185 BIOASSAYS TO DETERMINE OF APPLE THINNING RESPONSE FROM APPLICATIONS OF NAA
Auxin induction of ethylene, and fruit growth rates were investigated as early indicators of NAA thinning response for Golden Delicious, Red Delicious, McIntosh, Empire, and Tydeman's Red over a four period. Abscission at the end of the drop period was correlated with ethylene evolution from leaves 24-48 hours after NAA application and with changes in fruit growth at 2-3 day intervals through 10-14 days after application. Variation in ethylene evolution and fruit growth were also associated with environmental conditions prior to and at the time of NAA application to determine which factors have the greatest influence on response. Ethylene was a better predictor of final fruit drop than changes in fruit size for all varieties tested. However both performed very well. The ethylene bioassay requires more equipment, but the response is more-immediate. Bourse, and spur leaves as well as fruit were capable of producing ethylene in response to NAA application. Thinning response was greatest when all leaves and fruit were treated with NAA, followed by the bourse and spur leaves. Little or no response was produced when the fruit alone were treated. Concentration experiments and radioisotope data indicate that ethylene response is directly related to the amount of NAA absorbed. Regression analysis indicates that approximately 60% of the variation in response can be predicted by ethylene evolution