CIMMYTS APPROACH TO BREED FOR DROUGHT TOLERANCE

About 32% of the 99 million ha wheat grown in developing countries exp eriences varying levels of drought stress. Three major drought types h ave been identified: Late drought (LD) is common in the Mediterranean region, early drought (ED) is found in Latin America and wheat is prod uced on residual soil moisture (RM) in the Indian subcontinent and par t of Australia. Until 1983, CIMMYT selected all germplasm under near o ptimum conditions for its yield potential and tested only advanced lin es under drought. In spite of many critics, this approach proved to be successful, since in the mid 80's CIMMYT germplasm was grown on 45% o f the wheat area in LC with annual rainfall from 300-500 mm and on 21% in areas with less than 300 mm. Since 1983, CIMMYT's drought breeding methodology is to alternate segregating populations between drought s tressed and fully irrigated conditions (FI) and to test advanced lines under a line source irrigation system. To compare the efficiency of t hese approach, yield of four, mostly leading varieties, from each of t he regions with LD, ED, RM, and FI and twelve recent CIMMYT cultivars selected for high yield under Fl and RM conditions (ALT) were compared under four different moisture regimes (FI, LD, ED, and RM) in 89-90 a nd 90-91 in Yaqui Valley, Mexico. Genotypic correlation between yield and days to flowering, days to maturity, height, grains m(-2), TKW, te st weight and grain fill period were calculated. Mean grain yield of t he four best lines in the ALT group was highest under all moisture str ess regimes, followed by the FI-group. However, the highest yielding c ultivar within each moisture regime was from the FI-group under FI, fr om the LD-group under LD, and from the ALT-group under ED and RM condi tions. Estimates for genetic advance suggest that FI is the best envir onment for increasing grain yield even in all three drought environmen ts. This indicates that yield potential per se is beneficial also in d rought environments. The highest yield in drought environments was rea lized by the CIM cultivars selected under FI and RM. Simultaneous eval uation of the germplasm under near optimum conditions, to utilize high heritabilities and identify lines with high yield potential, and unde r stress conditions to preserve alleles for drought tolerance seem at present the best strategy.