the Symposium on Wheats for More Tropical Environments - cimmyt

209III. AgronomyPhysiological Limitations to Producing Wheatin Semitropical and Tropical Environments andPossible Selection CriteriaR.A. Fischer, Division of Plant Industry, Commonwealth Scientificand Industrial Research Organization, Canberra, AustraliaAbstract''Tropica.l'' wheat environments are characterized by short winter photoPeriods(11 and 12.5 hours) with high temperatures, <strong>the</strong> mean temperaturefor <strong>the</strong>coolest month varyingfrom warm (l5°C) to hot (20°C) to very hot (25°C).Diurnal temperature range, solar radiation, vapor-pressure deficit and watersupply (irrigated or rainfed) vary considerably between regions:frosts and hotwinds constitute meteorological hazards in some locations. In suchenvironments, wheat may establish poorly because of high soil temperatures.More seriously, development is accelerated (approximately in proportion totemperature), while growth rate is stable or may decline, so that leaf size,tillering, spike size and yield potential suffer, even under irrigation. Excessiverespiration and possible direct effects of high temperature on sinkformationmayfur<strong>the</strong>r reduce potential, while kernelfilling is curtailed by hasteneddevelopment and/or carbohydrate shortage. There is little physiologicalexperience with wheat at high temPeratures, but some o<strong>the</strong>r plant genera showremarkable <strong>the</strong>rmal adaptation. It is possible to leng<strong>the</strong>n <strong>the</strong> seeding-tofloweringinterval in wheat with daylength and cold-sensitivity genes, <strong>the</strong>rebyincreasing biomass production. Selecting in such materialfor harvest index(currently very low) would seem a rational physiological approachforimprovement of yield potential. In order to avoid high temperatures at criticalstages, and to maximize water-use efficiency, it is desirable to have seeding dateand cultivar maturity class such thatflowering is around <strong>the</strong> coolest point of <strong>the</strong>year and, if <strong>the</strong> crop is rainfed, subsoil moisture reserves arefully explotted.Crop research comprises <strong>the</strong>complementary activities of adaptingenvironments to plants, and adaptingplants to environments. At first glance,<strong>the</strong>re is not much that agronomists cando about <strong>the</strong> <strong>the</strong>rmal regimes of <strong>the</strong>tropics, but genotypic adaptation tosuch conditions may offer possibilitiesto plant breeding. Genotypic adaptationto superoptimal temperatures is also ofinterest to physiologists; ecophysiologistsalready recognize manyadaptations to hot regimes in o<strong>the</strong>rplants (37). The tropics being a newenvironment for wheat. it might permiteasier prediction of advances than <strong>the</strong>well-trodden field of geneticimprovement in more temperate areas.In this paper. important features ofwheat climates in <strong>the</strong> tropics and ofyield determination in wheat will besummarized, before passing to specificphysiological problems of tropicalwheat and possible genotypicadaptations. Possible selection criteriaand interactions with agronomy wUlalso be discussed. Disease and weedfreeconditions and adequate fertilitywill be assumed. The indulgence isasked of researchers in <strong>the</strong> tropics for<strong>the</strong> author's haVing ventured fromtemperate agriculture into a field inwhich he has had little researchexperience.