Abscission of apples

Apple trees are now often seen with apples but no leaves into autumn. This is because of climate change.

The change in prolonging warmer temperatures later into the year is accentuating a trait already selected by nature and Man. In Tien Shan the apple trees' homeland, it was advantageous to have fruit remain on the tree as long as possible for dispersal reasons. Apple seeds are dispersed widely by having them encased in the tasty apple which is attractive to bears and other now extinct mega flora. These large animals can reach high into the tree branches for the fruit and by the time they are excreted in the dung, the animal will have travelled a distance aiding the seed dispersal. The seeds survive the bears digestive tract.

If the drooped onto the ground there is a higher chance of being eaten by a ruminant which because of the multiple stomachs needed for grass digestion would kill the seeds.

This has been further selected for by Man. Apples plucked from the tree being more saleable than from the orchard sward. And less damaged too! Not a trait needed however for cider apples, which are often picked from the orchard sward.

Both leaves and apples are shed by abscission but both respond to different environmental stimuli. Leaf fall is controlled by day length, fruit fall by temperature - now pushed back by climate change. Multiple genes are involved in the process and the process is quite complex. Auxin a plant hormone is produced in the growing tip of the branch and acts to stop fruit abscission before the pips are ripe by making the abscission zone insensitive to ethylene. As the pips ripen, auxin production drops and abscinic acid increases this acts synergistically with ethylene leading to fruit fall by invoking changes in the abscission zone, a specialised region of cells in the abscission zone. AZs are laid down early in development and often have a band-like appearance.

The cells in an AZ are smaller than the surrounding cells and have a densely packed cytoplasm. Once abscission is triggered, AZ cells expand and the middle lamella (the pectin layer that glues two cells together) is dissolved via hydrolytic enzymes, allowing cell separation. After abscission has occurred, a new, protective epidermal layer is laid down over the abscission ‘scar’.