Drought tolerance improvement in tropical maize source populations: Evidence of progress

The objectives of this study were to evaluate direct and correlated responses to recurrent selection for drought tolerance in two CIMMYT maize (Zea mays L.) source germplasm populations,'DTP1' and 'DTP2', adapted to the lowland and mid-altitude tropics. Selection was primarily based on grain yield, ears per plant, anthesis-silking interval, and leaf senescence under drought. Cycles C-0, C-3, and C-6 of DTP1 and C-0, C-3, C-5 and C-9 of DTP2 were evaluated under drought, low N, and optimal conditions. In both populations, significant yield gains were observed under drought conditions, associated with a significant increase in numbers of ears per plant and grains per ear, and significant reductions in anthesis-silking interval, ovule number and abortion rate during grain filling. Abortion rate was positively correlated with the number of ovules at silking and with anthesis-silking interval. In DTP1, recurrent selection under drought was associated with a derease of tassel and stem dry weight and with an increase of ear dry weight and harvest index. This study confirms the effectiveness of recurrent selection under drought as a means of improving tropical maize source populations for performance under water deficits and to a lesser extent under low N. The primary mechanism underlying these changes appears to be improved partitioning of assimilates to the ear at flowering, at the expense of tassel and stem growth.