Divergent phenotypic recurrent selection for nitrate reductase activity in maize. I. selection and correlated responses

The use of biochemical or physiological traits as accessory criteria for selection of high-yielding maize (Zea mays L.) genotypes has been proposed. In this study, direct and correlated responses to eight cycles of divergent phenotypic recurrent selection for high and low levels of postanthesis leaf-lamina nitrate reductase activity (NRA) in the maize synthetic Super Stiff Stalk (BSSS) were valuated fertilizer-N rates of 112, 224, and 336 kg ha⁻¹, Cycle 0, 3, 6, and 8 of the high-and low-NRA strains were evaluated in 1985 for NRA to determine the direct response to selection. Cycles 0, 3, and 6 of the high-and low-NRA strains were evaluated for correlated responses in grain yield, yield components, harvest index, days to anthesis, and days to silk emergence in 1983, 1984, and 1985 at one location each year. Cycle 8 was an additional treatment in 1985 only. Relative to Cycle 0, eight cycles of selection resulted in a 46% increase in NRA in the high-NRA strain and a 62% decrease in the low-NRA strain, averaged over supplemental-N rates. Divergence of NRA was maintained late into the grain-fill period. Cycle 6 of the high-and low-NRA strains had grain-yield reductions of 5.6 and 16.8%, respectively, relative to Cycle 0, averaged over supplemental-N rates. The large grain-yield reduction for Cycle 6 of the low-NRA strain was due primarily to a reduction in kernels plant⁻¹. Estimates of inbreeding depression could account for essentially all of the yield reduction for Cycle 6 of the high-NRA strain, but for only a fraction of the yield reduction for Cycle 6 of the low-NRA strain. Both NRA strains showed reductions in the harvest index and increases in days to flower.