Genetic improvement of pre-anthesis growth of turnip rape (Brassica rapa L.) at low temperature

It has been shown that higher growth rates at low temperature before anthesis in B. rapa L. lead to greater biomass production at anthesis in the field. This study aims to understand inheritance of the traits associated with higher pre-anthesis growth at low temperature and to evaluate responses to selection methods for those traits. A triple test cross was used to determine genetic effects and heritability of the traits associated with higher preanthesis growth at low temperature. There were significant (P < 0.05) additive genetic effects for relative growth rate (RGR) in the field during winter and for potassium leakage from leaf tissue at 4<degrees>C. Low potassium leakage is correlated with higher pre-anthesis growth at low temperature. Dominance genetic effects were not important for these two traits. Narrow-sense heritability was 0.55 for RGR and 0.48 for potassium leakage, indicating potential for gain from selection in early generations. Seedling emergence rate in the field during winter was largely determined by epistatic effects, suggesting that selection for rapid seedling emergence should start in advanced generations. Selection for low potassium leakage at 4 degreesC in an F-2 population significantly (P < 0.05) increased dry weight of F-2:3 plants grown at 13/4<degrees>C (day/night) at 26 days after emergence, whereas selection for rapid seedling emergence at 4 degreesC had no effect on plant growth. Pollen selection at low temperature improved subsequent plant growth at low temperature. The dry weight ratio of plants grown at 13/4 degreesC to that at 18/13 degreesC was higher in progenies derived from fertilisation at 10/2 degreesC than those from 18/13 degreesC fertilisation. The ability to use pollen selection in combination with pedigree selection for low potassium leakage should increase genetic gain of plant dry weight at low temperature in segregating populations.