Drought tolerance at seedling and flowering growth stages in maize: Abscisic acid and other traits as selection tools

Expansion of maize (Zea mays L.) growing into unfavorable areas, driven by increasing food demand, exposes over 60 million hectares in the tropics and subtropics to drought stress at the seedling and flowering growth stages causing about 17% yield losses annually. Breeding for improved drought tolerance will require methodology and techniques to be developed, environments for selection identified, selection criteria determined, and their genetics and effectiveness understood. Studies were undertaken on tropical maize germplasm to identify traits, methodologies, and environments for evaluation and selection, and to understand the gene action involved in control of putative secondary traits [leaf abscisic acid (L-ABA) concentration, anthesis-silking interval (ASI), and leaf senescence] for selection. Under drought at the seedling stage, seedling weight was increased and leaf abscisic acid concentration decreased as indirect responses to selection for drought tolerance at the flowering stage, even though these traits were not part of the index used in that selection. There was no reduction in seedling biomass under well-watered conditions. This gives hope that selection at the flowering stage confers some drought tolerance at the seedling stage. Pre- and post-flowering L-ABA concentrations under drought at the flowering stage were negatively correlated with grain yield (r = −0.67** and r = −0.54*, respectively) and were shown to decrease with cycles of selection for drought tolerance. L-ABA concentration under seedling drought stress is controlled by dominance gene action, while at flowering, both additive and dominance effects were important. Reciprocal recurrent selection or any selection involving tester crossing would be appropriate to reduce ABA concentrations at the seedling stage. Reduction of ABA concentration at the flowering stage could be achieved through either inter- or intra-population improvement procedures. ASI and leaf senescence are controlled by additive gene action, and recurrent selection or pedigree breeding procedures were suggested as suitable approaches for improving tropical maize using these traits. Index selection rather than selection based on individual traits was suggested as an appropriate approach for improvement of maize for drought tolerance considering the additive contributions of the traits studied in drought tolerance at seedling and flowering stages. Other considerations were the correlation coefficients between the putative secondary traits and grain yield and among the secondary traits (L-ABA concentration, leaf rolling, ASI, leaf senescence, and ears per plant). Drought environments are imperative for expression of the putative secondary traits; their expression in non-stress environments is inadequate for effective selection of drought tolerant genotypes.