Variation in the pre-anthesis development of spring rape (Brassica napus L.)

Fifteen cultivars of spring rape (Brassica napus L.) were each sown at four different times between April and October at Perth in either natural photoperiods or continuous light. Under natural photoperiods, flowering of the Japanese cultivars Chisaya and Isuzu was delayed in later sowings, but the flowering times of all other cultivars were advanced with each successive delay in sowing. When plants were grown under continuous light another two groups of cultivars could be recognized. One group comprised early-flowering Canadian cultivars which flowered later in the June sowing than in the other sowings. The other group comprised late-flowering European cultivars characterized by a slight delay in flowering in the last sowing. The direct effects of temperature on growth rate were more important than inductive responses in determining time of flowering in all cultivars except Chisaya and Isuzu, which had substantial vernalization requirements. Other cultivars such as Komet, Bronowski, Masoweicki and Norin 16 had low vernalization requirements which were largely masked by the increase in the rate of growth with higher temperatures in later sowings. In controlled environments, considerable variation in response to vernalization, temperature and photoperiod was detected between six spring rape cultivars. The early-flowering Canadian cultivars Target and Oro were the least responsive to vernalization and under continuous light did not respond at all. European and Japanese cultivars used here responded more markedly to vernalization, and their responses were greatly influenced by temperatures experienced after the completion of vernalization. Under continuous light and high temperature, the Japanese cultivars responded markedly to 4 weeks' vernalization which apparently saturated the system. The European cultivars Bronowski and Masoweicki, on the other hand, only responded moderately.