the Symposium on Wheats for More Tropical Environments - cimmyt

981984, and a high percentage wereconfirmed as being heat tolerant. Since<strong>the</strong> program is in <strong>the</strong> initial stage ofheat-tolerance screening, no crosseshave yet been made for this purpose.However, <strong>the</strong>re are plans to initiatebreeding for heat tolerance within <strong>the</strong>next two years.Drought resistanceDrought resistance is defined here as"<strong>the</strong> ability of one genotype to be moreproductive with a given amount of soilmoisture than o<strong>the</strong>r genotypes." From<strong>the</strong> beginning, CIMMYT's objective inthis area has been to producegermplasm which combinesresponsiveness under both droughtconditions and high-yieldingenvironments. To achieve this goal, abreeding program has beenimplemented in which spring x wintermaterials are subjected alternately toreduced moisture levels and optimumirrigated conditions. The shuttle takesplace between Ciudad Obregon in <strong>the</strong>winter season, and <strong>the</strong> highlandlocations of EI Batan and Huamantla in<strong>the</strong> summer.The performance of 12 advanced linesin Ciudad Obregon (two irrigations) andEI Batan (rainfed conditions) are givenin Table 4. It appears from this tablethat <strong>the</strong>se lines will perform well underboth environmental conditions. Thesematerials are in <strong>the</strong> new DroughtScreening Nursery for internationalyield testing, which will provideadditional data on <strong>the</strong>ir stability.ConclusionsThe CIMMYT Tropical WheatImprovement Program has completedtwo years of research to identifysuitable lines for tropical conditions.The crossing program has continuouslyutilized adapted germplasm as parentallines in crosses, resulting in a largenumber of segregating populations.Materials selected from <strong>the</strong> breedingprogram, as well as germplasm fromcountries around <strong>the</strong> world, have beendistributed to target areas throughCIMMYT's international nurserynetwork. The materials are classified fortraits and entered into <strong>the</strong> follOWingnurseries:• Helminthosporium ScreeningNursery• Drought Screening Nursery• Heat Tolerance Screening Nursery• Early Screening Nursery• Scab Resistant LinesThese nurseries serve as a means toverify resistance and as mechanisms forgermplasm introduction into nationalprograms.In cooperation with CIMMYTpathologists, techniques for massscreening for such major tropicaldiseases as helminthosporium havebeen developed. These techniques arenow being fur<strong>the</strong>r refined.In <strong>the</strong> years to come, CIMMYT willendeavor to recombine genes for highyield, early maturity under hightemperature conditions and resistanceto leaf rust and helminthosporium. Inspite of <strong>the</strong> fact that only about 4 % ofits total wheat budget is directly relatedto tropical wheats, CIMMYT willcontinue to integrate and test all of itsadvanced lines for <strong>the</strong> improvement ofgermplasm for <strong>the</strong> tropical wheat areas.