Studies on locating the signal for fruit abscission in the apple tree

The abscission of young fruit occurring in apple trees was measured under various conditions in order to locate the possible source of the signal that induces fruit drop. The effect on fruit abscission of shading of leaves, girdling of conducting tissue, extracting seeds and defruiting pedicels was investigated. Complete shading of whole trees for 5 days, steam girdling of defoliated fruiting wood proximal to the abscission zone and removal of seeds from attached fruit resulted in 100% fruit drop when treatment was carried out prior to the June drop period. When the same treatment was applied after June drop no fruit abscised. However, fruit abscission took place before and after June drop when steam girdling was applied to the pedicel distal to the abscission zone. The same effect was observed in defruited pedicels, which always abscised after fruit removal throughout fruit development. The results show that during the pre-June drop period, abscission can be induced by treatments which reduce or block nutrient supply to the abscission zone of fruit. After that period the growing fruit becomes a sugar storage organ which is resistant to abscission. It is though likely that inhibition of abscission is related to the presence of stored carbohydrates already accumulated in the young fruit. An indication for the involvement of the sugar level, in particular glucose, in the control of the abscission process was provided by the results of measurements of individual sugar levels in conducting tissue distal and proximal to the abscission zone. It was found that during fruit development, glucose concentration in the pedicel was normally above the concentration found in fruiting wood proximal to the abscission zone. However, when abscission during the June drop period was induced, the glucose level dropped to a minimum in the pedicel, reaching the same value as in the fruiting wood. It is suggested that the glucose level in the pedicel of growing fruit may therefore be a factor in the control of the abscission process in young apple fruit, abscission occuring only in those fruit in which the glucose content in the pedicel is below a critical level. Fruit which remain on the tree have most likely reached a stage in their development at which sufficient amounts of sugar have accumulated to prevent abscission.