Genetic regulation of seed dormancy in Purshia tridentata (Rosaceae)

Seeds of Purshia tridentata, a shrub of semi-arid North America, require ch illing 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 degrees C. Differences among ovule parents accounted for most o f the variance in chilling response, and these differences (2 to 83% germin ation) were consistent across years. Differences among pollen parents were also significant, producing a two- to five-fold difference in mean germinat ion percentage. Differences among years were significant but small. Ovule p arent by par interactions Showed that among-year variation in ripening envi ronment did not affect ovule parents equally. There was no significant poll en parent by year interaction, suggesting that the effect of maturation env ironment was mediated through maternal tissues. In reciprocal crosses, two plants that showed contrasting dormancy levels as ovule parents produced se eds with similar dormancy as pollen parents, indicating that the genetic di fference between them was at the testa level. Two plants that produced seed s 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 endo sperm genotype was primarily responsible for chilling response of intact se eds, while embryo genotype affected chilling response and also exercised pr imary control over low temperature germination rate, whether of excised emb ryos or of intact seeds.