the Symposium on Wheats for More Tropical Environments - cimmyt

219developmental periods. to around goCfor grain filling (3.36). The relationshipImplies that duration declines lessrelatively and absolutely astemperature rises (e.g.. ifTb = 5. <strong>the</strong>acceleration from 15 to 20°C will be33%. that from 20 to 25°C. only 25%).Although data for wheat attemperatures above 20°C Is scarce. Itdoes show this tendency for duration tobe reduced less at temperatures above20 to 25°C and sometimes but notalways (35.39). to approach a minimumvalue (3.26); this fact Is not predictedby Equation 5 and is worthy of fur<strong>the</strong>rInvestigation.Although Tb and. hence. <strong>the</strong> sensitivityof duration to temperature. variesbetween species for given developmentalphases (2). <strong>the</strong>re Is no evidence ofgenotypic variation within wheat. In anycase. it Is not so much <strong>the</strong> accelerationof development with higher temperaturewhich Is critical. but ra<strong>the</strong>r <strong>the</strong> actualduration reached under hot conditions.Three mechanisms exist by whichlonger duration under hot conditionscan come about. The most striking oneIs vemaUzation sensitivity. wherebydevelopment in <strong>the</strong> first and. to a limitedextent. <strong>the</strong> second. but not laterdevelopmental periods (Table 2). Isslowed when <strong>the</strong> sensitive genotypeexperiences no vernalizing cold (26.40);<strong>the</strong> universal effect of highertemperature apparently Is overridden(Figure 4). Vernalization in this contextappears to operate with minimumtemperatures below about 13°C.increasing in intensity down to aminimum of 2 to 8°C (23). In so-calledspring wheats. a small number of genesprobably control vemaUzationresponsiveness; however. <strong>the</strong>re appearsto be a continuous gradient inresponsiveness. Pitic 62. Cajeme 71.Gabo. Odzi. Soltane. Oxley and C306 arecultivars with moderate responsiveness.The second mechanism leading to longduration in all periods before an<strong>the</strong>sis.even under hot tropical conditions. Isseen WIth genotypes such as Manitouand Era. which are highly sensitive tophotoperiod (Figure 4). With moderatelysensitive ones (e.g.• Kloka and Anza).<strong>the</strong>re is a smaller but still signtflcanteffect. Sensitivity to photoperiod Isunder simple genetic control.FInally. It can be suggested thatsignificant variation In pre-an<strong>the</strong>slsduration exists under hot conditions.even in cultivars Insensitive tovernalization and minimally sensitive tophotoperiod (this might be conSidered asgenotypic variation in <strong>the</strong> day degreeconstant of Equation 6). A comparison ofSiete Cerros and Hira. both insensitivecultivars. shows that Siete Cerros has aconsistently longer sowmg-to-an<strong>the</strong>slsduration (61 days versus 51 days. +20%) at sowmgs at hot sites in centralMexico (25). O<strong>the</strong>r workers have alsopointed to <strong>the</strong> relative lateness ofSieteCerros or Kalyansona. which appears tobe associated with <strong>the</strong>ir tendency toproduce an extra leaf on <strong>the</strong> main stem(25. H.M. Rawson. personalcommunication).WhUe difTerences in vernalization andphotoperiod sensitivity do not operatein <strong>the</strong> grain-filling period (Figure 4).small genotypic differences (20%) Induration under hot conditions do occurand should be likened again todifferences in <strong>the</strong> constant value inEquation 6. Also. larger differences(35%) reported for o<strong>the</strong>r cultivars undercooler conditions should persist in arelative sense under hot ones (35).Accelerated leaf senescence Is ano<strong>the</strong>raspect of <strong>the</strong> high temperature-rapiddevelopment syndrome. This Is not wellunderstood. and may not be a totallynegative effect; for example. leavesmust senesce if all of <strong>the</strong>ir nitrogen is tobe mobUtzed into growing grains. Evenso. ifa phenomenon such as <strong>the</strong>genetically controlled delayed leafsenescence in soybeans (29) were to befound in wheat. It might be especiallyuseful under hot gram-mUngconditions.