Kernels of 20 corn (Zea mays L.) inbred lines were examined for 3 years to determine variation for black layer maturity, filling period, and associated characters. The color of the black layer and the rate at which it developed at maturity varied among the inbreds. For each inbred, the black layer was observed at a time that was coincident with the observed maximum dry weight accumulation. Moisture content at the time of black layer maturity varied among the inbreds from 15.4 to 35.0%. Differences among inbreds were significant but year means did not differ. There was a significant year ✕ inbred interaction.The growing degree days required to reach black layer maturity varied from 1337 to 1808 and the growing degree days required for kernel filling varied from 512 to 821. Year differences were significant; however, interaction of inbreds with years was found to be minor. The variability among inbreds was always much greater than among years. The growing degree days required for the filling period had positive phenotypic and genotypic correlations with the growing degree days required from planting to pollination, but the correlations were small enough to suggest possible selection for types with long filling periods and short time to pollination. The rate of kernel dry weight accumulation during the filling period was significantly different among inbreds and years but had a significant inbred ✕ year interaction. The rate of kernel dry weight accumulation was not correlated with any character other than dry weight at black layer maturity.

The rate of kernel dry weight accumulation and filling period duration of corn (Zea mays L.) have been suggested as significant factors that influence yield potential. To determine the importance and usefulness of these kernel characteristics, environmental and genetic variations should be investigated. The objectives of this study were to measure the effects of plant density on the kernel growth components and observe the inheritance of these characteristics in the F1 hybrids. Kernel dry weight accumulation rate and effective filling period duration (EFPD) were examined in three corn inbreds and their three F1 hybrids at two plant densities. Kernel dry weight accumulation rate was not affected by plant density; however, EFPD was 2.5 days less at the high plant density. The resultant, smaller kernels along with a reduced number of kernels resulted in a 20% yield reduction per plant at the higher plant density. Yield per plant and per unit area in response to changes in plant density were altered more by kernel number per plant than by kernel size.A significant increase in kernel size contributed to the greater F1 yields although the major advantage of the hybrids was due to more kernels per plant. In two of the three F1 hybrids, the larger kernel size was due solely to longer EFPD's since the rate of dry weight accumulation of one of the parental inbreds was the same as that of the F1's. In the other F2 hybrid, both rate and EFPD were greater in the F1 genotype than for either parental inbred.These data show that EFPD but not the rate of kernel growth is influenced to a limited extent by plant density and both are under genetic control. Thus genetic modification of these kernel growth components should be feasible.

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Variability in recombination frequency has been reported in several plant populations. The objectives of the present research were to establish the range in variability in recombination among genotypes in the important corn population Iowa Stiff Stalk Synthetic and to identify individual genotypes which produced increased or decreased recombination frequencies. Approximately 150 individual S0 plants were testcrossed to measure male recombination frequency on three chromosomes: 4, sul-c2; 5, a2-btl-pr1; and 9, sh1-bz1-wx1. Although the variance component for individuals accounted for only 20-33% of the total variation, highly significant variability among individuals was present at all chromosome regions. Thus the environmental effects did not prevent measurement of differences between S0 individuals. At each chromosome region, individual genotypes with recombination frequencies at least two standard deviations above or below the population mean were isolated. Reports in the literature suggest that the variability reported here for the BSSS population should be representative of that present in other corn breeding populations. Recombination frequencies were positively correlated between adjacent regions of chromosome 9 and also between adjacent regions of chromosome 5. Recombination frequencies were positively correlated between both regions on chromosome 5 with the su1-c2 region of chromosome 4. Negative correlations were observed between chromosome 9 recombination and recombination in each region of chromosomes 4 and 5. Thus rankings of S0 individual recombination frequencies were not consistent for all three chromosomes.

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