Physiological, molecular and practical aspects of fruit abscission

Many fruit species bear an abundance of flowers which produce a surplus of fruit that the tree is unable to support. In anticipation of this the major fruit species developed an immature fruit physiological drop as a self regulatory mechanism. The physiological fruit drop leads to a reduction of the fruit number per tree, but this is still insufficient to guarantee, at harvest, fruits of good marketable size and maximum commercial yield. For this reason supplementary fruit thinning is performed to achieve the optimum fruit load. Taking into account the practical importance of the thinning operations, the elucidation of the molecular mechanisms underlying natural fruitlet abscission is important for improving fruit thinning techniques and setting up molecular strategies useful for screening new chemical thinners or for selecting self thinning cultivars. Fruitlet abscission is a highly co-ordinated event. It involves multiple changes in cell structure, metabolism and gene expression leading to cell separation occurring in an abscission zone (AZ). Anatomical, biochemical and molecular events associated with cell separation at fruit AZ level have been described. Ethylene and IAA are deeply involved in the regulation of abscission. The general interaction between the two hormones is manifested in their antagonistic relation when the IAA status controls sensitivity of the tissue to ethylene. On the other hand, ethylene is a potent inhibitor of IAA interfering with its polar transport. Recent advances on molecular aspects referring to ethylene biosynthesis and action, as well as IAA polar transport, in relation to apple and peach fruitlet abscission are discussed.