the Symposium on Wheats for More Tropical Environments - cimmyt

216sites. but field work at CIANO (15) andelsewhere suggests <strong>the</strong> followingrelationship, as shown forrepresentative cultivars (Figure 3):Grain weight (mg) = 64-1.6 x T(Equation 2)Here T is <strong>the</strong> mean temperature duringgrain filling (30 to 40 days followingan<strong>the</strong>sis).Equations 1 and 2 can be multiplied togive <strong>the</strong> aforementioned simple index ofgrain yield:Yield (g/m2) = corrected Q x(700-17 x T) (Equation 3)This equation contains <strong>the</strong> two majornegative influences of increasedtemperature on grain yield. namely thatof temperature on duration of <strong>the</strong> spikegrowth period and on grain weight.Increased radiation, through Q. has apositive effect on yield.It is possible to use Equation 3 toestimate grain yield as a function ofan<strong>the</strong>sis date for <strong>the</strong> locations inFigure 1 (Table 3). Q was calculated for<strong>the</strong> month prior to an<strong>the</strong>sis. assumingfull light interception, and T for <strong>the</strong>month after. Q tends to peak inFebruary (August in <strong>the</strong> sou<strong>the</strong>rnhemisphere). but <strong>the</strong> steepness withwhich T rises before and. especially.after January also influences an<strong>the</strong>sisdate for maximum yield (Figure 5). Thisis seen to be January at all tropicallocations except Los Banos. Philippines.where temperature changes less40......t::'c./)....Il)~l::~ ....d3020DOPoza Rica...TlaltizapanDEI Batan"'Toluca0o... ...... ...... ....... ...... ... .............ooo16 18 20 22Mean filling temperature (0C)24 26Figure 3. The relationship of individual grain dr.y weight to mean grainfiningtemperature for <strong>the</strong> sowings of Figure 2, and <strong>the</strong> linear relationshipderived from disease-free sowings at CIANO, Ciudad Obregon, Mexico.Source: R.A. Fischer, unpublished