ha−1, 20 kg K ha−1, and.5 kg Zn ha−1 in the WS. Nitrogen in the form of urea was split-pplied: 60 kg ha−1 at basal, 40 kg ha−1 at mid-tillering, 60 kg ha−1

t panicle initiation, and 40 kg ha−1 at flowering in the DS;0 kg ha−1 at basal, 20 kg ha−1 at mid-tillering, 30 kg ha−1 at panicle

nitiation, and 20 kg ha−1 at flowering in the WS. Standard culturalanagement practices were followed to achieve maximum grain

ield. The field was flooded at 4 days after transplanting and 5–10-m water depth was maintained until 7 days before maturity forach entry when the field was drained. Insects, diseases, and weedsere intensively controlled by using approved pesticides to avoid

ield loss.Twelve hills (0.48 m2) were sampled from each plot at flow-

ring to measure plant height, stem number, leaf area index, andboveground total dry weight. Plant height was measured from thelant base to the tip of the highest leaf. Plants were separated intoreen leaves and stems. Green leaf area was measured with a leafrea meter (LI-3000, LI-COR, Lincoln, NE, USA) and expressed as leafrea index. The dry weight of each component was determined afterven-drying at 70 ◦C to constant weight. Total dry weight was theum of the weights of green leaves and stems.

At maturity, 12 hills were taken diagonally from a 5-m2 areahere grain yield was determined in each plot to measure above-

round total dry weight, harvest index, and yield components.lant height was measured from the plant base to the tip ofhe highest leaf and panicle. The difference in plant height mea-ured to the tip of the highest leaf minus that measured to theip of the panicle was used to judge the relative height of pani-les within a canopy. Panicle number of each hill was counted toetermine the panicle number per m2. Plants were separated intotraw and panicles. Straw dry weight was determined after oven-rying at 70 ◦C to constant weight. Panicles of all 12 hills wereand-threshed and filled spikelets were separated from unfilledpikelets by submerging them in tap water. Three sub-samplesach of 30-g filled spikelets and 2-g unfilled spikelets were takeno count the number of spikelets. Dry weights of rachis and fillednd unfilled spikelets were determined after oven-drying at 70 ◦Co constant weight. Aboveground total dry weight was the total dry

atter of straw, rachis, and filled and unfilled spikelets. Spikeletser panicle, grain-filling percentage (100 × filled spikelet num-er/total spikelet number), and harvest index (100 × filled spikeleteight/aboveground total dry weight) were calculated. Grain yieldas determined from a 5-m2 area in each plot and adjusted to the

tandard moisture content of 0.14 g H2O g−1 fresh weight. Grain

oisture content was measured with a digital moisture tester

DMC-700, Seedburo, Chicago, IL, USA).Daily weather records were obtained from the weather station

djacent to the experimental site. Data underwent analysis of vari-nce (SAS, 2003) for each season. Means of varietal groups were

Fig. 1. Ten-day average of daily minimum temperature (A), daily maximum tem-perature (B), and daily solar radiation (C) in the dry seasons (DS) of 2009 and 2010and the wet season (WS) of 2009.

compared based on the least significant difference test (LSD) at the0.05 probability level.

3. Results

Fig. 1 shows the 10-day averages of daily minimum and max-imum temperature, and solar radiation from transplanting tomaturity in 2009 DS, 2009 WS, and 2010 DS. Higher daily minimumtemperature and lower radiation were observed in the WS thanin the DS (Fig. 1(A) and (C)). The WS had higher daily maximumtemperature than the DS only during the early vegetative stage

(Fig. 1(B)). Daily minimum temperature was consistently lower in2010 DS than in 2009 DS. 2010 DS had higher radiation than 2009DS during the ripening phase. There was no significant difference indaily maximum temperature between the two DS. Seasonal meandaily minimum temperatures were 23.9 ◦C in 2009 DS, 24.9 ◦C in

W. Yuan et al. / Field Crops Research 121 (2011) 168–174 171

Table 3Grain yield of rice breeding lines and check varieties grown in an agronomic trial at the IRRI farm in three consecutive seasons.

Genotypic group Genotype name Grain yield (t ha−1)

2009 dry season 2009 wet season 2010 dry season

Check IR72 6.48 5.21 9.14NSICRc158 6.95 5.48 8.69Hybrida 6.69 5.95 9.48Mean 6.71 a 5.55 a 9.10 a

Plant traitsb IR72892-77-2-2-2 6.41 5.49 8.69IR75287-19-3-3-3 6.57 5.76 8.62IR77533-29-2-2-2 6.30 4.83 8.37IR79201-101-1-2-2 5.55 4.50 8.02IR79218-43-2-1-2 6.41 5.26 8.80IR79218-69-2-2-2 5.94 5.14 8.74IR79233-28-2-1-2 4.54 5.42 7.12IR79525-20-2-2-2 5.51 4.88 7.79IR80655-33-3-2-1 6.73 5.73 8.20IR80864-57-1-5-3 4.82 4.67 7.28IR80894-18-2-2-3 5.99 5.54 8.64IR81336-39-3-3-3 6.17 5.25 7.50IR81852-120-2-1-3 5.77 5.33 7.95Mean 5.90 b 5.22 a 8.13 c

Grain yieldc IR71700-247-1-1-2 5.77 5.82 8.97IR73012-15-2-2-1 5.47 5.52 8.01IR73885-1-4-3-2-1-6 4.82 5.28 8.38IR74646-96-2-3-3 5.48 5.70 7.67IR75298-59-3-1-3 6.55 5.33 8.08IR77498-127-3-2-3-2 6.29 5.15 8.61IR78581-12-3-2-2 7.30 5.78 8.94IR79505-51-2-2-2 6.45 5.30 9.28IR80397-87-1-2-3 6.20 4.77 8.15IR80404-28-2-3-2 6.66 4.96 8.94IR80658-67-2-1-2 5.85 4.84 8.19IR81330-19-2-1-3 6.11 5.43 8.35IR81330-29-3-1-2 6.17 5.41 8.22Mean 6.09 b 5.33 a 8.45 b

Overlapd IR77032-47-2-3-3 6.32 5.54 7.70IR77700-84-2-2-2 6.31 5.77 8.52IR78585-98-2-2-1 5.65 4.67 8.17Mean 6.09 b 5.33 a 8.13 c

Within a column, means of varietal groups followed by the same letter are not significantly different according to LSD (0.05).a

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Hybrid variety was SL-8H in the dry season and Mestizo7 in the wet season.b These lines were selected based on plant traits.c These lines were selected based on grain yield.d The 3 lines were selected based on both plant traits and grain yield.

009 WS, and 23.2 ◦C in 2010 DS. Seasonal mean radiation was 16.4,4.2, and 18.1 MJ m−2 day−1 in 2009 DS, 2009 WS, and 2010 DS,espectively. There was about a 15% difference in radiation duringhe growing season between the DS and WS in 2009, and about a0% difference between the two DS.

Grain yield of 32 entries in four genotypic groups in three sea-ons is shown in Table 3. Large variation in grain yield was observedmong the genotypes and across the three seasons, ranging from.50 to 9.48 t ha−1. Average yield was higher in the DS than in theS and higher in 2010 DS than 2009 DS. There was no significant

ifference in grain yield among the four genotypic groups in 2009S. In the DS, the average yield of check varieties was significantly

igher than the average yield of breeding lines. The hybrid vari-ty produced the highest yield among the 32 entries in 2009 WSnd 2010 DS. The average yield of trait-based selection was notignificantly higher than that of yield-based selection in all threeeasons. In fact, yield-based selection produced significantly higherverage yield than trait-based selection in 2010 DS. Furthermore,he highest yielding line was among the yield-based selection and

he lowest yielding line was among the trait-based selection in allhree seasons.

Plants from trait-based selection and overlapped lines were con-istently taller than plants from yield-based selection and checkarieties at flowering (Table 4). The average difference in plant

height at flowering between trait- and yield-based selection was10.3 cm across the three seasons. Average stem number per m2

was lower in trait-based selection and overlapped lines than inyield-based selection and check varieties at flowering. Trait-basedselection had the highest leaf area index among the four genotypicgroups at flowering. Leaf area index of 2010 DS was the highest, fol-lowed by 2009 DS and 2009 WS. The difference in total dry weightamong the four genotypic groups was similar to that in plant heightat flowering. Total dry weight of trait-based selection was 9% higherthan that of yield-based selection. Higher total dry weight was pro-duced in the DS than in the WS at flowering. There was no differencein total dry weight at flowering between 2009 DS and 2010 DS.

Growth duration of trait-based selection was on average 4 dayslonger than that of yield-based selection (Table 5). The differencein plant height among the four genotypic groups at maturity wassimilar to that at flowering. The difference in plant height measuredto the tip of the highest leaf minus that measured to the tip of thepanicle was smallest in yield-based selection. This suggests thatpanicles of lines selected based on grain yield were located more

in the upper layer of the canopy compared with other genotypicgroups. The difference in total dry weight among the four geno-typic groups was smaller at maturity than at flowering. Trait-basedselection had 4% higher total dry weight than yield-based selec-tion at maturity. The two DS did not show a significant difference

172 W. Yuan et al. / Field Crops Research 121 (2011) 168–174

Table 4Growth analysis at flowering of rice breeding lines and check varieties grown in an agronomic trial at the IRRI farm in three consecutive seasons.

Genotypic group 2009 dry season 2009 wet season 2010 dry season Mean

Plant height (cm)Check 106.9 b 102.9 b 106.6 b 105.4 bPlant traits 119.0 a 117.1 a 116.3 a 117.5 aGrain yield 110.0 b 107.1 b 104.5 b 107.2 bOverlap 120.9 a 117.5 a 115.4 a 117.9 a

Stem number (m−2)Check 487.0 a 385.8 a 470.3 a 447.7 aPlant traits 426.4 b 325.9 c 418.2 b 390.2 bGrain yield 482.6 a 361.3 b 460.5 a 434.8 aOverlap 434.5 b 333.2 c 416.2 b 394.6 b

Leaf area indexCheck 5.34 b 3.39 b 6.22 b 4.98 bPlant traits 5.77 a 4.23 a 6.76 a 5.59 aGrain yield 5.53 b 3.61 b 6.00 b 5.05 bOverlap 5.60 ab 3.82 b 6.28 ab 5.23 b

Total dry weight (g m−2)Check 1065 b 830 b 1113 ab 1003 b

39 a52 b06 ab

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Plant traits 1194 a 9Grain yield 1099 b 8Overlap 1179 a 9

ithin a column for each parameter, means followed by the same letters are not si

n total dry weight at maturity. A consistently lower harvest indexas observed in trait-based selection than other genotypic groups,hereas check varieties showed the highest harvest index among

he four genotypic groups. Harvest index was higher in the DS thann the WS. Average harvest index was 41.9% in 2009 DS and 44.5%n 2010 DS.

Differences among the four genotypic groups in panicle numberer m2 at maturity were similar to those of stem number per m2 atowering (Table 6). On average, yield-based selection produced 8%

ore panicles than trait-based selection. A small and inconsistent

ifference in spikelet number per panicle was observed among theour genotypic groups. Spikelet number per m2 was higher in yield-ased selection than in trait-based selection, but the difference was

able 5rowth analysis at maturity of rice breeding lines and check varieties grown in an agrono

Genotypic group 2009 dry season 2009 w

Growth duration (days)Check 117 119Plant traits 123 125Grain yield 117 122Overlap 121 121

Plant height (cm)Check 113.3 b 109.6 cPlant traits 122.6 a 124.6 aGrain yield 115.3 b 115.4 bOverlap 125.4 a 122.6 a

Height difference (cm)a

Check 6.9 ab 1.8 bPlant traits 5.9 b 5.5 aGrain yield 3.5 c 3.5 bOverlap 8.5 a 7.3 a

Total dry weight (g m−2)Check 1641 ab 1305 abPlant traits 1629 ab 1354 aGrain yield 1598 b 1274 bOverlap 1693 a 1262 b

Harvest index (%)Check 44.5 a 42.7 aPlant traits 39.2 c 37.8 bGrain yield 42.4 ab 41.0 aOverlap 41.3 bc 39.5 ab

ithin a column for each parameter, means followed by the same letters are not significaa Plant height from the base to the tip of the flag leaf minus the height from the base to

1173 a 1102 a1071 b 1008 b1140 a 1075 a

ntly different according to LSD (0.05).

significant only in the DS. Spikelet number per m2 was higher inthe DS than in the WS and higher in 2010 DS than in 2009 DS.Grain-filling percentage of check varieties was the highest amongthe four genotypic groups and the other three groups did not showa significant difference. Grain weight of overlapped lines was thehighest among the four genotypic groups. Trait-based selection hadhigher grain weight than yield-based selection, but the differencewas significant only in 2010 DS.

4. Discussion

The criteria of trait-based visual selection in this study weremoderately tall plants (approximately 120 cm), erect top three

mic trial at the IRRI farm in three consecutive seasons.

et season 2010 dry season Mean

114 117118 122115 118117 119

113.4 b 112.1 b121.6 a 123.0 a112.8 b 114.5 b123.8 a 123.9 a

7.3 a 5.3 b6.3 a 5.9 b3.4 b 3.4 c7.9 a 7.9 a

1717 a 1554 a1696 a 1560 a1626 b 1499 b1631 ab 1529 ab

48.1 a 45.1 a41.6 c 39.5 d45.2 b 42.9 b43.0 bc 41.3 c

ntly different according to LSD (0.05).the tip of the panicle.

W. Yuan et al. / Field Crops Research 121 (2011) 168–174 173

Table 6Yield components of rice breeding lines and check varieties grown in an agronomic trial at the IRRI farm in three consecutive seasons.

Genotypic group 2009 dry season 2009 wet season 2010 dry season Mean

Panicles (m−2)Check 411.1 ab 329.3 a 382.8 a 374.4 aPlant traits 383.5 b 289.5 c 351.1 b 341.4 bGrain yield 418.9 a 311.1 b 372.6 a 367.5 aOverlap 381.9 b 287.3 c 340.3 b 336.5 b

Spikelets panicle−1

Check 97.4 a 96.1 b 116.7 a 103.4 aPlant traits 101.1 a 105.6 a 119.3 a 108.7 aGrain yield 99.4 a 100.3 ab 117.3 a 105.7 aOverlap 101.3 a 102.9 ab 115.8 a 106.7 a

Spikelets m−2 (×103)Check 38.8 b 31.5 a 43.5 ab 37.9 abPlant traits 38.6 b 30.4 a 41.6 bc 36.9 bcGrain yield 41.5 a 31.1 a 43.5 a 38.7 aOverlap 38.5 b 28.8 a 39.2 c 35.5 c

Grain filling (%)Check 80.2 a 75.5 a 79.5 a 78.4 aPlant traits 72.3 b 71.2 ab 70.9 b 71.5 bGrain yield 73.2 b 72.7 ab 73.2 b 73.1 bOverlap 73.3 b 68.4 b 69.5 b 70.4 b

Grain weight (mg)Check 23.5 ab 23.4 b 24.0 bc 23.6 bcPlant traits 23.1 b 23.8 b 24.2 b 23.7 b

23.3 b25.5 a

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Grain yield 22.5 bOverlap 24.8 a

ithin a column for each parameter, means followed by the same letters are not si

eaves, panicles located inside the canopy, large and compact pan-cles, and large grain size. Many of these traits were the key

orphological traits of NPT lines developed by IRRI in early 1990sPeng et al., 1994) and were proved to be ideotype traits by the suc-ess of China’s “super” rice breeding program (Cheng et al., 2007;eng et al., 2008). A recent study reported that a large-grain variety,kita 63, had 22–58% higher grain yield than the check varieties,hich was associated with its 35% larger grain size than the check

arieties (Mae et al., 2006). Katsura et al. (2007, 2008) also sug-ested that large grain size was partially responsible for higherrain yield of China’s “super” rice varieties. The 52 lines selectedisually based on the ideotype traits were further reduced to 16ines because an agronomic trial cannot accommodate so manyntries. Quantitative traits of total dry weight and grain filling per-entage were used as the selection criteria because these two traitsere identified as the main reasons why IRRI’s NPT lines did not

ncrease grain yield as expected (Peng et al., 2008).Plant height, leaf area index, total dry weight, and grain weight

ere increased and panicle height relative to plant height wasecreased by trait-based selection. However, spikelet number peranicle and grain-filling percentage were not improved by trait-ased selection. Indirect selection for yield based on plant traits didot increase grain yield compared with direct selection based onield per se across three seasons, although longer growth durationas observed in the lines selected based on plant traits. There waslarge difference among the three seasons in climatic yield poten-

ial due to variations in night temperature and solar radiation (Pengt al., 2004). In 2010 DS when climatic yield potential was the high-st, grain yield was significantly lower in trait-based selection thann yield-based selection. Among all 29 tested breeding lines, max-mum yield was produced by yield-based selection and minimumield came from trait-based selection in all three seasons. There-

ore, lines selected based on ideotype traits did not express higherield than the lines selected based on yield per se under the optimalrop management conditions of this study. Yield-based selectionas as effective in increasing grain yield as trait-based selection in

he late generations of the breeding cycle.

23.3 c 23.0 c25.8 a 25.4 a

ntly different according to LSD (0.05).

Why was grain yield not improved by trait-based selection inthis study? First of all, compensation among different plant traitswas responsible for the lack of effectiveness of trait-based selec-tion (Gravois and McNew, 1993). For example, the increase in totaldry weight by trait-based selection was offset by the decrease inharvest index. Selection for tall plants might have resulted in areduction in panicle number per m2. Second, visual selection oflarge panicle size in the breeder’s replicated yield trial did not leadto an actual increase in spikelet number per panicle compared withyield-based selection. Third, target plant traits were not improvedenough to result in a significant increase in grain yield probablybecause the variations in the traits among 196 breeding lines wererelatively small. To break the yield barrier, for example, averagepanicle weight of 5 g per panicle was the target in the “super” ricebreeding program (Yuan, 2001). However, average panicle weightof the lines reached a maximum of only 3 g per panicle in this study(data not shown). Finally, trait-based selection was done near theend of the breeding cycle when plant traits were stabilized and uni-form in this study. However, several studies reported that selectionbased on yield components was more efficient than yield-basedselection for increasing grain yield, especially in the early gener-ations (Kumar and Bahl, 1992; Saadalla, 1994; Takeda and Frey,1976; Totok et al., 1998). This suggests that trait-based selectioncould be effective if it is done in earlier generations.

The effectiveness of trait-based and yield-based selection forincreasing crop yield was compared in many studies (Gravois andMcNew, 1993; Kumar and Bahl, 1992; Saadalla, 1994; Totok et al.,1998). The outcome of the comparisons is diverse depending onspecies, target traits, and the generation of the breeding cycle. Pos-itive responses to selection for yield based on yield componentswere reported in several studies (Kumar and Bahl, 1992; Saadalla,1994; Takeda and Frey, 1976; Totok et al., 1998). Our result is con-

sistent with that of Gravois and McNew (1993), who reported thatselection for yield components was less effective in increasing grainyield than selection for yield per se.

None of the breeding lines produced consistently higher yieldthan the best check variety across three seasons (Table 3). This

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s probably because there was no substantial difference in thelant traits between the breeding lines and the check varieties. Thereeding lines did not demonstrate superior plant type traits thatonfer high yield potential. In fact, the check varieties had consis-ently higher grain-filling percentage and harvest index than thereeding lines. Peng et al. (2008) stated that, in order to achieve a0% increase in yield potential under tropical conditions, the fol-

owing are the target traits: 330 panicles per m2, 150 spikelets peranicle, 80% grain filling, 25 mg grain weight (oven-dry), 22 t ha−1

boveground total biomass (at 14% moisture content), and 50% har-est index. The breeding lines of this study had much lower valuesor these target traits except for panicle number per m2. Furtherncreases in panicle size and biomass production are required formprovement in the grain yield of these breeding lines.

Selection for yield per se has been responsible for genetic gainn yield potential in many crop species (Austin et al., 1980; Evans,993; Khush, 1987). Richards (2000) stated that selection for yielder se will continue to contribute to yield improvement in cropreeding programs in spite of developments in molecular technol-gy such as marker-assisted selection. Ideotype traits with a closeelationship with grain yield may still be useful for visual selec-ion in early generations when a breeding population is still in theegregating stage. Furthermore, ideotype traits can guide breedersn the selection of donor parents for crossing. Evaluation of high-ielding lines and associated plant type traits provides insight intonderstanding the underlying physiological mechanisms of highield potential.

cknowledgments

We wish to thank the China Scholarship Council (CSC) of theinistry of Education in China for accepting the senior author in

he “Sandwich” Doctoral Degree Training program at IRRI. Financialupport was partially provided by the Major Project of Interna-ional Cooperation, National Natural Science Foundation of ChinaNo. 30821140349).

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