the Symposium on Wheats for More Tropical Environments - cimmyt

103hence, select for drought tolerance. Theexperiment consisted of 45 treatmentcombinations, five varieties and nineirrigation treatments (Table 2).Due to wide variations betweenreplicates of different treatments, <strong>the</strong>rewas no significant difference between<strong>the</strong> varieties. Interaction betweenirrigation treatments and varieties wasalso not significant. Of all <strong>the</strong> varieties.only HD832-55 x eNO-JAR wasseriously affected when irrigation waswithheld twice consecutively at <strong>the</strong>jointing stage (T7). Missing twoirrigations at <strong>the</strong> flowering stage(T8) affected all of <strong>the</strong> varietiesconsiderably. Varying yield reductionswere observed in all o<strong>the</strong>r treatments,but it appeared that <strong>the</strong> two mostcritical stages for moisture stress are T7and T8, Jointing and an<strong>the</strong>sis.Characteristics AffectingSelection for Heat and DroughtToleranceEarlinessAn understanding of a stress-tolerancemechanism is necessary in order tocorrectly suggest useful characteristicsby which a plant can tolerate stress.Under Nigerian conditions, it appearsthat <strong>the</strong> main mechanism leading togood performance in wheat is escape oravoidance. The variety should grow andmature within <strong>the</strong> cold period, Le., withmaturities of 70 to 120 days. Thus.only early or medium-early lines!varieties will perform well. This is alsotrue for drought tolerance. sinceearliness enhances water economy.YieldIn addition to earliness. superiority inyield under optimal conditions isnecessary for both heat and droughttolerance. According to Blum (4)varieties with superior yield at optimallevels will also yield relatively wellunder sub-optimal levels.AdaptabiUtyWide adaptability is a necessary factor,since it is believed that. when stresstolerance is present in a variety. it maybe expressed as an unidentifiedcomponent of stability in performanceover various environments (4.11.17).Developmental synchronyPlants should tiller uniformly, holdmost of <strong>the</strong>ir leaves intact and matureuniformly.Test weightThe grains of plants should have goodtest weight, which results from goodand uniform grain filling.Spike fertUltyIncreased fertility of <strong>the</strong> spikes assuresthat most florets will be fertile understress conditions, as well as at optimallevels.Table 2. Effects of variety and moisture stress on grain yield of bread wheat, Bakura,Nigeria, 1982·83Grain yield (kg/ha) according to i'rrigation treatment!./Variety T1 T2 T3 T4 T5 T6 T7 T8 T9HD832-55 x CIANO-JAR 2747 1907 1720 1813 2087 1687 833 1587 2220Super X 2627 1893 1960 1667 1887 1987 1433 1147 1820(TF x CIAN067/Bluebird-CIANO)ZA 1840 2033 1860 1887 2527 1960 1733 1187 1867NP876-164 x Roch/CIANO"S"-Bluebird 2380 1960 1960 2020 1860 1600 1793 1453 1720Pi62·Frond/Pi62-Mazoe x Mexipak 65 2293 2427 1867 1487 2427 1860 1667 1333 2120!,/ T1 = control, 19 irrigations; T2 = one irrigation withheld at tillering; T3 = one withheld at jointing; T4 = one withheldat flowering; T5 = one withheld at grain filling; T6 = two irrigations withheld at tillering; T7 = two withheld at jointing;T8 = two withheld at flowering; T9 = two withheld at grain fillingSource: Aremu and Orakwe (2)