the Symposium on Wheats for More Tropical Environments - cimmyt

75Recently, <strong>the</strong> pedigrees of 125improved varieties from <strong>the</strong> mid-lowerYangtze Valley were analyzed. Twentyeightof <strong>the</strong>m were derived fromindigenous varieties, and 92 fromItalian varieties; about three-fourths of<strong>the</strong> improved varieties were derivedfrom Italian varieties. There were also afew varieties derived from varietiesfrom o<strong>the</strong>r countries; eight were from<strong>the</strong> USA, seven from Chile, three from<strong>the</strong> USSR, three from Australia and twofrom Mexico.Major WheatProduction ConstraintsWheat production in <strong>the</strong> sou<strong>the</strong>rnwinter wheat region has developedrapidly in <strong>the</strong> last ten years, but <strong>the</strong>factor of yield instability still exists. Forinstance, during <strong>the</strong> past 34 years,production increased 18 years in <strong>the</strong>sou<strong>the</strong>rn part of Jiangsu province anddecreased 16 years. This was caused byvarious production constraints in <strong>the</strong>region.First, <strong>the</strong>re is excessive and poorlydistributed rainfall dUring <strong>the</strong> wheatgrowingseason, particularly in <strong>the</strong>spring. It generally considerablyexceeds wheat's physiologicalrequirements. and seriously influencesgrowth and development. For instance,in <strong>the</strong> winter wheat region of <strong>the</strong> midlowerYangtze Valley, averageprecipitation from <strong>the</strong> seedling stage tomaturity us.ually amounts to 430 to 755mm, of which 224 to 626 mm is in <strong>the</strong>spring (March to May), making up 48.5to 82.9% of <strong>the</strong> total rainfall during <strong>the</strong>wheat-growing period. According tometeorological data for <strong>the</strong> sou<strong>the</strong>rnpart of Jiangsu province from 1951 to1980, average rainfall dUring <strong>the</strong> periodof elongation to maturity was 270 to320 mm; <strong>the</strong>re was one rainy day inevery 2 to 2V2 days. In <strong>the</strong> years ofover-abundant rain, <strong>the</strong>re may be up to400 mm during this period. Excessivemoisture is <strong>the</strong> most important reasonfor <strong>the</strong> instability of wheat production,and it also enhances scab infection.Statistics from <strong>the</strong> Suzhou PrefecturalAgricultural Research Institute for <strong>the</strong>last 28 years shows that serious scabepidemics occurred in five of thoseyears; <strong>the</strong> rate of infection reached 50to 100% and caused yield losses of 20to 40%. There were ten years withmedium epidemics, with a rate of scabinfection of 20 to 40% and yield lossesof 10 to 20%; light or zero scab damageoccurred in 13 years. The averagefrequency of scab epidemics was53.3%, with one year of serious ormoderate scab epidemics out of everytwo years. For this reason. varietieswith high tolerance to excessive soilmoisture and good resistance to wheatscab are urgently reqUired for <strong>the</strong>region.The second limiting factor in <strong>the</strong> regionis high temperature dUring <strong>the</strong> lategrowing period of wheat. The averagenumber of days when air temperaturegoes above 30°C is usually 2.2 to 9.1days dUring <strong>the</strong> ripening period.Sometimes, before harvest, <strong>the</strong>re maybe high temperatures with lowhumidity, high evaporation and hot,dry southwest winds. This combinationof factors dries <strong>the</strong> grain so qUickly thatpremature senescence or even greendeath dUring <strong>the</strong> milk stage can occur;this, of course is a major reason for lowgrain weight. Thus. it is necessary todevelop a wheat variety with quickgrain filling, early maturity and a longperiod of seed dormancy in order toresist germination in <strong>the</strong> head causedby <strong>the</strong> excessive precipitation and hightemperatures in <strong>the</strong> late growing stage.High and stable yields of wheat are alsolimited by poor drainage facilities, lowsoil fertility, rough tillage, weeds anddamage by diseases and insect pests.There is also a large amount of redyellowsoils in Jiangxi, Hunan, Yunnanand Guizhou provinces. This type ofsoil is not suited to <strong>the</strong> growth ofwheat, and yield per unit area in <strong>the</strong>seprovinces is very low.