Genetic Regulation of Seed Dormancy in Purshia tridentata(Rosaceae)

Abstract

Seeds of Purshia tridentata, a shrub of semi-arid North America, require chilling to become non-dormant. Using seeds produced from controlled crosses in a common garden, we examined effects of ovule parent, pollen parent, and year of production on germination percentage in response to chilling for 2 weeks at 2°C. Differences among ovule parents accounted for most of the variance in chilling response, and these differences (2 to 83% germination) were consistent across years. Differences among pollen parents were also significant, producing a two- to five-fold difference in mean germination percentage. Differences among years were significant but small. Ovule parent by year interactions showed that among-year variation in ripening environment did not affect ovule parents equally. There was no significant pollen parent by year interaction, suggesting that the effect of maturation environment was mediated through maternal tissues. In reciprocal crosses, two plants that showed contrasting dormancy levels as ovule parents produced seeds with similar dormancy as pollen parents, indicating that the genetic difference between them was at the testa level. Two plants that produced seeds with contrasting dormancy as pollen parents showed a similar but stronger pattern of contrast as ovule parents, showing that the genetic difference between them was at both embryo and endosperm or testa level. Testa or endosperm genotype was primarily responsible for chilling response of intact seeds, while embryo genotype affected chilling response and also exercised primary control over low temperature germination rate, whether of excised embryos or of intact seeds.