Selection for Drought Tolerance Increases Maize Yields across a

Published July, 1999

Selection for Drought Tolerance Increases Maize Yields across a Range of Nitrogen Levels M. Banziger,* G. O. Edmeades, and H. R. Lafitte ABSTRACT It is not known whether selection for improved tolerance to a specific abiotic stress leads to correlated changes in performance under other stresses. Drought and N deficiency are important constraints to production in the tropics. We examined the effect of selection for drought tolerance on performance of tropical maize (Zea mays L.) under a range of N levels. Original and advanced selections of four populations, improved for tolerance to midseason drought for two to eight recurrent selection cycles each, were evaluated in two experiments under severe N stress, one experiment under medium N stress, and two well-fertilized experiments. Nitrogen accumulated in the aboveground biomass at maturity averaged 52, 63,105,151, and 163 kg N ha"1 in the five experiments, and grain yields of 3.0,2.9,5.2,6.0, and 6.5 Mg ha ' were obtained. Selection for tolerance to midseason drought stress increased grain yields by an average of 86 kg ha ' yr"1 with nonsignificantly larger gains under severe N stress (100 kg ha"1 yr"1). Drought-tolerant selections had increased biomass and N accumulation at maturity, the changes being largest under severe N stress. Additionally, drought-tolerant selection cycles were associated with delayed leaf senescence and an increased or unchanged N harvest index, indicating that leaf N was used more efficiently for grain production. Selection for tolerance to midseason drought stress appears to increase grain yield across a range of N stress levels and may lead to morphological and physiological changes that are of particular advantage under N stress.

M

AIZE YIELDS in farmers' fields in many tropical countries average from 1 to 2 Mg ha"1 (CIMMYT, 1994), in stark contrast to yields ranging from 4 to 12 Mg ha"1 reported on breeding stations in those same countries (e.g., CIMMYT, 1995, 1996). This indicates that farmers in the tropics are growing maize under conditions that differ from those used by many researchers during crop improvement. Abiotic stresses in farmM. Banziger and G.O. Edmeades, Intl. Maize and Wheat Improvement Center (CIMMYT), Apdo. Postal 6-641, 06600, Mexico D.F., Mexico; H.R. Lafitte, Intl. Rice Res. Inst. (IRRI), P.O. Box 933,1099 Manila, Philippines. Contribution from CIMMYT, Apdo. Postal 6-641, 06600, Mexico D.F., Mexico. Received 3 Mar. 1998. *Corresponding author ([email protected]). Published in Crop Sci. 39:1035-1040 (1999).

er's fields, principally from drought and low soil fertility, and often exacerbated by competition from intercrops and weeds, are major reasons for this yield gap (Banziger et al., 1997; Edmeades et al., 1989; Simmonds, 1991). Breeding under high-yielding, nonstressed conditions may not be the best approach to increasing yields where severe abiotic stresses are encountered in the target environment (Rosielle and Hamblin, 1981; Simmonds, 1991). Even though heritability for grain yield usually decreases as stress intensifies, breeding progress may be increased if abiotic stresses in the target environment are included during selection (Atlin and Frey, 1990; Banziger et al., 1997; Ceccarelli et al., 1992; Ud-Din et al., 1992; Zavala-Garcia et al., 1992). Selection studies have shown that the tolerance of tropical maize to drought and N stress can be improved more rapidly when selection environments comprise managed levels of those stresses than when the same germplasm is selected only under high-yielding, nonstressed conditions, or under randomly occurring levels and types of stresses (Bolanos and Edmeades, 1993; Byrne et al., 1995; Edmeades et al., 1997; Lafitte and Edmeades, 1994). These studies were conducted for several cycles of selection and with diverse germplasm, but each of these studies were aimed at improving the tolerance of maize to only one particular stress type. Farmers' fields are rarely characterized by only one abiotic stress, and it would be desirable to increase the tolerance of crops to several stresses that occur in a target environment. In areas where the probability of drought stress is high, farmers often respond by reducing the application of N fertilizer (McCown et al., 1992). Resource-constrained farmers in many parts of the tropics may apply no fertilizer at all. In seasons when rainfall is plentiful, maize crops are often severely N deficient; therefore, tolerance to drought and low N would be desirable. Our study examines in detail an observation reported by Lafitte and Edmeades (1995), indicating considerable progress in grain yield under N stress in a population

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improved for tolerance to midseason drought stress. That study was limited to a single drought-tolerant population and the considerably improved performance under N stress could have been a random event, however. The objective of our study was to examine whether selection under midseason drought stress in several unrelated maize populations has indeed resulted in a concomitant increase in grain yields across a range of N levels and in the absence of plant water deficits. MATERIALS

AND METHODS

Germplasm Four CIMMYT lowland tropical maize populations were used for the study. Population21 (Tuxpefio-1)and Population 43 (La Posta) are late maturing with white dent grain, and were formed in the 1960s. Population 21 is a broad-based compositeof collections of the Tuxpefiorace, whereasPopulation 43 is based on a synthetic of 16 elite Tuxpefio inbred lines. Pool 26 (late maturing) and Pool 18 (early maturing) both possess yellow dent grain, and are populations formed in the early 1970sfrom collections madein Mexico,Central and South America, Asia and the Caribbean. While being selected for drought tolerance, these four populations were renamedas Tuxpefio Sequia (derived from Population 21), La Posta Sequia (derived from Population 43), Pool 26 Sequia (derived from Pool 26), and Pool 18 Sequfa (derived from Pool 18; "sequla" is the Spanish wordfor drought). Tuxpefio Sequla was improved for drought tolerance at CIMMYT’s station at Tlaltizap~in, M6xico(18°N, 940-melevation) in the dry winterseason, using a full-sib recurrent selection schemewith a selection intensity of 30%.Oneselection cycle took two crop cycles, or 1 yr, to complete. Details of the selection methodologyare provided by Bolafios and Edmeades(1993). La Posta Sequia, Pool 26 Sequla, and Pool 18 Sequia were improvedfor drought tolerance using an $1 recurrent selection schemewith a selection intensity that varied between3 and 10%.In each cycle, 600 to 1500$1 families per population were evaluated in unreplicated trials under summerheat stress in both well-wateredand drought-stressed conditions at CiudadObreg6n,M6xico(27°N, 39-melevation). The best 13 to 30%(i.e., 200-250families) were reevaluated in replicated trials at Tlaltizap~nduringthe winter dry season under three water regimes: flowering drought stress, grainfilling droughtstress, and well-wateredconditions. The best 50 $1 families were then recombinedin all possible combinations, and a newset of S~ progenieswere formedfor the next cycle of selection. One$1 selection cycle took four cropcycles, or 2 yr, to complete. Thus, in all four populations, drought was applied during floweringand grain filling (defined here as midseasondrought stress) at one or twolevels of intensity such that grain yields of progenies were reduced to 15 to 60%of those of the wellwatered control plots. Selection wasfor an index that sought to maintaintime fromsowingto anthesis; maintainor increase grain yield underwell-wateredconditions; increase grain yield under drought; and decrease the anthesis to silking interval, the rate of leaf senescence, and leaf rolling under drought. TuxpefioSequiawasadditionally selected for an increasedrate of stemand leaf extension and for lower canopytemperatures under drought stress (Bolafios and Edmeades,1993). EvaluationSite andCulturalPractices Original and advancedselection cycles of each population, namely Tuxpefio Sequfa Co and CO, La Posta Sequia Co and C3, Pool 26 Sequ~aCo and C3, and Pool 18 Sequia Co and C~,

were grown in five experiments at CIMMYT’s experiment station at Poza Rica, Veracruz,M6xico(21°N, 60-melevation) during the summerseason of 1992 (under low and high N), the 1992-1993winter season (under low and high N), and the 1993-1994winter season (under low N). Except for summer 1992, low N experiments were conductedin fields that had been depleted of N for five to nine previous crop seasons by not applying N fertilizer and by cutting and removingthe biomass after each crop season. The 1992 low N experiment was sownafter a crop of velvet beans (Mucunaspp.). No chemical N fertilizer was applied to the low N experiments. In the high N experiments, 125 kg N ha-~ were incorporated as ammonium sulfate prior to sowing, and 75 kg N ha-~ as urea were side-dressed =30 d after sowing. Wherelow and high N experiments were grownin the sameseason (1992 and 1992-1993),experimentswere sownin adjacent blocks of the same field on the samedate. All experimentsreceived 18 kg P ha-~ prior to sowingand were kept free of insects, weeds, and foliar diseases. Long-terma__veragemaximum and minimumdaily temperatures at this site are 32.-2-~ and 21.5°C during the summerseason (June-October) and 26.3°C and 15.9°Cduring the winter season (November-April).Rainfall during the crop cycle was supplementedas neededwith furrow irrigation to meet the evaporative demandof the crop. The soil is a Tropofiuvent,with a sandyloamtexture, a pHof 7.4, and an organic C content of 11 g C kg-~. Seeds were oversown in rows 5 min length and 0.75 mapart, and later thinned to -2. a density of 5.3 plants m Measurements All measurementswere taken on well-bordered plants in the central tworowsof four rowplots. Using30 plants per plot, days from sowing until 50%of plants had extruded anthers (anthesis) and silks (silking) weredetermined.Leaf area index at silking was determined by measuring maximum leaf width and length from12 plants per plot. Areaper leaf wascalculated as (maximumwidth × length × 0.75) (Montgomery,1911). The fraction of incident radiation intercepted by the crop was measuredin the first 2 wkafter silking using a bar sensor 0.9 m long (Ceptometer, Decagon Devices, Pullman, WA) placed across the interrowspace at groundlevel in 10 bordered locations per plot. Thefraction intercepted was: 1 - (the mean of these 10 values/incident radiation measuredoutside the plot). Chlorophyllconcentration of the ear leaf was determinedphotometricallyon 10 plants per plot 2 wkafter silking, using a portable photometer(Hardacreet al., 1984) that had previously beencalibrated against extracted chlorophyll contents. Total leaf numberwas determined by markingLeaves 5 and 10 whenthe plants wereyoung, and taking a final count immediatelyafter anthesis. The numberof green leaves below the ear wascountedand estimated to the nearest 0.1 of a leaf at 3 and 5 wkafter silking. Plant height (distance fromground to flag leaf) was measuredon 10 plants per plot. Leaf area and leaf chlorophyll concentration were determined in the summer1992 and winter 1992-1993 experiments only, and radiation interception was determinedonly in the winter 19921993 experiments. A well-bordered area of 3.38 m2 was harvested fromeach plot at maturity. Plants were cut at ground level, dividedinto stover and ears, and dried to constantweight at 80°C. Ears wereshelled and the cob was addedto the stover fraction. Dry weights and N concentration (micro-Kjeldahl) were determinedfor stover and grain. Nitrogen accumulation by plants and grain N content were calculated. Experimental DesignandStatistical Analyses The eight entries were grown with seven (1992) or four (1992-1993and 1993-1994)other check entries in an alpha

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(0,1) lattice design with four replicates (Patterson and Williams, 1976).Results fromcheckentries will not be considered in this study. Withineach experiment,lattice-adjusted entry means were calculated using the PROCMIXEDprocedure of SAS(SASInstitute, 1994), with entry as a fixed factor and replicate and incompleteblocks within replicates as random factors. Across-experimentanalysis was conductedfrom lattice-adjusted entry meansusing populationand selection cycle as fixed factors, and experimentas a randomfactor. Selection cycles were considered either "original" (Co) or "droughttolerant" (advancedcycle). Systematicinteractions of selection cycleswith the level of Nstress wereexamined by calculating linear regression of the meantrait value per experiment on meangrain yield of the sameexperiments. For each trait, this resulted in one regression per population by selection cycle, eight regressions in total. Regressionsof original and drought-tolerant selection cycles were then comparedby conductingan analysis of varianceof slope and intercept values, using populationand selection cycle as fixed factors. RESULTS Nitrogen Stress Intensity and Selection in Grain Yield

Gains

Nitrogen accumulation at maturity averaged 52, 63, 105, 151, and 163 kg N ha-1 for experiments with grain yields of 3.0 (1992-1993 low N), 2.9 (1993-1994 low 5.2 (1992 low N), 6.0 (1992-1993 high N), and 6.5 (1992 high N) Mgha-~. Thus, N stress reduced grain yields in the low N experiments by 20% in 1992 and by 50% in 1992-1993 compared with the high N experiments. The 1993-1994 experiment was conducted under low N only and yielded similarly to the 1992-1993 low N experiment. Selection for tolerance to midseason drought stress consistently increased grain yields by an average of 86 kg ha-~ yr -1 across populations and N levels (Table 1). Gains per cycle of $1 recurrent selection, with a higher selection intensity, were about twice the gain per cycle from full-sib recurrent selection. Population × selection cycle interaction and experiment× selection cycle interaction for grain yield were not significant. Regression analysis confirmed that selection gains were similar across N levels, as the slopes of linear regressions of grain yield on mean grain yields of the experiment did not differ between original and drought-tolerant selection cycles (Table 2). In fact, grain yields of all populations increased by 100 kg ha-~ yr-I under severe N stress -1) (in the two experiments with grain yields