Constancy of Population Parameters for Life-History and Floral Traits in Raphanus sativus L. II. Effects of Planting Density on Phenotype and Heritability Estimates
To determine the effect of growing conditions on population parameters in wild radish, (Raphanus sativus L.: Brassicaceae), we replicated maternal and paternal half-sib families of seed across three planting densities in an experimental garden. A nested breeding design performed in the greenhouse produced 1,800 F1 seeds sown in the garden. We recorded survivorship, measured phenotypic correlations among and estimated narrow-sense and broad-sense heritabilities (h2) of: days to germination, days to flowering, petal area, ovule number/flower, pollen production/flower, and modal pollen grain volume. Survivorship declined with increasing density, but the relative abundances of surviving families did not differ significantly among densities. Seeds in high-density plots germinated significantly faster than seeds sown in medium- or low-density plots, but they flowered significantly later. Plants in high-density plots had fewer ovules per flower than those in the other treatments. Petal area and pollen characters did not differ significantly among densities. Densities differed with respect to the number and sign of significant phenotypic correlations. Analyses of variance were conducted to detect additive genetic variance (Va) of each trait in each density. At low density, there were significant paternal effects on flowering time and modal pollen grain volume; in medium-density plots, germination time, flowering time and ovule number exhibited significant paternal effects; in high-density plots, only pollen grain volume differed among paternal sibships. The ability to detect maternal effects on progeny phenotype also depended on density. Narrow-sense h2 estimates differed markedly among density treatments for germination time, flowering time, ovule number and pollen grain volume. Maternal, paternal and error variance components were estimated for each trait and density to examine the sources of variation in narrowsense h2 across densities. Variance components did not change consistently across densities; each trait behaved differently. To provide qualitative estimates of genetic correlations between characters, correlation coefficients were estimated using paternal family means; these correlations also differed among densities. These results demonstrate that: a) planting density influences the magnitude of maternal and paternal effects on progeny phenotype, and of h2 estimates, b) traits differ with respect to the density in which heritability is greatest, c) density affects the variance components that comprise heritability, but each trait behaves differently, and d) the response to selection on any target trait should result in different correlated responses of other traits, depending on density.