Evaluation of drought resistance indices under various environmental conditions
Evaluation of drought resistance indices under various
environmental conditions
A.Sio-Se Mardeh a,A.Ahmadi b,*,K.Poustini b,V.Mohammadi b
a College of Agriculture,Kurdistan University,Sanandaj,Iran
b Department of Agronomy and Plant Breeding,College of Agriculture,Tehran University,Karaj3158777871,Iran
Received7February2006;received in revised form12February2006;accepted13February2006
Abstract
Drought is a wide-spread problem seriously in?uencing wheat(Triticum aestivum L.)production and quality,but development of resistant cultivars is hampered by the lack of effective selection criteria.The objective of this study was to evaluate the ability of several selection indices to identify drought resistant cultivars under a variety of environmental conditions.Eleven bread wheat cultivars differing in yield performance were grown in separate experiments under rain-fed(non-irrigated)and irrigated conditions at two locations in2001–2003.Non-irrigated experiments experienced different levels of water stress due to variable rainfall over the years and locations.Nine selection indices including stress susceptibility index(SSI),stress tolerance index(STI),tolerance(TOL),regression coef?cient of cultivar yield on environmental index(b),yield index(YI),yield stability index(YSI),mean productivity(MP),geometric mean productivity(GMP), and superiority measure(P)were calculated based on grain yield under drought-stressed and irrigated conditions.The results showed that under moderate stress,MP,GMP and STI were more effective in identifying high yielding cultivars in both drought-stressed and irrigated conditions(group A cultivars).Under severe stress,none of the indices used were able to identify group A cultivars,although regression coef?cient(b)and SSI were found to be more useful in discriminating resistant cultivars.It is concluded that the effectiveness of selection indices in differentiating resistant cultivars varies with the stress severity.Wheat breeders should,therefore,take the stress severity of the environment into account in choosing an index.
#2006Elsevier B.V.All rights reserved.
Keywords:Drought;Resistance indices;Wheat;Rain-fed;Irrigated condition
1.Introduction
Insuf?cient water is the primary limitation to wheat production world-wide(Ashraf and Harris,2005).Wheat is widely grown as a rain-fed crop in semi-arid areas,where large?uctuations occur in the amount and frequency of rainfall events from year to year and among sites within years.The development of resistant cultivars,however,is hampered by low heritability for drought tolerance and a lack of effective selection strategies(Kirigwi et al.,2004).
The relative yield performance of genotypes in drought-stressed and more favorable environments seems to be a common starting point in the identi?cation of traits related to drought tolerance and the selection of genotypes for use in breeding for dry environments(Clarke et al.,1992). According to Fernandez(1992),genotypes can be divided into four groups based on their yield response to stress conditions:(1)genotypes producing high yield under both water stress and non-stress conditions(group A),(2) genotypes with high yield under non-stress(group B)or (3)stress(group C)conditions and(4)genotypes with poor performance under both stress and non-stress conditions (group D).The question is:should breeding for stress-prone environments rely on selection under both potential and stress conditions or on selection in either environment alone?
Some researchers believe in selection under favorable condition(Richards,1996;Van Ginkel et al.,1998;Rajaram and Van Ginkle,2001;Betran et al.,2003).Selection in the target stress condition has been highly recommended too (Ceccarelli,1987;Ceccarelli and Grando,1991;Rathjen, 1994).Several researchers have chosen the mid-way and
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Field Crops Research98(2006)222–229
*Corresponding author.Tel.:+98212246074;fax:+98212227605.
E-mail address:ahmadia@ut.ac.ir(A.Ahmadi).
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doi:10.1016/j.fcr.2006.02.001
believe in selection under both favorable and stress conditions(Fischer and Maurer,1978;Clarke et al.,1992; Nasir Ud-Din et al.,1992;Fernandez,1992;Byrne et al., 1995;Rajaram and Van Ginkle,2001).
To differentiate drought resistance genotypes,several selection indices have been suggested on the basis of a mathematical relationship between favorable and stress conditions(Clarke et al.,1984;Huang,2000).Tolerance (TOL)(McCaig and Clarke,1982;Clarke et al.,1992),mean productivity(MP)(McCaig and Clarke,1982),stress susceptibility index(SSI)(Fischer and Maurer,1978), geometric mean productivity(GMP)and stress tolerance index(STI)(Fernandez,1992)have all been employed under various conditions.Fischer and Maurer(1978) explained that genotypes with an SSI of less than a unit are drought resistant,since their yield reduction in drought condition is smaller than the mean yield reduction of all genotypes(Bruckner and Frohberg,1987).Bansal and Sinha (1991)evaluated wheat accessions based on the stability in grain yields of various species grown across a range of soil moisture conditions,and concluded that species with a smaller linear regression coef?cient(b)have a higher drought resistance.The superiority measure(P)proposed by Lin and Binns(1988)is another indicator that compares the productivity of genotypes across different environments. This technique utilizes the highest-yielding genotypes within each environment as a reference point.Cultivars with the largest yield difference than the reference would have the highest P value.
The suitability of indicators seems to depend on the timing and severity of stress in drought-prone environments. The objective of this study was to test this hypothesis in order to identify the most suitable indices/cultivars for each environment.
2.Materials and methods
Eleven bread wheat cultivars including Sardari,Sabalan, Roushan-Back cross,Alvand,Mahdavi,Agosta-Se?d,Toos, Cross-Shahi,Azar2,Glinson and M-75-7were chosen for study based on their reputed differences in yield perfor-mance under irrigated and non-irrigated conditions.The experiments were conducted at two dry land research stations,Gerizeh and Ghamloo,in Kurdistan Province (northwest of Iran)in2001–2003.Gerizeh(1373m above sea level,358160N,47810E)has440mm annual rainfall on a long-term average.The soil texture was clay-loam(27% clay,36%silt and37%sand)with0.82%organic matter and a pH of7.4.Absorbable P and K were7and356ppm, respectively.Ghamloo(1850m above sea level,358230N, 478140E)has an annual rainfall of350mm.The soil texture was clay-loam(37%clay,27%silt and36%sand)with 0.62%organic matter and a pH of7.5.Absorbable P and K were14and320ppm,respectively.In the Ghamloo experiments,the11cultivars were planted in a randomized complete block design with four replications and grown under rain-fed conditions.At Gerizeh,cultivars were planted as subplots within the irrigation plots in a randomized complete block design with four replications.Irrigated plots were watered at planting,tillering,jointing,?owering and grain?lling stages.Non-irrigated plots received no water other than rainfall.The cumulative precipitation of3years is shown in Fig.1.Sowing was done in November in all experiments and seedling density was300seeds mà2.Plots consisted of six,6m long rows spaced20cm apart. Fertilizer was applied before sowing(100kg haà1P2O5and 25kg haà1Zn)and at stem elongation(50kg haà1N).
The total dry weight and grain yield were measured by harvesting4.2m2of the central part of each plot at crop maturity.Ten plants were randomly chosen from each plot to measure the number of grains per spike(grains/spike),grain weight and plant height.
Drought resistance indices were calculated using the following relationships:
(1)SSI?1àeY s=Y pT
ˉsˉp
(Fischer and Maurer,1978)
where Y s is the yield of cultivar under stress,Y p the yield of cultivar under irrigated condition,ˉY s andˉY p the mean yields of all cultivars under stress and non-stress conditions, respectively,and1àeY s=Y pTis the stress intensity.The
A.Sio-Se Mardeh et al./Field Crops Research98(2006)222–229
223 Fig.1.Cumulative precipitation during3years in Grizeh(left)and Ghamloo(right)stations.
irrigated experiment of Gerizeh station was considered to be a non-stress condition in order to have a better estimation of optimum environment.
(2)MP ?Y p tY
s 2(Hossain et al.,1990).(3)TOL =Y p àY s (Hossain et al .,1990).
(4)STI ?Y p tY
s Y 2p (Fernandez,1992).
(5)GMP =(Y p ?Y s )0.5(Fernandez,1992).
(6)Yield index eYI T?Y s
ˉs (Gavuzzi et al.,1997).(7)P i ?P n j ?1eX i j àM j T2
2n
(Clarke et al.,1992;Lin et al.,1986).
where n is the number of environments,X ij the grain yield of i th genotype in the j th environment and M j is the yield of the genotype with maximum yield at location j .
(8)Yield stability index (YSI)=Y s
Y p
(Bouslama and Scha-paugh,1984).
(9)b :The coef?cient of linear regression of grain yield of
a cultivar in each environment on the environmental index (mean yield of all cultivars at any environment)(Bansal and Sinha,1991).
Data were analyzed using SAS for the analysis of variance and Duncan’s multiple range test or the LSD value was employed for the mean comparisons.
3.Results
The results of analyses of variance for grain yield,grain weight,plant height and grains/spike in Gerizeh and
Ghamloo stations are presented in Table 1.There was a signi?cant difference among years for grain yield.The highest yield in 2002was at Gerizeh station (due to the high yield of non-irrigated plots)and,in 2003at Ghamloo station.The cultivars showed signi?cant differences in grain yield and other traits except plant height.The cultivar ?year interaction was highly signi?cant,indicating that cultivars performance changed from 1year to another.
At Gerizeh station,the main effect of moisture regimes and the interaction between cultivars ?moisture were highly signi?cant for the measured traits.Grain yield of cultivars varied,particularly under stress conditions,with the locations and years.This variation can be explained,in part,by the fact that traits suitable for a given environment with its own weather conditions may be unsuitable in another environment (Austin,1987;Van Ginkel et al.,1998).Alvand,Mahdavi and M-75-7were the most productive cultivars in irrigated and the least productive ones in non-irrigated conditions.Sardari,Agosta-Se?d,Azar2and Sabalan performed visa versa (Table 2).Grain yield under irrigated condition was adversely correlated with rain-fed condition (Fig.2)suggesting that a high potential yield under optimum condition does not necessarily result in improved yield under stress condition.Thus,indirect selection for a drought-prone environment based on the results of optimum condition will not be ef?cient.These results are in agreement with those of Ceccarelli and Grando (1991)and Bruckner and Frohberg (1987)who found that landraces of barley and wheat with low yield potential were more productive under stress condition.The lack of response to improved environmental conditions may be related to a
A.Sio-Se Mardeh et al./Field Crops Research 98(2006)222–229
224Table 1
Mean squares for agronomic traits of 11wheat cultivars in 2001–2003S.O.V .
d.f.Mean square Grain yield
1000-Grain weight Harvest index Plant height Grains/spike (a)Gerizeh station Year (y)252928**426**917**253.5192.6*Error
95210
58.58652.336.3Moisture (m)13672113**6924**2301**859.4398**M ?Y 2121856**411**553**185.7251**Error
9166432.849.536.424.7Cultivar (C)109900**302**50.9**155.437.7**C ?Y 207004**13.8n.s.48.6**8.518.4*C ?M 1015989**18.3*68.0**51.644.8**C ?M ?Y 204013n.s.9.4*19.5n.s.24.820.1*Error 180
29659.1919.67.1610.85CV
18
9.215.3 5.719.3(b)Ghamloo station Year (y)237100**87.2*107.8n.s.71.3*26.5Error
986320.1536.416.2 6.7Cultivar (C)107152**214.1**63.4**54.3**23.5**Y ?C 201292**9.6n.s.31.7n.s.11.65* 6.23**Error 90
412.79.519.9 6.13 3.27CV
14.9
10.5
13.8
4.7
14.3
*p <0.05.**
p <0.01.
lack of adaptation to high-moisture conditions (Clarke et al.,1992).The poor yielding cultivars in the present study were tall,sensitive to lodging,and with small number of grains/spike (Table 3),the desirable traits for drought condition but undesirable for high-moisture condition.Several studies indicated that semidwarf stature is preferred in late season drought condition (Fischer and Maurer,1978;Richards,1996;Van Ginkel et al.,1998).Van Ginkel et al.(1998)also found that many grains/spike was critical to high yield only in irrigated condition and it was negatively correlated with yield under late season drought condition.
Resistance indices were calculated on the basis of grain yield of cultivars over the years and locations (Table 4).As shown in Tables 2and 4,the greater the TOL value,the larger the yield reduction under stress condition and the higher the drought sensitivity.A positive correlation between TOL and irrigated yield (Y p )and a negative correlation between TOL and yield under stress (Y s )(Table 5)suggest that selection based on TOL will result in reduced yield under well-watered conditions.Similar results were reported by Clarke et al.(1992)and Rosielle and Hamblin (1981).Rizza et al.(2004),however,showed that a selection based on minimum yield decrease under stress with respect to favorable conditions (TOL)failed to identify the best genotypes.
Yields under irrigated condition were about three times higher than yields under stress in the present study.Since MP is mean production under both stress and non-stress conditions (Rosielle and Hamblin,1981),it will not be correlated with yield under stress (Table 5).For this reason,MP was not able to differentiate cultivars belonging to group A from the others.As described by Hohls (2001)selection for MP should increase yield in both stress and non-stress environments unless the correlation between yield in contrasting environments is highly negative.This is the condition found in our experiment.Toos,Roushan-Back cross and Mahdavi,for example,with relatively low yields under stress conditions,exhibited high MP values.The MP can be related to yield under stress only when stress is not too severe and the difference between yield under stress and
A.Sio-Se Mardeh et al./Field Crops Research 98(2006)222–229
225
T a b l e 2G r a i n y i e l d (g m à2)o f t h e c u l t i v a r s i n i r r i g a t e d (I R )a n d n o n -i r r i g a t e d (N I R )e n v i r o n m e n t s i n 2001–2003a t G e r i z e h a n d G h a m l o o s t a t i o n s
C u l t i v a r
G e r i z e h
G h a m l o o
2001
2002
2003
3-Y e a r m e a n
2001
2002
2003
3-Y e a r m e a n b
I R
N I R
Y .R .(%)a
I R
N I R
Y .R .(%)I R
N I R
Y .R .(%)a I R b
N I R b Y .R .(%)
N I R
A l v a n d 422103.075.647220357.0418141.766.1467.32
149.211
65.9111.986.1177.6125.27
C r o s s -s h a h i 385110.571.348926545.8409147.963.8427.77
174.510
59.2106.8105.8121.9115.59
M a h d a v i 494118.776.049024550.0432171.560.3472.01
178.49
62.2105.696.4166.5122.88
M -75-7437127.570.847225446.2427166.061.1445.34
182.57
59.0112.877.2144.2111.410
T o o s 54094.282.637926530.1393199.349.3437.35186.25
57.490.498.8143.9111.011
R o u s h a n -B a c k c r o s s 486120.575.241925638.9469166.764.4458.03
181.08
60.5123.1138.1173.3144.85
G l i n s o n 394114.271.046926443.7441201.454.3434.76
193.92
55.5106.2130.8139.0125.36
S a b a l a n 424107.574.637226030.1401195.551.2399.09
187.74
52.0105.4144.6195.5148.53
A z a r 2432165.261.835219046.0420199.352.5401.38
184.86
53.9162.0192.8206.1178.01
A g o s t a s e ?d 390138.564.530425217.1327177.145.8340.311
189.23
44.4119.2114.9202.3145.54
S a r d a r i
373146.2
60.8273
22617.2376
225.0
40.2340.710
199.11
41.6
150.1
154.6181.4
162.02
M e a n 434
122.4
71.8408
24440.1414181.056.3418.7
182.556.5
117.6
121.8168.4136.0L S D (5%)6139.163
54
50.429.953.232.4
24.326.233.127.3
L S D :l e a s t s i g n i ?c a n t d i f f e r e n c e a t 5%l e v e l o f s i g n i ?c a n c e .a P e r c e n t a g e o f y i e l d r e d u c t i o n u n d e r n o n -i r r i g a t e d c o n d i t i o n .b S u p e r s c r i p t v a l u e s a r e r a n k i n g o f c u l t i v a r s
.Fig.2.Association between grain yield of irrigated and non-irrigated wheat cultivars.Each point is the mean yield over the years and locations.
non-stress conditions is not too much.This case,for example,was observed in the Gerizeh experiment in2002 (Tables2and6).Cultivars with a high MP would belong to group A in these situations.Hossain et al.(1990)used MP as a resistance criterion for wheat cultivars in moderate stress conditions.
SSI showed a negative correlation with yield under stress (Table5).The tall cultivars Sardari,Agosta-se?d and Azar2 with high yield under stress produced a lower yield under non-stress conditions and showed the lowest SSI.Winter et al.(1988)also reported that tall wheat cultivars had a lower SSI.No signi?cant correlation was found between yield under stress and SSI in moderate stress conditions such as Gerizeh in2002(Table6),showing that SSI will not discriminate drought sensitive cultivars under such condi-tions.SSI was adversely correlated with both harvest index and grain yield under stress(Table5),suggesting that these traits can contribute to increased yield under stress and reduce stress susceptibility(Fernandez,1992).SSI has been widely used by researchers to identify sensitive and resistant genotypes(Clarke et al.,1984,1992;Fischer and Maurer, 1978;Winter et al.,1988).In the present study,the mean SSI over locations and years appeared to be a suitable selection index to distinguish resistant cultivars.Sardari,Agosta-se?d, Azar2and Sabalan with a lower SSI were identi?ed as resistant cultivars whereas Alvand,Mahdavi and M-75-7, with the highest SSI were sensitive(Tables2and4).
There was not a signi?cant correlation between either STI or GMP and yield under stress.However under less severe stress condition in2002,GMP and STI were signi?cantly correlated with both stress and non-stress yields(Table6). We conclude that GMP and STI are able to discriminate group A cultivars only under moderate drought stress conditions.
The difference between the highest and lowest yielding cultivars was about1318and480kg haà1in non-stress and
A.Sio-Se Mardeh et al./Field Crops Research98(2006)222–229
226
Table3
Grain weight,harvest index,plant height and grain/spike of the cultivars(averaged over3years)at Gerizeh and Ghamloo stations
Cultivar1000-Grain weight(g)Harvest index(%)Plant height(cm)Grains/spike
Ghamloo Gerizeh Ghamloo Gerizeh Ghamloo Gerizeh Ghamloo Gerizeh
NIR NIR IR NIR NIR IR NIR NIR IR NIR NIR IR Alvand30.325.536.431.123.934.946.249.567.513.419.426.4 Cross-shahi23.125.035.129.026.531.464.169.179.614.317.519.0 Mahdavi31.328.340.032.525.134.243.745.070.513.414.030.2 M-75-727.125.936.730.625.134.644.144.662.38.619.323.5 Toos26.124.535.430.128.933.242.245.573.614.016.024.0 Roushan-Back cross26.523.236.432.024.433.049.849.583.010.815.134.6 Glinson23.723.731.934.828.034.248.847.763.218.618.025.9 Sabalan30.729.241.229.924.030.156.657.182.69.213.021.0 Azar233.830.839.935.226.829.764.362.977.913.917.321.1 Agosta se?d34.334.142.233.727.327.761.263.091.512.616.220.4 Sardari35.234.141.935.625.727.865.362.687.49.810.614.1 Mean29.327.737.932.226.031.953.354.076.312.615.023.7 LSD(5%) 6.1 3.6 4.77 4.3 5.8 4.577.512.811.5 4.6 4.88.7 LSD:least signi?cant difference at5%level of signi?cance.
IR:irrigated;NIR:non-irrigated.
Table4
Resistance indices of the cultivars(averaged over3years)
Cultivar YSI YI P STI GMP SSI MP TOL Alvand0.3190.85931100.346241 1.127287300 Cross-shahi0.3330.88235650.364243 1.090285285 Mahdavi0.3340.93123020.400260 1.082311322 M-75-70.3350.90636080.376249 1.080296298 Toos0.3550.89629650.374249 1.022300302 Roushan-back cross0.370 1.04114480.435272 1.020312292 Glinson0.387 1.00035270.4152600.985299272 Sabalan0.439 1.04231130.3792530.889284231 Azar20.478 1.23328650.4242710.866294215 Agosta se?d0.508 1.04752150.3222340.766254173 Sardari0.547 1.16254390.3512450.713261160 Mean0.400 1.00033780.3802520.967289259 LSD(5%)0.0670.10818620.056160.151942
stress conditions,respectively (Table 2).Thus cultivars with the lowest yield under irrigated conditions exhibited the highest P value.This is con?rmed by a negative correlation between P and yield under non-stress conditions (Table 5).On the other hand,there was no association between P and yield under stress.Although Lin et al.(1986)have used P for differentiating stress resistance genotypes,it does not seem to be useful under severe stress conditions.
YI,proposed by Gavuzzi et al.(1997),was signi?cantly correlated with stress yield.This index ranks cultivars only on the basis of their yield under stress (Tables 2and 4)and so does not discriminate genotypes of group A.YSI,as Bouslama and Schapaugh (1984)stated,evaluates the yield under stress of a cultivar relative to its non-stress yield,and should be an indicator of drought resistant genetic materials.So the cultivars with a high YSI are expected to have high yield under both stress and non-stress conditions.In the present study,however,cultivars with the highest YSI exhibited the least yield under non-stress conditions and the highest yield under stress conditions (Table 4).
The linear regression of cultivar yield in each environ-ment on the mean cultivar yield over nine environments (3years ?3locations)was shown in Fig.3.The regression coef?cient (b )of Sardari,Agosta-Se?d,Azar2,Sabalan and Glinson were signi?cantly lower than those of other cultivars,being more stable.Mahdavi and Sardari had the highest and the lowest b ,producing the lowest and the
A.Sio-Se Mardeh et al./Field Crops Research 98(2006)222–229227
Table 6
Simple correlations coef?cients of stress indices with grain yield of 11wheat cultivars at Gerizeh and Ghamloo stations in 2001through 2003
YSI
YI STI GMP SSI MP TOL Stress intensity 2001
Grizeh-IR à0.74**à0.390.290.300.74**0.92**0.94**–Grizeh-NIR 0.90** 1.0**0.76**0.75**à0.90**0.01à0.65*0.72Ghamloo-NIR 0.91** 1.0**0.79**0.78**à0.91**0.01à0.65*0.73Grizeh-IR à0.80**0.270.87**0.88**0.80**0.95**0.93**–2002
Grizeh-NIR 0.33 1.00**0.70*0.69*à0.330.61*à0.100.40Ghamloo-NIR 0.91** 1.0**0.70*0.71*à0.91**à0.1à0.79**0.70Grizeh-IR à0.67*à0.180.440.440.67*0.79**0.86**–2003
Grizeh-NIR 0.85** 1.00**0.80**0.80**à0.85**0.45à0.66**0.56Ghamloo-NIR
0.89**
1.0**
0.81**
0.83**
à0.89**
0.41
à0.76**
0.59
*p <0.05.**
p <
0.01.
Fig.3.Association between mean grain yield and the environmental index (mean yield of all cultivars in each environment):(a)susceptible cultivars and (b)resistant cultivars.
Table 5
Simple correlation coef?cients between resistance indices and grain yield,grain weight,grains/spike and harvest index (HI)of 11wheat cultivars (averaged over years and locations)in irrigated (IR)and non-irrigated (NIR)conditions
b
YSI YI P STI GMP SSI MP TOL IR-yield 0.96**à0.91**à0.61*à0.85**0.61*0.450.91**0.94**0.97**NIR-yield à0.80**0.87**0.99**0.250.270.41à0.85**à0.32à0.78**HI (IR)0.92**à0.91**à0.72*à0.63*0.360.230.91**0.79**0.93**HI (NIR)
à0.510.590.590.410.110.16à0.61**à0.26à0.511000-Grain weight (IR)à0.67*0.69*0.540.38à0.35à0.18à0.68*à0.56à0.67*1000-Grain weight (NIR)à0.81**0.81**0.65*0.57à0.40à0.22à0.79*à0.68*à0.80**Grain/spike (IR)0.70*à0.62*à0.34à0.83**0.590.540.61*0.80**0.71*Grain/spike (NIR)
0.39
à0.48
à0.37
à0.18
0.15
0.04
0.49
0.29
0.42
*p <0.05.**
p <0.01.
highest yields under stress conditions,respectively (Table2).Bansal and Sinha(1991)used this method to assess the stability of wheat accessions over variable environments.Hohls(2001)showed that genotypes with a high stress tolerance had low b even when a range of stress and non-stress environments was used.
4.Discussion
Yield(and yield-related traits)under stress were independent of yield(and yield-related traits)under non-stress condition,but this was not the case in less severe stress condition.As STI,GMP and MP were able to identify cultivars producing high yield in both conditions.When the stress was severe,the linear regression coef?cient(b)and SSI were found to be more useful indices discriminating resistant cultivars,although none of the indicators could clearly identify cultivars with high yield under both stress and non-stress conditions(group A cultivars).It is concluded that the effectiveness of selection indices depends on the stress severity supporting the idea that only under moderate stress condition,potential yield greatly in?uences yield under stress(Blum,1996; Panthuwan et al.,2002).
Two primary schools of thought have in?uenced plant breeders who target their germplasm to drought-prone areas.The?rst of these philosophies states that high input responsiveness and inherently high yielding potential, combined with stress-adaptive traits will improve perfor-mance in drought-affected environments(Richards,1996; Van Ginkel et al.,1998;Rajaram and Van Ginkle,2001; Betran et al.,2003).The breeders who advocate selection in favorable environments follow this philosophy.Produ-cers,therefore,prefer cultivars that produce high yields when water is not so limiting but suffer minimum loss during droughty seasons(Nasir Ud-Din et al.,1992).The second is the belief that progress in yield and adaptation in drought-affected environments can be achieved only by selecting under the prevailing conditions found in target environments(Ceccarelli,1987;Ceccarelli and Grando, 1991;Rathjen,1994).The theoretical framework to this issue has been provided by Falconer(1952)who wrote,‘‘yield in low and high yielding environments can be considered as separate traits which are not necessarily maximized by identical sets of alleles’’.Austin(1987)and Van Ginkel et al.(1998)showed that the traits suitable for a given environment with its own weather conditions may be unsuitable(or even harmful)in another environment. The weakness of this approach is that input responsive-ness,so important in the wetter,admittedly less frequent but much more productive years cannot be easily maintained in the germplasm.The method also assumes yield crossover will occur below a certain yield threshold. Evidence for the existence of crossover(Ceccarelli,1989) and non-existence of crossover(Rajaram and Van Ginkle,2001)in environments ranging from moisture stressed to non-stressed has been reported.Blum(1996)and Panthuwan et al.(2002)believe that potential yield has a large impact on yield only under moderate drought stress conditions,before stress is severe enough to induce a genotype?environment(G?E)interaction for yield. Whether direct or indirect selection is superior depends upon the heritability of the selected trait in stress and non-stress environments and the genetic correlation between stress and non-stress environments(Nasir Ud-Din et al., 1992).Several researchers have concluded that selection will be most effective when the experiments are done under both favorable and stress conditions(Fischer and Maurer,1978;Clarke et al.,1992;Nasir Ud-Din et al., 1992;Fernandez,1992;Byrne et al.,1995;Rajaram and Van Ginkle,2001).Trethowan et al.(2002)showed that selection in alternating drought and non-drought environ-ments at the International Maize and Wheat Improvement Center(CIMMYT)has resulted in a signi?cant progress in the development of wheat germplasm adapted to dry areas globally.
When breeding for drought resistance is the aim,two situations seem to be clearly distinguished in order to choose a selection strategy:(1)where the drought condition is predominant over the years and wet years are infrequent,and (2)where the drought condition happens rarely and wet years are predominant.In the regions with the former situation(such as many parts of Iran),selection should be based on the yield in the target environments as Ceccarelli (1987),Ceccarelli and Grando(1991)and Rathjen(1994) have suggested.Where the latter situation exists(e.g.most parts of Europe),selection in favorable environments will be more effective because input responsiveness,so important in the wetter,admittedly less frequent,but much more productive years can be easily maintained in the germplasm (Richards,1996;Van Ginkel et al.,1998;Rajaram and Van Ginkle,2001;Betran et al.,2003).
5.Conclusions
If the strategy of breeding program is to improve yield in a small stress or non-stress environment,it may be possible to explain local adaptation to increase gains from selection conducted directly in that environment(Atlin et al.,2000; Hohls,2001).However,selection should be based on the resistance indices calculated from the yield under both conditions,when the breeder is looking for the cultivars adapted for a wide range of environments.
The?ndings of this study showed that the breeders should choose the indices on the basis of stress severity in the target environment.The linear regression coef?cient(b)and SSI are suggested as useful indicators for wheat breeding,where the stress is severe(northwest of Iran at the present study) while MP,GMP and STI are suggested if the stress is less severe.
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Acknowledgement
The authors thank the Research Council of Tehran University for providing the necessary facilities. References
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