Abstract:
Chilling requirements of 43 apple and 38 pear cultivars
were estimated over a 2 year period by cutting shoots in the
field at weekly intervals and forcing them in a warm
greenhouse for 4 weeks. For apples, the mean values ranged
from 490 Chill Units (CU) +- 1 for 'Dorsett Golden' to
1320.5 CU +- 8 for 'Cortland', 'Marshall Mclntosh' and
'Starking Delicious'. For pears, the mean values ranged from
749 CU +- 9 for 'Batjarka' to 1320.5 CU +- 8 for 'Poirier
Fleurissant Tard'. Interval of sampling, coastal effect and
involvment of other factors than CU accumulation in
releasing plants from dormancy could be among the possible
explanations for the variations in the year-to-year
estimation of the chilling requirement.
Budbreak and phytotoxicity (expressed as percent dead
buds) induced by hydrogen cyanamide in apple cultivars
depend on hydrogen cyanamide concentration, time of
application and chilling requirements of the cultivars.
'Arlet' (811.5 CU), having the lowest chilling requirement,
showed a different endodormancy pattern than 'Braeburn'
(1141 CU), 'Golden Delicious' (1277 CU) and 'Starking
Delicious' (1320.5 CU), resulting in a different response to
the chemical. During early to deep dormancy, 'Braeburn',
'Golden Delicious' and 'Starking Delicious' exhibited a
greater tolerance to hydrogen cyanamide, with an increase in
budbreak and a reduction in phytotoxicity. In 'Arlet'
however, the level of tolerance did not increase
significantly during this period. Towards the end of
endodormancy, the sensitivity to the chemical increased in
all cultivars, but at a higher rate in 'Braeburn', 'Golden
Delicious' and 'Starking Delicious' than in 'Arlet'. The
sensitivity to hydrogen cyanamide was also dependent on bud
and internode position in the shoot, with an increasing
tolerance from the top to the base. This gradient
disappeared after satisfaction of the chilling requirement
of the buds, while it remained for the internodes,
suggesting that other factors, unrelated to the dormant
period, may be involved in determining the differential
sensitivity of the stem along the shoot.
