Naphthaleneacetic Acid and Ethephon Are Florigenic in the Biennial Apple Cultivars Golden Delicious and York Imperial

摘要

'Golden Delicious' and 'York Imperial' are apple cultivars that are prone to develop a biennial bearing habit. A successful chemical thinning program with carbaryl plus 6-benzyladenine applied at the 10-mm fruit diameter stage reduced cropload and increased return bloom of 'York Imperial', although the improvement in return bloom resulting from chemical thinning was insufficient to ensure a commercial cropload in the year after treatment (fewer than 10% of spurs developing flowers). A chemical thinning program with multiple applications of a naphthaleneacetic acid (NAA) and ethephon mixture during the period from 36 to 73 days after bloom increased return bloom of 'York Imperial' trees to commercially acceptable levels (25% or greater of spurs flowering). NAA applied during the period from 50 to 100 days after bloom (summer NAA program) or from 110 to 140 days after bloom (preharvest NAA program) increased return bloom of 'Golden Delicious'. When aminoethoxyvinylglycine (AVG) was included with thefirst NAA spray in a summer program, the efficacy was reduced, indicating that ethylene may be partly involved in the florigenic activity of NAA. Dissection of 'Golden Delicious' buds sampled from three locations (Asheville, NC; Amherst, MA; Wenatchee, WA) at 14-day intervals beginning 50 days after bloom indicated that the time of floral transition (doming of the meristem apex) occurred during the period from 65 to 105 days after bloom at each location. Thus, NAA applications in a summer program for return bloom coincided with the period when floral determination normally occurred. Preharvest NAA programs effectively promoted return bloom in the experiments where a summer NAA program was also effective. These responses indicate that NAA can trigger floral development within vegetative buds relatively late in the summer and outside of the time period when it is generally believed possible to influence flower bud formation.