| Maize(Zea mays L.)is the first food in China. It is also used as fodder, industry material andrenewable energy and is very important in national economy. Corn production was affected severelybecause drought frequently occurred and pests and diseases aggravated increasingly. Therefore, newbreeding objective and scheme were needed to establish to improve diseases resistance and adaption tosevere environment. Experience of the nations with developed seed industries indicated thatretrospective study could afford useful advice and instruction for improving yield ability. Identificationexperiments on drought tolerance and disease resistance were carried out for spreading cultivars from1950to2000. The aim was to discover:(1)trend of drought tolerance of Chinese spreading cultivars,(2) trend of yield, plant trait and heterosis of spanning cultivars in full irrigation and drought stressenvironments in China,(3) trend of disease resistance of spreding cultivars. The research can explorenew breeding strategies to further improve yield potential in the future and also offord theory basis forhigh yield breeding.The material of the research was mainly spanning cultivars in maize central production region from1950to2000, including the4OPVs in1950s,4double-cross hybrids in1960s and27single-crosshybrids after1970s. Evaluation of drought tolerance in seedling stage was carried out in dry house in2010. Identification of drought tolerance in the flowering stage was developed in Xinjiang and Hainanin2010and2011. Evaluation on diseases resistance was lunched in Beijing, Jining and Jilin in2010.From these experiments, the results were acquired as follows:1. Drought tolerance in seedling stage slowed down along with the decades. It increased rapidlyfrom1950to1960and decreased after1960. It also declined with released years for single-crosshybrids after1970. Twenty seven single-cross hybrids were clustered into three categories, hybrids withbetter tolerance to drought including Ludan50, Qundan105, Nongda108, Yedan13, Yedan4, Zhengdan2;worse tolerant hybrids comprising Nongda138, Nongda60, Shendan16; the rest belonging to middletype. One of emphasis in maize breeding is to strengthen water stress in seedling stage, whichestablishes the basis for breeding new hybrids tolerant to drought.2. The change between grain yield of cultivars and decades was linear increase in full irrigation anddrought stress treatments. The yield gain was85.8Kg.ha-1.year-1(R2=0.96)in full irrigation and53.88Kg.ha-1.year-1(R2=0.94)in drought stress at Hainan location。At Xinjiang location, it was94.3Kg.ha-1.year-1(R2=0.97)in full irrigation and61Kg.ha-1.year-1(R2=0.99)in drought stress.The yield gain was higher in full irrigation treatment than in drought stress treatment. Thecoefficient of drought tolerance (Cd) of cultivars decreased with decades, that is to say, the droughttolerance declined with decades. Although the breeders always attached importance to the droughttolerance of cultivars, it declined in severe stress environments. Therefore, drought stress should bestrengthened in breeding progress including inbred lines and hybrids in the future for further improvingthe ability of adaption to stress environments.In drought treatments, improved agronomic traits included that ASI, leaf angle, tassel branches number, green leaves number, empty ear rate, ear length, ear diameter, kernel number per ear, weightper hundred kernels, barren tip length. The traits with no improvements included that photosynthesis,leaf rolling and shell percentage. The traits with decreased drought tolerance were comprised of leafangle and empty ear rate.The next tusk emphasized on diminishing tassel, empty ear rate and barren tip length, andimproving photosynthesis and shelling percentage in drought tolerance breeding in maize.3. The grain yield of inbred lines increased linearly with decades. The yield increased from1960to1980, decreased in1990, and then increased in2000. The grain yield of inbred lines in droughttreatment reduced comparative with full irrigation treatment. The yield gap of new inbred lines waslarger than that of old one. The Cd of inbred lines decreased with decades, but declined in2000.Therefore, the drought stress tolerance of inbred lines increased step by step, but backward in2000. Weshould introduce new germplasm and strengthen drought pressure in breeding lines in the future.In drought treatments, improved agronomic traits included ASI, plant and ear height, leaf angle,tassel branches number, empty ear rate, green leaves number and all the ear traits. The traits whosedrought tolerance declined were comprised of ear height and weight per hundred kernels.Therefore, most agronomic traits of parental lines and their drought tolerance were improved. Andwe should further improve weight per hundred kernels for breeding new inbred lines tolerance todrought.4. The absolute heterosis in drought stress was less than that of full irrigation. It indicated that theyield reduction range of hybrid was higher than parental lines. Absolute heterosis increased withdecades from1970to2000in drought stress and in full irrigation treatments. The relative heterosisdeclined from1970to2000slowly, so it kept steady generally. No obvious difference exists betweenrelative heterosis in drought stress and full irrigation treatments. Thus we can conclude that theimprovement of hybrids was impertinent to relative heterosis in drought stress.5. The resistance to maize dwarf mosaic virus (MDMV) and common smut disease (CSD)increased with decades. The resistance of hybrids was better than OPVs to the two diseases. Theresistance to head smut disease (HSD) increased from1950to1960, decreased in1970, and increasedagain from1980to2000. So the HSD resistance decreases in1970because of single hybrid occurrence.But it was improved from1980as the result of purposive breeding strategies. All of the entries werehighly susceptible to maize rough dwarf virus (MRDV). Therefore, breeding new varieties resistant toMRDV is a key problem to solve.
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