the Symposium on Wheats for More Tropical Environments - cimmyt

227105% that of C306 for <strong>the</strong> threesowings. respectively; <strong>the</strong> yield of C306was unaffected by sowing date. Clearly.Kalysansona developed too rapidly toan<strong>the</strong>sis with <strong>the</strong> October 15 sowing.and a cultivar like C306. with a smallvernalization response. was betteradapted to <strong>the</strong> early date.Early sowing. as described in this NewDelhi study. is often desirable for wheatsown on receding monsoon moisture.although improved agronomicmanagement (better tillage. deep-furrowdrills) can permit later seeding. Undersuch non-irrigated conditions. one oftwo key issues is <strong>the</strong> importance ofhaVing maximum crop growth coincidewith <strong>the</strong> period of lowest VPD.December to February (Figure 1). inorder to maximize WUE. Fortunately.this does not disagree with optimuman<strong>the</strong>sis date considerations whenwater is not limiting. The o<strong>the</strong>r issuewith growth on ample stored water is<strong>the</strong> importance of maximizing wateravailable for crop transpiration(Equation 4) by haVing deep rooting.Deep rooting is probably related tohaVing a long sowing-to-an<strong>the</strong>sisduration. and may be ano<strong>the</strong>r reasonwhy C306 performs well in <strong>the</strong> earlysowing when WUE may be prejudiced.Certainly. in a dry season. its yieldadvantage over Kayansona wasespecially evident (11). The juggling ofsowing date and cultivar-maturity classis both complex and profitable indryland Queensland. Australia(Woodruff. <strong>the</strong>se proceedings). Inconclusion. it should be emphasizedthat <strong>the</strong>re are genetic mechanisms forextending <strong>the</strong> duration up to an<strong>the</strong>sis.even under hot conditions. Yieldpotential will be approximatelyproportional to duration up to somelimit (i.e.• 60 to 100 days. depending on<strong>the</strong> crop growth environment). prOVidedthat <strong>the</strong> longer duration is accompaniedby earlier planting and that poorpartitioning to <strong>the</strong> spike and abnormalspike growth can be avoided. Also.water supply must be adequate tosupport <strong>the</strong> extra growth. Breeding for<strong>the</strong> new tropical environments shouldbe more efficient when optimuman<strong>the</strong>sis dates and durations are betterdefined.Of <strong>the</strong> o<strong>the</strong>r possible adaptations to hotconditions. none has reached <strong>the</strong> statusof recommended selection criteria;however. <strong>the</strong>re are many possibilitiesworthy of investigation in pilot breedingprojects. They are. along with someo<strong>the</strong>r relevant physiological questions:• Vernalization sensitivity versusphotoperiod sensitivity to extendduration;• Spikes cooled by increased waxiness;• Long sowing-to-an<strong>the</strong>sis period. fordeep rooting and full exploitation ofstored water;• Genetic variability in maintenancerespiration;• Long filling duration versus rapidfilling. and low grain-nitrogencontent to stabilize grain weight;• Grain set. as affected bytemperature. humidity and o<strong>the</strong>reffects;• The role of <strong>the</strong> Norio 10 dwarfinggenes in <strong>the</strong> tropics. where stature isreduced in any case. and• The likely magnitude of heterosis in<strong>the</strong> tropics.To this list of worthwhile researchtopics should be added <strong>the</strong> analysis andinterpretation of international yieldtrials in <strong>the</strong> tropiCS. something whichcould contribute significantly to <strong>the</strong>understanding of genetic adaptation.prOVided enough trials were conductedin a standard manner and with goodagronomic management.