Eight cycles of selection for drought tolerance in lowland tropical maize. III. Responses in drought-adaptive physiological and morphological traits
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Selection for grain yield under severe drought stress has often been considered inefficient because the estimate of heritability of grain yield has been observed to decline as yields fall. Under these conditions secondary traits may increase selection efficiency, provided they have adaptive value, high heritability, and are easy to measure. Increased relative stem and leaf elongation rate (RLE), delayed foliar senescence, reduced canopy temperatures and reduced anthesis-silking interval (ASI) were used to augment efficiency of selection for grain yield under drought during eight cycles of recurrent full-sib selection in the lowland tropical maize (Zea mays L.) population,'Tuxpeno Sequia'. Six cultivars comprising Cycles 0, 2, 4, 6 and 8 of Tuxpeno Sequia, and a check cultivar were grown for two consecutive years at Tlaltizapan, Mexico, under three moisture regimes that provided a well-watered control, a severe moisture stress during flowering, and a severe stress during grain-filling. Previous reports have documented significant improvements in grain yield and ASI in this population. When observed under drought, no significant differences were detected among cultivars in RLE or canopy-air temperature differentials, nor in chlorophyll per unit leaf area during grain-filling (an indicator of foliar senescence). Cultivars did not differ in seasonal pre-dawn or diurnal courses of leaf water potential, in leaf osmotic potential, in capacity to adjust osmotically, nor in their seasonal profiles of soil water content with depth to 140 cm. Selection significantly altered final plant height, total leaf number, and tassel primary branch number by -0.9%, -0.5% and -2.6% cycle-1, respectively. Observations on root growth in 2-m deep pots showed that eight cycles of selection had reduced root biomass in the upper 50 cm by 33%, consistent with a significant change of -1.2% cycle-1 in vertical root-pulling resistance. The lack of direct and correlated changes in traits related to plant water status due to selection suggests that in this population heritabilities of such traits are low, or that the traits are only weakly associated with grain yield under severe moisture stress. The present study indicates that improved drought tolerance in Tuxpeno Sequia was due to increased partitioning of biomass towards the developing ear during a severe drought stress that coincided with flowering, rather than to a change in plant water status.
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