Steven J. McArtney

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

Summary 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 Hawkes Bay, New Zealand (latitude 398S) 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.

Bud development and floral morphogenesis in four apple cultivars

Summary Buds were sampled from non-flowering spurs that had developed on 1 year-old wood of four apple (Malus domestica Borkh.) cultivars during the 1998-99 growing season in order to determine the effect of cultivar on appendage formation, doming and flower morphogenesis. Cultivars differed in their pattern of appendage formation over time. The rate of appendage formation was highest in ‘Fuji’ and ‘Pacific Rose™’ and lowest in ‘Braeburn’ during the first 60 d after bloom (DAB). A high proportion of buds were floral in all cultivars at the end of the growing season (75–100% depending on cultivar). However, the probability of observing doming was never greater than 0.13, indicating that flower morphogenesis proceeded rapidly once buds were committed to floral development. The four cultivars each exhibited a unique pattern of floral development, as determined by fitting response probabilities to each of five ordinal stages of development with time. Doming occurred significantly earlier in ‘Fuji’ buds (peaking 86 DAB) than in buds of the other cultivars (peaking 104–112 DAB). Doming was initiated at the same time in buds of ‘Braeburn’, ‘Royal Gala’ and ‘Pacific Rose™, but was completed earlier in ‘Braeburn’ than in ‘Royal Gala’. The timing of floral commitment was not related either to the time of flowering, or to the time of fruit maturity of the cultivar. These observations indicate that the timing of specific events during flower morphogenesis differed between cultivars.

Effects of 1-aminocyclopropane carboxylic acid on the rate of ethylene release from detached fruiting spurs and on fruit abscission in apple

Summary 1-Aminocyclopropane carboxylic acid (ACC) was applied to apple (Malus × domestica Borkh.) trees or to individual fruiting spurs between full bloom (FB) and 31 d after full bloom (DAFB; mean fruit diameter of 20 mm) to determine the effects of ACC on the ethylene released from detached spurs and on fruit abscission. The rate of ethylene release from detached fruiting spurs increased 1 d after applying 50 mg l–1 ACC at full bloom or 16 DAFB (mean fruit diameter of 10 mm), but ACC had a reduced effect on the rate of ethylene release when it was applied 31 DAFB (mean fruit diameter of 20 mm). ACC reduced fruit set in ‘GoldRush’ apple in a concentration-dependent manner in 2009 and in 2010, but had no thinning activity on ‘Pink Lady’ in 2010. ACC at 200 mg l–1 resulted in the yellowing and abscission of ‘GoldRush’ spur leaves in 2009, but not in 2010. The effects of combining ACC and 5 mg l–1 naphthaleneacetic acid on fruit set in ‘GoldRush’ apple were additive. The application of 200 mg l–1 ACC when fruit are ca. 10 mm in diameter may therefore be a useful chemical thinning treatment for apple fruit.