Gains from Selection under Drought versus Multilocation Testing in ...

Published January, 1995

Gains from Selection

under Drought versus Multilocation Tropical Maize Populations

P. F. Byrne,* J. Bolafios,

Testing in Related

G. O. Edmeades, and D. L. Eaton

ABSTRACT

well-watered conditions was greatly reduced under crop water deficits. 2. Selecting only under stress conditions. A drawback to this approach is that sometraits that contribute to survival under drought may lower productivity under favorable conditions (Blum, 1988; Ludlow and Muchow,1990). Another potential limitation is that heritability of grain yield, and thus the effectiveness of selection, is often reduced under moisture stress (Blum, 1988). Edmeades et al. (1992) reported that in maize this was not the case until yield fell below about 20%of its level under unstressed conditions. This selection strategy was employed reasonably successfully by ArboledaRivera and Compton (1974), who reported increased yield in both stressed and unstressed environments. 3. Selecting in a combination of stressed and unstressed environments.This is the intrinsic goal of multilocation testing schemes, although in practice results from low yielding, drought-stressed sites are sometimes discarded due to high coefficients of variation. Moreover,in the presence of a large genotype × stress-level interaction, progress from selection based on combined data maybe limited. Using this approach, Arboleda-Rivera and Compton (1974) reported a lower gain per cycle than whenselection was done either in the stressed or unstressed environments. Thus, the question of which selection strategy produces the best results remains unresolved.

Ideal maize (Zea maysL.) cuitivars for tropical areas should yield well both in the presenceandabsence of drought, but optimal selection strategies for accomplishingthis goal are not dear. This study evaluated progress from selection of two related tropical populations across a broad range of environmentalconditions. ’Tuxpeflo Sequia’ (TS) had undergonefuli-sib recurrentselection for eight cycles at one location under managedlevels of drought stress, while ’Tuxpefio 1’ (T1) was selected for six cycles in a modified full-sib selection scheme that relied heavily on muitilocation yield trial data. Combinedover 12 environments (with mean yields ranging from 0.30-7.93 Mgha-~), regression analysis revealedsignificantly different rates of change per cycle for TS and T1, respectively, for grain yield (1.68 and 1.06%, P < 0.10), anthesis-siiking interval (ASI) ( - 8.59 and 0%, P < 0.10), ears per plant (1.26 and 0%, P < 0.05), and plant height (- 0.83 and 1.29%, P < 0.01). Days to anthesis decreased in both TS and T1 (- 0.36 and -0.15% per cycle, respectively), but the difference betweenpopulations was not significant at P < 0.10. The interaction of environmentswith the linear rate of gain in grain yield was not significant in either population, indicating similar progress across the range of environmentalconditions sampled.Stability analysis indicated that TS Cycles 6 and 8 and the check variety ’La Posta Sequia Best’ werethe most stable and high yielding entries in the trial. Better yield gain in TS is likely related to its selection for reduced ASI under controlled stress at a single site. Selection undermanagedlevels of droughtstress at one location together with muitiiocation testing maybe desirable componentsof maize breeding programsfor drought-prone tropical areas.

D

ROtJ~H’rSa’Pa~SSis a major factor limiting maize yields in the tropics, affecting annually an estimated 80%of the lowland maize area (Edmeadeset al., 1989). Becauserainfall in muchof this drier zone is unpredictable in quantity and distribution, cultivars targeted there should perform well under a wide range of moisture conditions. Plant breeders and physiologists have investigated the following selection strategies for obtaining such broadly adapted cultivars.

In the mid-1970s, CIMMYT began selection for drought tolerance in the population Tuxpefio Sequfa (TS), formed from a set of families of the lowland tropical population ’Tuxpefio CremaI’. Full-sib (FS) recurrent selection, based on an index of traits believed to be related to drought tolerance and measured under three moisture regimes at one site (Fischer et al., 1989), was carried out for eight cycles (Bolafios and Edmeades, 1993a). A similar set of families from the same source was used to form the Tuxpefio 1 (T1) population, which underwent selection for six cycles of a modified FS recurrent selection schemethat relied heavily on results of multilocation yield trials. BecauseCycle 0 of TS and T1 were derived from the same source population, an evaluation of cycles of selection of the two materials allows a reasonable comparison of gains made under the two improvement schemes. In an evaluation of Cycles 0 and 3 of the two materials, Fischer et al. (1989) observed that T1 showed greater yield improvement under mild moisture stress, whereas TS had higher rates

1. Selecting only under well-watered conditions. Johnson and Geadelmann(1989) reported that yield gains from selection under irrigation were equal to those from selection under drought stress when evaluated in stress conditions, and that such gains were superior when evaluated in favorable conditions. Arboleda-Rivera and Compton(1974) and Martinez-Barajas et al. (1992), however, found that progress from selection for high yield under P.F. Byrne, USDA-ARS Plant Genetics Research Unit, 310 Curtis Hall, Univ. of Missouri, Columbia, MO65211; J. Bolafios and G.O. Edmeades, Intl. Maize and Wheat Improvement Ctr. (CIMMYT),Apartado Postal 6-641, 06600 M6xico,D.F., M6xico;D.L. Eaton, Dekalb Genetics Corp., Dekalb, IL 60115. Received 10 Feb. 1994. *Corresponding author (byrne @teosinte. agron.missouri.edu).

Abbreviations:ASI, anthesis-silking interval; DA,days to anthesis; EPP, ears per plant; PH, plant height; TS, Tuxpefio Sequia; T1, Tuxpefio 1; FS, full-sib.

Published i’n Crop Sci. 35:63-69 (1995).

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CROP SCIENCE, VOL.35, JANUARY-FEBRUARY 1995

of gain at intermediate and severe stress levels. Bolafios and Edmeades(1993a), however, reported that Cycle of TS yielded better than the same cycle of T1 under both well-watered and moisture-deficit conditions. The objective of our study was to compare yield and other characteristics of genotypes from a more representative set of selection cycles of the two populations, TS and T1, under a broader range of environmental conditions. Twoadditional materials developed under conditions of managed drought were also included in the study as check entries. MATERIALS Population

AND METHODS Improvement

Both the TS and T1 populations originated from Cycle11 of TuxpefioCremaI, a lowlandtropical late white dent population comprising Tuxpefio race collections from Mtxico(Johnson et al., 1986). Of the 250 FS families used in formingCycle 0 of each population, 165 families were common to both TS and T1 and 85 families were different, althoughfromthe same source population. Selection in TS, described in detail elsewhere(Fischer et al., 1989; Bolafios and Edmeades,1993a), wasbegunin 1975, and carried out during the virtually rain-free winter season at Tlaltizap~in, Mexico, (18°N, 940-melevation) under three waterregimes:(i) well-watered,(ii) stress duringgrain filling, obtained by suspendingirrigation from 2 wkbefore anthesis until maturity,and(iii) stress duringfloweringandgrain filling, obtained by withdrawingirrigation 3 wkafter emergenceand growingthe crop to maturityon stored soil water. Grainyields of FS families grownunder the three water regimes averaged approximately6, 4, and 1.5 Mgha-~, respectively. Population size, except in Cycle 0, was 256 FS families, and selection intensity was 30 to 40%.FS progenyfor the next round of testing were formedby recombiningan average of five plants per selected family during the following summerseason at Poza Rica, Mrxico, (60-m elevation, 19°N) under natural disease pressure. Onecycle of selection was completedeach year (two growingseasons). Selection in TS wasbasedon an index of traits whichaimed to increasegrain yield and the rate of stemand leaf elongation under drought; maintain constant days to anthesis and grain yield under unstressed conditions; and reduce canopytemperature, the anthesisosilkinginterval (ASI), and the rate of leaf senescence under drought. Each variable was expressed in standard measure,and was multiplied by the Euclideandistance froma selection target expressedin standard deviation units from the mean, and a weight (Fischer et al., 1989). The weightsfor grain yield underdrought, ASI, and anthesis date were approximately1.0, while those for other variables were 0.5. The selection index for a full-sib family was therefore these products summed over all traits, and the smallest value of the index indicated the genotypethat mostclosely approximatedthe desired plant ideotype. The improvement of TI (also knownas Population 21) was through a FS recurrent selection schemewhichincluded multilocation progenytesting (described in detail in Pandey et al., 1986).Thefirst twocycles of selection of T1involved two growingseasons per cycle. In the first season, 250 FS families plus six checkswereevaluatedfor yield and agronomic traits in tworeplicates of a simplelattice designof an international yield trial. Each trial was grownin Poza Rica and up to five other locations volunteered by national program cooperators in Africa, Asia, and Latin America.Simultane-

ously, the FSfamilies wereevaluatedfor stress, tolerance in disease, insect, and high-density nurseries at Poza Rica. In the secondseason, families selected primarily on the basis of yield trial results across environments, andto a les.,;er extent on their performancein the PozaRica stress nurseries, were recombined,and the next set of progenyproduced. Beginningwith Cycle3, the procedurewas modifiedso that one cycle of selection was completedin four croppingseasons (i.e., 2 yr), and evaluation of progeniesin stress nurseries wasdiscontinued.Thus,in the first season the yield trial was grownat up to six locations, as describedabove.In the following season, the 250 FS families were planted in a nursery, normallyat Poza Rica. Plants within families wereselected, primarily for resistance to fall armyworm (SpodopterafrugiperdaJ.E. Smith),and self-pollinated. At harvest, four to five S~ ears per family were selected. Meanwhile,basedon results of the yield trial across locations, the best fraction of the FS families was determined,and in the third season two to three St families from selected FS families were recombinedusing a bulk of pollen collected amongthem. To regenerate FS progenyfor the next cycle of selection, half-sib ears harvested in the third season were planted ear-to-row and reciprocal plant-to-plant crosses were madebetweenplants originating from different FS families. At harvest, the 250 best pairs of ears wereselected as entries in the next cycle’s yield trial. Theselection intensity in T1was35 to 40%during the first five cycles and about 20%thereafter. By 1985six cycles of selection were completed. Evaluation Thetrial contained16 entries arrangedin a square lattice design with four replications. Entries wereCycles0, 2, 4, 6, and 8 of TS; Cycles 0, 2, 4 and 6 of T1; and seven check varieties. Cycles were represented by a balanced composite of seed resulting from crosses amongFS families. For the analyses reported here, only data of the cycles of selection and the checks ’La Posta SequfaBest’ and ’La Posta Sequfa Worst’ were included. Those two synthetic varieties were formedby recombiningthe 10 $1 families of ’La Posta Sequfa Cycle1’ having,respectively, the best and worst scores for an index of droughttolerance traits rated at Tlaltizap~n, Mrxico. Thus, their selection was basedon a set of characters similar to that employedin TS. The population’La Posta Sequfa’ was developed from ’La Posta Cycle 5’ by selecting for traits associated with droughttolerance in an S~ recurrent selection scheme. The ’La Posta’ population is based on 16 lines from Tuxpefiocollections, is closely related to T1and TS, and was improvedthrough CIMMYT’s international testing system, as described above for T1. Seventeensets of the trial wereplanted in 1988and 1989. At four locations the trial was completely lost to extreme droughtconditions,, and at one on-farmsite, non-uniformmanagementpractices invalidated the results. Thus, data from 12 location-year combinations, each considered an environment, were combinedfor analysis (Table 1). Four replications data from each environmentwere used in the analysis, except for E1 Eden,Mrxico,whereonly two replications wereplanted. Each plot included four rows 5 mlong, with 0.75 mbetween rows and 0.20 mbetweenplants. Rowswere overplanted and -~. thinned after emergenceto approximately66 000 plants ha Waterregimesand quantity of fertilizer applied varied among environments(Table 1). Weedswere controlled chemically manuallyas required. Data were recorded only from the two central rows of each plot, and at mostlocations the twoplants at each end of the rowswerediscardedas borders.Thetrials wereeither irrigated,

65

BYRNEET AL.: SELECTION UNDERDROUGHT-VERSUS-MULTILOCATION TESTING IN MAIZE Table 1. Summaryof environmental characteristics, managementconditions, grain yields, environments, listed in descending order of mean maize grain yield.

Country Colombia Guatemala Mexico Mexico Mexico Thailand India Zimbabwe Guatemala Mexico Mexico Mexico

Location Cali Jutiapa Miacatl~ln Poza Rica Poza Rica Tak Fa Banswara Mzarabani Cuyuta El Eden Cd. Obreg6n Tlatizap~tn

Elevation Latitude m 965 900 860 60 60 87 716 480 48 40 40 940

3°N 14°N 18°N 20°N 20°N 15°N 23°N 16°S 14°N 20°N 26°N 18°N

and other traits

measured in 12 experimental Grain yield

Planting date

Soil type~"

Fertilizer~t

7 Jan. 1989 6 June 1989 4 July 1989 17 Nov. 1988 11 May1989 27 May 1989 1 July 1989 2 Oct. 1989 15 Dec. 1988 11 July 1989 11 May1989 22 Nov. 1988

CL CL CL SL SL CL CL SL SL SL CL CL

120-30-30 85-40-0 120-40-0 200-80-0 200-80-0 100-125-0 80-60-0 120-80-0 100-60-0 100-40-0 15046-0 150-50-0

Moisture

regime Mean sd§

Irrigated Rainfed Rainfed Irrigated Irrigated/rainfed Rainfed Rainfed Rainfed Irrigated Rainfed Limitedirrigation Limitedirrigation

- Mgha-~ 7.93 0.52 7.76 0.49 7.39 0.67 6.98 0.29 6.78 0.35 6.60 0.46 5.77 0.36 5.67 0.67 5.28 0.42 2.84 0.52 2.68 0.33 0.30 0.14

Othertraits measured¶ ASI, ASI, ASI, ASI, ASI, ASI,

DA, EPP, DA, EPP, DA, EPP, DA, EPP, DA, EPP, DA, PH

PH PH PH PH PH

ASI, EPP, EPP ASI, ASI,

DA, EPP, PH PH DA, EPP DA, EPP

SL = Sandy loam; CL = Clay loam. kg ha ° t N, P205, and K20,respectively. Standard error of the difference betweenentry means. ASI= anthesis-silking interval; DA= days to anthesis; EPP= ears plant-l; PH= plant height.

rainfed, or received a combinationof both. At Tlaltizap~in, M6xico,severe moisturestress was inducedby irrigation withdrawalfrom3 wkprior to floweringuntil 5 wkafter flowering. At Cd. Obreg6n, M6xico, an arid hot environment, water stress wasinducedby extendingthe interval betweenirrigations from the normal10 d to 3 wkat flowering. Daysfrom planting to anthesis (DA)and silking (DS) calculated from the date on which 50%of the plants had begun shedding pollen or had silks emergedfrom the husk. Anthesis-silking interval (ASI) was obtained by subtracting DAfrom DS. Plant height (PH), measuredfrom the base the plant to the first tassel branch, was averaged for five competitiveplants in eachplot. Grainyield, expressedat 150g H20kg-1, was obtained either from shelled grain, or by assuminga shelling percentage of 80%whereear weight was reported. Numberof ears per plant (EPP) was determined dividingthe total numberof harvestedears (cobs with at least one kernel) by the total numberof plants in the harvested section of the plot. Statistical Analysis Asubset of five variables (grain yield, DA,ASI, EPP,and PH)was chosen for detailed analysis. Yield data from one site, Mzarabani,Zimbabwe,were adjusted using plot stand as a covariate prior to analysis. Combined analysesof variance (ANOVA) were conducted with the SASGLM procedure (SAS Institute, Inc., 1988), basedon a randomizedcompleteblock design and a modelin whichgenotypeswere considered fixed effects and environmentsrandom.Single degree of freedom contrasts wereused to test the difference betweenthe initial cycles of the two populations and betweenthe two checks. The genotype × environment interaction mean square was used to test the effect of entries and to computeLSDvalues. For each population the genotype x environmentinteraction sums of squares was partitioned into environment× linear, environment× quadratic, and residual components to test the consistency of regression relationships across environments. Because error variances were not homogeneousacross all environmentsaccording to Bartlett’s test (P = 0.05), some tests of significancewill not be at the exactprobabilitiesgiven. Changesover cycles of selection for each population were estimated by regressing genotypemeanson cycle of selection, and fitting linear, quadratic, and cubic modelssequentially (SASInstitute, Inc., 1988). After each modelwasfitted, the significance of the last-added parameter was determinedby

an F-test using as the error term the genotype× environment interaction mean square from the combined ANOVA. Quadratic and cubic terms werenon-significant (P > 0.10) in all cases. Whenthe linear componentwas also non-significant, we concludedthat no changes had occurred with selection. Percent changeper cycle was calculated as the ratio of the linear regressioncoefficientto the intercept, multipliedby 100. Standarderrors of regression coefficients and homogeneity of linear regression coefficients for the two populations were determined according to Gomezand Gomez(1984). Simplephenotypiccorrelation coefficients amongthe traits werecalculatedfor each populationand for all entries together, using entry meanscombinedacross all environmentsin which both membersof a pair of traits were measured. Yieldstability of the 11 entries wascompared using a method based on principal coordinate analysis (Westcott, 1987). Briefly, this descriptive methodcalculates similarity measures for all pairs of entries basedon performancerelative to the highest yielding genotype. The analysis, based on genotype meansfor each environment,wasperformedin cycles, starting with the lowestyieldingsite and sequentiallyaddingindividual environments to the data set in ascendingorder of yield. Results fromeach cycle of analysis weredisplayedin a two-dimensional diagramin whichentries least similar to the highest yielding genotypeare represented by points clustered near the center of the diagram.By this method,stable and high-yieldinggenotypes are those with consistent above-averageperformancein consecutive analyses, and are represented as points that are remotefromthe central cluster in the diagram. RESULTS AND DISCUSSION Meanyields in the 12 environments varied from 0.30 to 7.93 Mgha-1 (Table 1), reflecting the broad range of growing conditions encountered. In the nine higher -~, yielding environments, which averaged 6.68 Mgha the trial received generally adequate water supplies throughout the growing season via rainfall, irrigation, or both, and thus did not suffer significant droughtstress. In each of the three less productive environments (mean yield of 1.94 Mgha-l), the trials experienced some degree of moisture stres s, whichwassevere at Tlaltizap~in and moderate at Cd. Obreg6n and E1 Eden. Combined analyses of variance including all 12 environments (not

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CROPSCIENCE, VOL. 35, JANUARY-FEBRUARY 1995

Table 2. Mean performance of nine cycles of selection and two check varieties in an international maize yield trial. Data are combined over 12 environments. Entry Tuxpefio Sequia Cycle 0 Tuxpefio Sequia Cycle 2 Tuxpefio Sequia Cycle 4 Tuxpefio Sequia Cycle 6 Tuxpefio Sequia Cycle 8 Tuxpefio 1 Cycle 0 Tuxpefio 1 Cycle 2 Tuxpefio 1 Cycle 4 Tuxpefio 1 Cycle 6 La Posta Sequia Best La Posta Sequia Worst Mean LSD(0.05) No. of environments with data

Grain yield

Days to anthesis

Mgha- t 5.31 5.57 5.70 5.90 6.05 5.23 5.50 5.50 5.61 5.88 5.52 5.61 0.24 12

66.3 66.1 65.0 64.7 64.6 66.5 66.0 65.7 65.9 66.2 66.0 65.7 0.7 9

Ears Plant ASIa" plant- t height

d 4.4 3.3 2.4 1.3 1.9 5.3 4.4 5.1 3.6 2.6 4.6 3.5 2.5 9

no. 0.87 0.89 0.93 0.95 0.96 0.88 0.89 0.85 0.90 0.95 0.89 0.91 0.06 10

cm 211 202 207 198 195 206 211 218 222 226 227 211 6 8

~" Anthesis-silkinginterval.

shown)indicated significant (P < 0.05) effects due environmentsand genotypesfor all variables. The genotype x environmentinteraction wassignificant only for DAand ASI. Combinedmeansfor TS Cycle 0 and T1 Cycle 0 did not differ significantly (P > 0.05) for anyof the traits (Table2), confirmingthe similarity of the starting points of the two selection schemes.Theleast squaresestimates of yield gain per cycle were 0.090 Mgha-1 (1.68%) TS and 0.056 Mgha-l (1.06%) in T1, values which differed significantlyat the 0.10probabilitylevel. Within each population the environmentx linear interaction wasnon-significant (P > 0.10), indicating the samegeneral trends across the range of environmentssampled. Whendata were analyzedseparately for the sets of nine higher yielding and three lower yielding environments, the relative yield gain advantage of TS over T1 was similar: 0.101 vs. 0.063 Mgha-~ in higher yielding environments and 0.052 vs. 0.029 Mgha-1 at lower yieldingsites; however,with these smallerdata sets, the regressioncoefficientswerenot significantlydifferent at P = 0.10. In the lower yielding environments,for unknown reasons, TSCycle 8 yielded less (P < 0.05) than Cycle 6, thus reversing the trends observedin Cycles0 to 6. Yield gains observedin both TS and T1 are generally lower than those reported in other studies. However, gains in the more productive environments(1.58 and 1.00%for TS and T1, respectively) comparerelatively

well with previously estimated improvement rates at the 6 Mgha-1 yield level of 2.29%per cycle for TSCycles 0 to 8 (Bolafios and Edmeades,1993a) and 1.21%per cycle for T1Cycles0 to 5 (Pandeyet al., 1986).Observed yield gains in the loweryielding sites weresurprisingly small. Thecalculatedgain per cycle for TSin this analysis (3.26%) contrasts with previous reports of 6.67%for Cycles0 to 8 (Bolafios and Edmeades,1993a)and 8.05 for Cycles0 to 3 (Fischer et al., 1989), at similar yield levels. The previously reported results are fromtrials grownonly at Tlaltizap~in, and mayreflect somedegree of specific adaptationto conditionsthere. Alternatively, the gains mayreflect specific features of the low yielding sites at whichthe trial wasgrown;excessive droughtat Tlaltizap~in (meanyield 0.30 Mgha-l), relatively poor crop management at El Eden, and excessive heat stress at Cd. Obreg6n, where daily maximumtemperatures mayexceed 40°Cduring the flowering period. Gains in low yielding environmentswere reduced also because of the poor performanceof TS Cycle 8. Regressionanalysis of other traits (Table 3) showed that days to anthesis decreasedin TSabout twice as fast as in T1 (-0.36 vs. -0.15%per cycle, respectively), and that ASI decreased and EPPincreased in TS, but were unaffected by selection in T1. The environment× linear interaction wassignificant (P < 0.05) for ASIand DAin TS and for ASI and EPPin T1. The interaction for ASIwasduealmostentirely to data fromTlaltizap~in, whereextreme drought accentuated differences in ASI amongcycles. A notable divergent but indirect response betweenthe two populations is that TS becameshorter by 1.76 cm per cycle (P < 0.01), whereas T1 became taller by 2.67 cmper cycle (P < 0.01), probablybecause T1 had beenselected for tolerance to Spodopterafrugiperda (S. Pandey, 1993, personal communication). Highlevels of correlation amongmost pairs of traits were observed in TS(Table 4). Yield correlated negatively with DA,ASI, and PH, and positively with EPP. In contrast, yield in T1 correlated significantly (P 0.10) only with increasing PH,although the correlation with DAwasnearly significant at that probability level. (Because the numberof degrees of freedom for T1 is less than for TS, the magnitudeof the correlation must be larger for significance.) Whenall 11 entries were included in the data set, the correlation betweenyieJd and ASIwas -0.88 (P < 0.01), very near the value -0.94 (P < 0.01) reported by Jensen (1971) for a group

Table 3. Linear regression coefficients (b~ and b2) with standard errors, and percent change per cycle for several traits combinedacross environments for Tuxpefio Sequia and Tuxpefio 1. If regression was not significant at P < 0.10, percent change per cycle was assumed to be equal to zero. Tuxpefio Sequ~ %

bt + SE -t) Grain yield (Mgha Daysto anthesis (d) ASI§(d) Ears plant-I (no.) Plant height (cln)

0.090** - 0.24** - 0.35* 0.011"* - 1.76"*

Tuxpefio 1

+0.014 +0.04 +0.14 +0.004 +0.36

1.68 - 0.36 - 8.59 1.26 - 0.83

b2 :[: SE 0.056** - 0.10 + ns ns 2.67**

**, *, +, ns Significant at the 0.01, 0.05, and 0.10 levels of probability,andnot significant, respectively. Significanceof the difference betweenregression coefficients of the twopopulations. Anthesis-silkinginterval.

+0.019 +_0.06 +0.51

%

hi -- b2"~

1.06 - 0.15 0 0 1.29

+ ns + * **

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BYRNEET AL.: SELECTION UNDERDROUGHT-VERSUS-MULTILOCATION TESTING IN MAIZE Table 4. Simple phenotypic correlation coefficients amongtraits for five cycles of selection of Tuxpefio Sequfa (above diagonal; N = 5) and four cycles of selection ofTuxpefio 1 (below diagonal; N=4). Grain yield Grain yield Daysto anthesis ASI Ears plant- 1 Plant height

Daysto anthesis

- 0.93* - 0.89 0.39 - 0.77 - 0.58 0.14 0.90 ÷ - 0.75

ASIa" - 0.95* 0.93* - 0.60 - 0.75

Ears plant- 1

Plant height

0.96** - 0.97** - 0.90*

- 0.88* 0.81 + 0.84 + - 0.78

- 0.39

+, *, ** Significantat the 0.10, 0.05, and 0.01levels of probability,respectively. Anthesis-silkinginterval.

0.8 ¯ LPSBest 0.4

&TS C8 -0.4 ¯ TS CG -0.8 -1 .

of 15 experimental hybrids varying in drought tolerance and evaluated in Nebraska. The synthetic variety La Posta Sequfa Best yielded more grain and EPP and had a reduced ASI compared to La Posta Sequfa Worst (Table 2). The consistency this relationship was shownby the non-significant (P 0.10) interaction of the check varieties with environment for all traits. In the series of stability analyses, whichbeganwith the lowest yielding site and added environments in ascending order of mean yield, TS Cycles 2 and 6 and La Posta Sequfa Best were initially located far from the center of the diagram in consecutive analyses. As additional environments were added, TS Cycle 2 dropped out of the group of stable varieties and TS Cycle 8 was added. From the fifth analysis onward, TS Cycles 6 and 8 and La Posta Sequfa Best were consistently identified as the most stable and high yielding genotypesin the trial (Fig. 1). All three stable genotypes had a history of selection for drought tolerance. Althoughselection criteria for TS included maintaining grain yield under full irrigation, selection was heavily weighted toward improving yield under drought (Fischer et al., 1989; Bolafios and Edmeades, 1993a). Nonetheless, our results showedthat selecting in a low-moisture environmentwas effective in increasing yield under wellwatered conditions, a finding contrary to the observations of Johnson and Geadelmann (1989), but in agreement with those of ArboledaoRivera and Compton (1974). This outcomeis likely related to a concomitant reduction in ASI (Table 2). Recent studies have indicated that the maintenance of silk growth rates under stress is a consequenceof high spikelet growth rates, indicative of increased biomasspartitioning to the developing ear shoot (Bolafios and Edmeades, 1993b; Edmeadeset al., 1993). This in turn results in fewer aborted kernels in the early stages of grain filling under drought, and a higher harvest index and yield both in the presence and absence of drought stress. In contrast, a significant trend for ASIin T1 was not found (nor was it selected for), although there was a net decrease from Cycle 0 to Cycle 6 (Table 2). Because variation in ASI and EPPare increased under drought at flowering, a uniform low-moisture environment provides more efficient selection conditions for these traits. Edmeadeset al. (1992) reported that the heritability of grain yield fell rapidly at yield levels < 20%of unstressed potential, but that this decline was

-0.5

0

0.5

Coordinate 1 Fig. 1, Plot of similarity distances(¥,estcott, 1987) among maize entriesrelativeto thefirst twoprincipalcoordinates in thestability analysisincludingall 12 environments. Dueto overlapping,not all points in the cluster are shown. TS = Tuxpefio Sequfa, LPS = La Posta Sequia.

less for ASI, while heritability of EPPactually increased. They also demonstrated that genetic correlations between grain yield and ASI, and between grain yield and EPP, approach -0.7 and 0.9, respectively, in this yield range. These observations suggest that in severely droughted environments where selection for grain yield is ineffective, efficient selection for ASI and EPPcan be carried out. In these circumstances selection for reduced ASI and increased EPP under severe drought at flowering can help maintain the efficiency of selection for grain yield. Further indication of the selection value of ASI and other traits measured under drought is suggested by the superior performance of La Posta Sequfa Best compared to La Posta Sequfa Worst across all environments. While selection for reduced ASI apparently has been effective in increasing yield potential in TS and several other tropical maize populations (G.O. Edmeades, 1993, unpublished data), it is uncertain whether other populations wouldbehave similarly. For example, selection for reduced ASI may not be as effective in germplasmwith a long history of intensive improvementthat included selfing under heat and drought. Most U.S. maize growing areas are subject to periodic droughts of varying length and intensity, and breeders there have long been aware of the relationship of ASI and drought tolerance (e.g., Jensen, 1971). Whether through direct selection for improved ASI or as a correlated response to selection for improved performance under dry conditions or under high population densities (Troyer, 1967), breeders may have improved ASI sufficiently and reduced variation in the trait in elite germplasm,so that further gains from reductions in ASI may be less likely. Whether ASI is a useful indicator of other types of environmental stress is also unclear; relatively weak associations have been observed between ASI and grain yield under acid soil conditions (S. Pandey, 1993, personal communication). Under low N (R. Lafitte, 1993, personal communication) or under artificial shade (G. Edmeades, 1993, unpublished data) ASI-grain yield associations are of a similar magnitude to those observed under drought.

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Although PH and DA were not selection criteria in TS, reductions in these traits have been associated with greater drought tolerance (Fischer et al., 1983), and thus the correlated response is understandable. However, tall plants with later maturity are also generally associated with higher yield potential in favorable environments. Reduction in these traits, accompanied by yield gain in high yielding environments, indicate that sufficient variability existed to permit concurrent improvements in all three traits. A similar result was reported by Johnson et al. (1986) from a recurrent selection study for reduced plant height in Tuxpeno Crema I. While selection for drought tolerance resulted in general improvement across a range of moisture conditions, that result must be qualified by the interaction of TS Cycles 6 and 8 with the higher and lower yielding groups of environments. Whereas Cycle 8 was the higher yielding genotype in the favorable environments, Cycle 6 was superior in the stressed locations. Stability analyses suggested that further selection for reduced ASI beyond Cycle 6 may result in decreasing yield stability under stress. This implies that after yield limitations due to poor partitioning to the ear at flowering have been partially overcome, other factors may become more important in improving yield potential. Among these is specific environmental adaptation as revealed by multilocation testing. In contrast to TS, selection in Tl was based on performance in generally favorable moisture conditions, with mean yields in multilocation trials averaging approximately 5 Mg ha'1 (CIMMYT, 1993, unpublished data). The slower rate of yield gain observed for this population in high yielding environments (1.00% per cycle vs. 1.58% per cycle for TS) is likely due to a combination of causes. Crossa and Gardner (1989) found that the mean ratio between genotype x environment and genotype variance components for yield was 2.78 for Cycle 0 through Cycle 4 of Tl. Such a large ratio indicates a high degree of heterogeneity in the testing environments and limits progress from selection. In addition to yield, Tl was selected for resistance to fall armyworm, resistance to stalk and ear rots, and other agronomic characteristics (Pandey et al., 1986), which somewhat reduced selection intensity for yield and related characters. It should be noted that CIMMYT's international testing system has been quite effective in improving yield in other tropical populations (Pandey et al., 1986, 1987). Results of this evaluation bring together previous findings concerning the value of selecting for drought tolerance and the importance of multilocation testing in evaluating and improving yield performance in lowland tropical maize (Bolanos and Edmeades, 1993a,b; Pandey et al., 1986, 1987, 1991). Together they suggest a breeding strategy that first attempts to reduce ASI by evaluating progenies or lines at few sites under drought stress that is carefully managed to coincide with flowering. Where tolerance to other biotic and/or abiotic stress is also required, selection is most efficient under managed uniform stress environments, rather than those which occur randomly during multilocation testing. Elapsed time per selection cycle is often less when testing under a few

managed environments than under multilocation testing. As a final step, selection for adaptation to a specific ecology, for yield potential and for yield stability can be accomplished by testing in multiple sites carefully chosen to represent the target ecology. This step also serves as a means of delivering germplasm products with enhanced stress tolerance to the many national program cooperators who conduct multilocation trials. ACKNOWLEDGMENTS The authors are indebted to many colleagues, within CIMMYT and in national research programs, who cooperated in conducting this study or provided helpful comments on the manuscript. The contributions of R. Arias, H. Baretto, H. Cordova, J. Crossa, L. Darrah, C. DeLeon, J. Deutsch, C. Kitbamroong, R. Lafitte, M. Smith, S. Pandey, H. Pham, N.N. Singh, S.K. Vasal, W. Villena, and J.L. Zea are especially appreciated.

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