Effect of temperature on vegetative growth, flower bud formation, bolting and flowering of pakki-hood (Raphanus sativus L.) introduced from northern Thailand

The present study was conducted to analyze the vegetative growth and flowering behavior of Pakki-hood (Raphanus sativus L.) introduced from northern Thailand, in response to temperature under both plastic tunnel and controlled conditions. When the seeds of Pakki-hood were sown three times (December, January and February) under a plastic tunnel, in the earlier sowing initiation of the increase of the leaf number and leaf area was accelerated. Comparison of the relative growth rates in terms of leaf number, leaf area and plant height revealed that plants sown earlier showed lower growth rates for these parameters. The duration of the time to bolting and flowering after sowing decreased with the delay in sowing. Using thermal times based on the calculation of temperature sums, flowering in Pakki-hood was considered to start at 730 degrees C d after sowing. There was no remarkable difference in the duration of the period until the start of flower bud formation among the three sowing times, although longer days were needed for germination in the earlier sowing. The seedlings of Pakki-hood were transferred to temperature-controlled growth chambers (5, 10, 15 degrees C) to analyze the effect of different growing temperatures. At 5 degrees C, the growth of the seedlings was interrupted after the development of the cotyledons. Seedlings started to bolt 60 days after transfer to a 10 degrees C chamber, and 80% of the seedlings bolted, although flowering did not occur during the experimental period. A temperature in the range between 5 and 10 degrees C is likely to play a direct role in the formation of flower buds, but is probably insufficient for the induction of bolting and flowering. Incubation at 15 degrees C resulted in 100% bolting and 80% flowering, and the duration of the period until bolting decreased. It is considered that the delay in bolting at a low temperature resulted in an increase in the number of flower buds