The importance of the anthesis-silking interval in breeding for drought tolerance in tropical maize

Selection for improved performance under drought based on grain yield alone has often been considered inefficient, but the use of secondary traits of adaptive value whose genetic variability increases under drought can increase selection efficiency. In the course of recurrent selection for drought tolerance in six tropical maize (Zea mays L.) populations, a total of 3509 inbred progenies (S₁ to S₃ level) were evaluated in 50 separate yield trials under two or three water regimes during the dry winter seasons of 1986–1990 at Tlaltizapán, México. In over 90% of the trials, ears plant⁻¹, kernels plant⁻¹, weight kernel⁻¹, anthesis-silking interval (ASI), tassel branch number and visual scores for leaf angle, leaf rolling and leaf senescence were determined. Low scores indicated erect, unrolled or green leaves. Canopy temperature, leaf chlorophyll concentration and stem-leaf extension rate were measured in 20–50% of the trials. Across all trials, linear phenotypic correlations (P < 0.01) between grain yield under drought and these traits, in order listed, were 0.77, 0.90, 0.46, −0.53, −0.16, 0.06^NS, −0.18, −0.11, −0.27, 0.17 and 0.10. Genetic correlations were generally similar in size and sign. None of physiological or morphological traits indicative of improved water status correlated with grain yield under drought, although some had relatively high heritabilities. Genetic variances for grain yield, kernels ear⁻¹, kernels plant⁻¹ and weight kernel⁻¹ decreased with increasing drought, but those for ASI and ears plant⁻¹ increased. Broad-sense heritability for grain yield averaged around 0.6, but fell to values near 0.4 at very low grain yield levels. Genetic correlations between grain yield and ASI or ears plant⁻¹ were weak under well-watered conditions, but approached −0.6 and 0.9, respectively, under severe moisture stress. These results show that secondary traits are not lacking genetic variability within elite maize populations. Their low correlation with grain yield may indicate that variation in grain yield under moisture stress is dominated by variation in ear-setting processes related to biomass partitioning at flowering, and much less by factors putatively linked to crop water status. Field-based selection programs for drought tolerance should consider these results.