Graduate Thesis Or Dissertation
 

Chilling requirement affects plant response to hydrogen cyanamide in apple cultivars

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  • 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.
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