the Symposium on Wheats for More Tropical Environments - cimmyt

Screening for tropical adaptationThe world wheat germplasm collectionis also being screened for materials withdesirable characteristics which will becontinuously introgressed into <strong>the</strong>breeding program for <strong>the</strong> tropics.In carrying out all of <strong>the</strong> abovementionedobjectives, CIMMYT utilizesa multidisciplinary, coordinatedapproach, involVing <strong>the</strong> participation ofscientists trained in various disciplines.Program ResultsEarly maturityCIMMYT's breeding objective for <strong>the</strong>last few years has been to select anddevelop wheats with earlier maturity. Intrying to obtain such lines. <strong>the</strong> earliestmaturingsegregates are selected fromF2 segregating populations. Forcomparison, <strong>the</strong> variety Sonalika fromIndia is used as <strong>the</strong> early check variety.When a low percentage of plants in asegregating population reachphysiological maturity, <strong>the</strong>y areselected and harvested. Then, as F3s.<strong>the</strong>y are handled in a separate nursery,toge<strong>the</strong>r with o<strong>the</strong>r segregating andadvanced early materials. This systemof handling <strong>the</strong> materials is more forpractical reasons. since damage frombirds and o<strong>the</strong>r pests is particularlysevere on early plants left in <strong>the</strong> field.Using this selection method. advancedlines that are as early as Sonalika havebeen obtained. Two hundred sixty earlylines were tested for yield in CiudadObregon in <strong>the</strong> 1983-84 cycle. Table 2presents 14 lines that were as early asSonalika or earlier. but which havebetter yield potential. These materialsare now in <strong>the</strong> First International EarlyScreening Nursery. toge<strong>the</strong>r with o<strong>the</strong>rearly germplasm in <strong>the</strong> program.HelmiDthosporium resistanceEfforts to improve resistance toHelmtnthosportum spp. began in <strong>the</strong>early 19808. Helminthosporiumscreening work is done at Poza Rica. in<strong>the</strong> tropical area of Mexico. and also at<strong>the</strong> Tlaltizapan station. It is hoped thatgermplasm with both helminthosporiumresistance and heat tolerancewill result from selections at <strong>the</strong>selocations. Helminthosporium resistanceis evaluated on <strong>the</strong> basis of <strong>the</strong>follOWing characteristics:• Slow development of <strong>the</strong> disease;• Acceptable agronomiccharacteristics;• "Good finish," <strong>the</strong> ability to haveclean, fertile and bright heads atmaturity. and• Good seed quality.Table 3 presents <strong>the</strong> best varieties andadvanced lines which were foundresistant to helminthosporium in PozaRica in 1983-84. These lines are now in<strong>the</strong> new Crossing Block and ThirdHelminthosporium Screening Nursery.All of <strong>the</strong>se lines showed betterresistance than BH1146. a resistantcheck variety from Brazil.Heat toleranceIn <strong>the</strong> breeding program. testing hasbeen initiated to identify varieties andadvanced lines haVing tolerance to hightemperature. Heat tolerance isevaluated in Ciudad Obregon byplanting advanced lines in mid­January. so that early growth stagesoccur during a period of rapidly risingtemperatures. The final performance of<strong>the</strong> lines is evaluated in late April. ThefollOWing characteristics are used asselection criteria for heat tolerance:• Longer leaf duration. <strong>the</strong> "staygreen"characteristic• High tillering capacity• Acceptable spike fertility• Relatively high grain weight• High hectoliter test weight• Relatively high yieldMore than 200 advanced lines wereidentified as haVing some heattolerance in 1983. These same lineswere reevaluated in Ciudad Obregon in