STRESS TOLERANCE IN TROPICAL MAIZE IS LINKED TO CONSTITUTIVE CHANGES IN EAR GROWTH-CHARACTERISTICS

The tolerance of maize (Zea mays L.) grain yields to abiotic stresses is largely determined by events that occur at or shortly after floweri ng. Our objective was to evaluate ear growth after silking under unstr essed conditions or low N environments in two late lowland tropical ma ize populations, previously selected for tolerance to drought or to lo w N, Selection under low N or drought reduced florets per ear by 12 an d 11 per cycle when evaluated under unstressed conditions. The lag per iod between pollination and the onset of linear grain growth increased by 1.7 d cycle(-1) in the low N tolerant population grown under unstr essed conditions and by 0.6 d cycle(-1) in the drought tolerant popula tion under low N. Selection for tolerance to low N increased biomass p er kernel at the end of the lag and linear grain filling periods by 40 and 7% per cycle and by 10 and 4% per cycle in the drought tolerant p opulation, across N environments. The duration of linear grain filling declined with selection in the low N tolerant population when grown u nder unstressed conditions. Selection for tolerance to either stress d id not significantly increase grain yield under unstressed conditions, though improvement for drought tolerance increased yield by 7.5% per cycle (P < 0.05) under low N. Selection for tolerance to both stresses increased kernels per plant across environments. Apparently, selectio n for tolerance to low N or drought results in more kernels achieving the minimum biomass needed to prevent kernel abortion at the onset of the linear growth phase, resulting in greater grain sink strength unde r stressed and unstressed environments.