the Symposium on Wheats for More Tropical Environments - cimmyt

224Unfortunately. <strong>the</strong> significance of <strong>the</strong>secontrolled-environment findings to fieldconditions. where little attention hasbeen paid to grain set per se. is notclear. Soil water deficit-induced malesterility can be very striking in <strong>the</strong>greenhouse. but is less evident.although important. in <strong>the</strong> field; <strong>the</strong>same may apply to high temperatureand humidity-induced sterility. In anycase. <strong>the</strong> subject needs attention and<strong>the</strong> approach outlined here ofmeasuring spike (or chaff) weight andgrains per unit weight (or grain set) isrecommended. Should grain setproblems be evident in tropicalconditions. genotypic variation in thisresponse is likely. and careful visualselection against <strong>the</strong> problem isfeasible.Reduced lDdJridual grain weightOver <strong>the</strong> range of 12 to 26°C. increasedtemperature during grain filling reducesgrain weight from 4 to 8% per degree(10.12.39,41.42.43.44); underlyingcausal mechanisms were alluded to inTable 2. Potential grain weight(unlimited assimilate) appears to bereduced by higher temperatures (arange of 15 to 21°C). according to fieldstudies at CIANO (15). Potential kernelweight may be determined by <strong>the</strong>number of endosperm cells formed in<strong>the</strong> early cell-division stage (17.39.41);however. increasing temperature to24°C did not affect final cell number inone study (41). Actual grain weight isusually below potential. because ei<strong>the</strong>rfilling duration and/or filling rate areinadequate. The depressing effect ofhigher temperature on duration hasbeen discussed. It should be noted that<strong>the</strong> termination of grain filling andonset of spike senescence may occurindependently of leaf senescence. asdemonstrated by spike warming (7)and. <strong>the</strong>refore. is under differentcontrolling mechanisms. The rate ofgrain filling becomes greater withhigher temperature. but is unable tofully compensate for <strong>the</strong> reduction induration of filling and. in any case. doesnot increase much above 20°C (35.39).The supply of assimilate for grain fillingcomprises pre-an<strong>the</strong>sis reserves pluscurrent net assimilation (17). Underhotter conditions. increased respirationmay contribute to reduced supply. butit cannot fully explain grain weightreductions (10.35). Increased leafsenescence is probably more importantand can precede grain maturity.whereas. under cooler conditions. itoften follows it. It has been suggested(39) that hastened leaf senescencearises because <strong>the</strong> protein (nitrogen)demand of grains increases morerapidly with temperature than <strong>the</strong>ircarbohydrate demands; this is satisfiedby a correspondingly faster withdrawalof nitrogen from <strong>the</strong> leaves. Theincrease in grain-nitrogen percentagewith higher temperatures is wellestablished (10.36). Finally.photosyn<strong>the</strong>tic rate is unlikely to beaffected by higher temperatures. asalready discussed. but would bereduced by low radiation during grainfilling in parts of <strong>the</strong> humid tropics.It would seem that higher temperatureshifts <strong>the</strong> balance towards grain weightlimitation through reduced assimilatesupply (15.39); thus. leaf-areamaintenance (e.g.. avoiding prematureleaf firing) is probably more critical.This makes more difficult <strong>the</strong> study orscreening of genotypes for resistance toreduced grain weight under controlledenvironmentconditions. and <strong>the</strong> fewstudies available have not foundconvincing genotypic differences in thistemperature response. or that of <strong>the</strong>components involved. It is conceivable(39) that genotypes with inherently lowgrain-nitrogen percentages would showlower <strong>the</strong>rmal sensitivity. It is alsopossible that waxy genotypes would bebetter. since <strong>the</strong>ir leaf and. especially.spike temperatures are lower. (R.A.Richards. personal communication). Itis worth noting that grain weight in