Development of 6-benzyladenine as an apple thinner

Abstract

The road from discovery through evaluation, field testing, and then marketing of 6-benzyladenine (6-BA) is one of the most interesting and longest of any of the presently used plant growth regulators. Its ability to stimulate cell division in the calyx end of apples resulted in the development of a commercial product to elongate apples that also contained an equal amount of GA4+7. Abscission is not a physiological characteristic that is normally attributed to cytokinins. However, it was soon discovered that this proprietary product to elongate apples (Promalin®) could cause some thinning. Initially, it was assumed that the thinning was due to the GA4+7 component, since GAs were known to cause some apple fruit thinning. It was subsequently shown that 6-BA could cause apple fruit abscission. 6-BA was evaluated alone as an agent to stimulate lateral branch development on young apple trees. It was during this evaluation process that the potential of 6-BA as an apple thinner was recognized. Not only did it thin apples but it also increased fruit size and enhanced return bloom for a crop the following year. The first 6-BA product to be released as an apple thinner was not successful because it contained a small amount of GA4+7. It was not until several years later when an all 6-BA product was introduced that 6-BA was accepted and adopted by the apple industry as an important and impactful chemical thinner.