Seasonal variation in the onset and duration of flower development in 'Royal Gala' apple buds

The formation of bud scales, transition leaves, true leaves, bracts and flower primordia were observed in buds removed from non-flowering sites on one year old shoots of apple 'Royal Gala'. Buds were sampled from trees in Hawke's Bay, New Zealand (latitude 39°S) during six successive seasons. The onset of flower development varied between seasons from 72 to 99 d after full bloom. Floral development was poorly synchronized within the populations of sampled buds, lasting 22 to 50 d. Differences in the onset and duration of flower development could not be related to seasonal differences in heat accumulation from full bloom. There was significant variation between seasons in the minimum number of appendages observed within buds that exhibited doming, which indicates floral development. In the 1998/99 season, doming was observed in buds that had only 15.7 appendages, whereas in the 1999/2000 season the minimum number of appendages within a bud that had initiated flowers was 17.5. When the data from all seasons were considered, the rate of appendage formation within developing buds was not consistently related to heat accumulation.