the Symposium on Wheats for More Tropical Environments - cimmyt

119• Resistance to shattering• Variability as to cycle, preferablyearly maturity• Tolerance to frost at <strong>the</strong> reproductionstage• Efficiency in phosphorus use• Resistance to greenbug (Schizaphisgramtnum R.)• Good industrial qualityThe information provided in this paperrefers only to breeding in relation to soilproblems.Breeding WheatsApproprIate for <strong>the</strong> ProblemSoils of Rio Grande do SuiTolerance to crestamentoTolerance to crestamento was firstidentified in 1914, among wheatsintroduced by immigrants to Brazil (7).In that year. <strong>the</strong> cultivar Pollisu wasselected for its exceptional growth inacid soils. This cultivar is part of <strong>the</strong>genetic background of nearly allvarieties recommended for Rio Grandedo SuI.In 1948. Araujo (2) determined thatcrestamento was caused by <strong>the</strong>occurrence of exchangeable aluminumand iron in <strong>the</strong> soil, as part of its totalabsorption capacity; this constituteswhat is called noxious acidity. In 1949,crestamento symptoms werereproduced by Oliveira after steriliZing<strong>the</strong> soil by heat, thus eliminating <strong>the</strong>presence of biological agents as possiblecauses of <strong>the</strong> disease.responsible for tolerance, a dominantcharacteristic. Nodari (6), in geneticstudies contrasting <strong>the</strong> tolerance ofseveral cultivars to crestamento underfield conditions, concluded thattolerance is controlled by two dominantgenes.Tolerance to crestamento involvesseveral factors, including tolerance totoxic aluminum and to manganese.When interactions among <strong>the</strong>se factorsoccur, <strong>the</strong> interpretation of resultsobtained in <strong>the</strong> field is difficult. The useof nutrient solutions has permitted <strong>the</strong>separation of cultivar reactions to eachof <strong>the</strong>se toxic factors. Using thismethodology, Lagos et al. (5)determined that <strong>the</strong> tolerance gene toaluminum in <strong>the</strong> cUltivar BH 1146 islocated'in chromosome 40.All wheat cultivars recommended inRio Grande do SuI are tolerant tocrestamento. Among <strong>the</strong>m, BR6, CNT1and IAC5 (Maringa) show <strong>the</strong> bestresistance (8). The wheat-breedingprogram maintains such tolerancethrough plant selection in segregatinggenerations (normally F2 or F3) in soilareas where lime has never beenapplied. In this way, tolerant plants areeasily identified and, in subsequentgenerations, <strong>the</strong>y are screened in soilshaVing better fertility. At <strong>the</strong> end of <strong>the</strong>selection process, tolerance tocrestamento is confirmed by againgrOWing <strong>the</strong> lines in soils withcrestamento.The first information availableconcerning <strong>the</strong> genetics of <strong>the</strong>transmission of tolerance tocrestamento is attributed to Beckman(1954), who indicated a gene as