Manipulation of the winter chilling requirement of apple trees

Plant growth regulators were tested over two seasons to determine whether they could modify the response of the mature apple tree to inadequate winter chilling. Cytokinins, benzyl adenine (BA) and forchlorfenuron (CPPU) and the growth inhibitor, abscisic acid (ABA) were sprayed on Golden Delicious trees at varying concentrations and at various times prior to and during dormancy. Chemicals were applied using a high pressure handgun to the point of runoff.

The response of the tree to the treatments was determined by calculating the time interval for 1-year-old shoots to reach 50% budburst under controlled conditions. A graph depicting the progression of dormancy for each treatment was drawn. The percentage budburst and shoot length was recorded at the end of the growing season.

The results from both the 2006 and 2007 seasons indicate that BA, CPPU and ABA did not significantly change the dormancy patterns or the exit from dormancy of mature Golden Delicious apple trees. There were no statistical differences between the treatments and the control with regard to percentage budburst and shoot growth. This is in contrast to results obtained from work done on nursery trees (Cook NC, Verhaegen K, Keulemans J, Jacobs G. 2000. Manipulation of acrotony in one-year-old apple shoots. S. Afr. J. Plant Soil 17: 108-112). In nursery trees lateral budburst was dramatically increased with autumn BA applications. It should be noted that this was with multiple hand applications with a paint brush.

A possible explanation for the lack of response could be that the compounds were not able to adequately penetrating the target organs, i.e., the buds, at the current rates. Penetrants and other adjuvants may improve uptake and could be investigated as a possible solution. The rates used previously on nursery trees would be prohibitively expensive in commercial orchards.