Eight cycles of selection for drought tolerance in lowland tropical maize. II. Responses in reproductive behavior
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Drought stress can greatly reduce grain yields of maize (Zea mays L.) if it coincides with flowering. Eight cycles of full-sib recurrent selection in the Population 'Tuxpeno Sequia' were carried out with the objective of improving performance under moisture stress targeted to coincide with flowering and grain-filling. Selection was based on an index comprising increased grain yield, the maintenance of a constant number of days to 50% anthesis, reduced anthesis-silking interval (ASI, or days from 50% anthesis to 50% silking) and other drought-adaptive traits. The objectives of this study were to evaluate the direct and correlated selection responses in days to flower, ASI, and biomass of reproductive organs at anthesis. During two consecutive winter seasons, Cycles 0, 2, 4, 6 and 8 of Tuxpeno Sequia and a check representing six cycles of international multilocation testing in the same population, were evaluated under three moisture regimes at Tlaltizapan, Mexico. Selection resulted in significant per cycle responses in days to 50% anthesis (-0.4 days or -0.5%), time to 50% silking (-3.4 days or -3.2%) and in ASI (-3.0 days or -16.1%) under drought. Comparable figures under well-watered conditions were -0.4 days or -0.5%, -0.8 days or -0.9%, and -0.4 days or -23.0%, respectively. Grain yield and its components, especially kernel number per plant, showed a strong statistical dependence upon ASI. The regression of best fit between grain yield and ASI across cultivars and moisture regimes showed that 76% of the variation in grain yield was accounted for by variation in ASI. Grain yield declined by an average of 8.7% day-1 increase in ASI up to 10 days. Selection resulted in a significant increase in ear biomass at 50% anthesis (EB) of 0.004 Mg ha-1 (14.7%) cycle-1 and a significant reduction in tassel biomass at 50% anthesis of -0.008 Mg ha-1 (-1.7%) cycle-1 under drought. Comparable figures under well-watered conditions, also significant, were 0.007 Mg ha-1 (14.7%) and -0.014 Mg ha-1 (-2.6%) cycle-1, respectively. ASI and the percentage biomass distributed to the tassel and ear at anthesis were not significantly affected by selection in the check entry. These data suggest that selection for reduced ASI under carefully managed moisture stress imposed at flowering provides an effective and rapid route to higher and more stable grain yield in lowland tropical maize.
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