the Symposium on Wheats for More Tropical Environments - cimmyt

133must also demonstrate a yieldadvantage. Fur<strong>the</strong>r. farmer preferencefor Sonalika is so pervasive that it isnearly considered synonYmous withwheat.Against this backdrop of variable datesof planting. water availability andvarietal preference. white-seededvarieties must be developed which canbe planted late and/or on drylands.while also responding favorably to earlyplanting and/or irrigated conditions. Inall probability. <strong>the</strong> perfect. miraclevariety will not be found that <strong>the</strong>separameters suggest is needed;obViously. some compromises will benecessary.The development of a number ofdifferent varieties. each with itscharacteristics favorable to a givensituation. is always suggested.However. 75% of <strong>the</strong> annual wheatseed requirement is stored by farmersand. suprisingly. a good percentage ofsuch seed survives <strong>the</strong> humid monsoonseason with a high level of germination.They cannot store seed safely for morethan six months. nor are <strong>the</strong>y willing tostore a number of varieties. selected for<strong>the</strong> different kinds of situations <strong>the</strong>ymay have on <strong>the</strong>ir own land (and whichmay change annually). Fur<strong>the</strong>r. <strong>the</strong>national seed production program.although producing good quality seed.is not sufficiently sophisticated toefficiently grow and distribute anumber of different varieties insufficient quantities that demandrequires. The present program. instead.is directed toward <strong>the</strong> selection of <strong>the</strong>maximum amount of flexibility ingermplasm. in terms of its droughttolerance/favorable response toincreased water availability. andtolerance to late planting/favorableresponse to earlier planting.Material is available with maturitiesand drought resistance comparable toSonalika. but with no particular yieldadvantage. Yields. of course. generallydecline with decreasing time-tomaturity.The probability of selecting avariety with similar duration anddesirable characteristics of Sonalika.and with a consistent yield advantage.is quite low. Ra<strong>the</strong>r than to exclusivelyselect for varieties that are similar toSonalika. selection is also made forvarieties which. under both irrigatedand water-stress conditions. are ofsomewhat longer duration thanSonalika (but no more than a weeklonger). They must also be tolerant toheat-forced maturation while stillmaintaining a reasonable yield andseed quality and. if planted at a morefavorable date. yield consistently better.All pre-trial material. irrigated ordryland. is selected at a favorable dateof planting (November 15 to 30) and alate date (December 15 to 30). at leastonce before being allowed to progress to<strong>the</strong> national screening nursery. <strong>the</strong>Bangladesh Screening Nursery (BSN).Individual plants from international andlocally generated segregating materialare alternately selected at favorable andlate dates of planting up to <strong>the</strong> F6generation (e.g.• plants are selected at afavorable date in <strong>the</strong> F2. F4 and F6generations. and at a late date in <strong>the</strong> F3and F5 generations). Lines may be cutin bulk if selected for <strong>the</strong> BSN startingwith F6; <strong>the</strong> F7 and F8 generations arealso planted at a favorable date.However. all F2 material is first selectedunder irrigated conditions; seeds ofselected plants are divided and <strong>the</strong>populations are <strong>the</strong>reafter tested atalternate dates. simultaneously underboth irrigated and water-stressconditions.Lines selected for <strong>the</strong> BSN are plantedat seven locations; three of <strong>the</strong> sets areplanted under water-stress conditions.Lines of good appearance. with leaf rustresistance. absence of physiogenic leafand spike problems. and yielding wellformed.clean seeds in a majority of