Photosynthesis Inhibition as a Tool for Apple Fruitlet Thinning

The possible mechanisms of apple fruitlet abscission are presented in this paper. Prevailing hypothesis on hormonal triggered processes in the event of “correlative dominance effects” of adjacent fruitlets or nearby shoot tips has the opponent theory of assimilate supply dependent fruitlet abscission. Assimilation shortage may also be involved in the correlative auxin processes but the connections of both statements are not clear yet. On last years a lot of evidence is provided in support of assimilation dependent fruitlet abscission process. At the time just after the end of the flowering, a strong competition between fruitlets and shoots for photosynthates exist. Shading experiments comparable for a few days of cloudy weather provoked strong natural apple fruitlet abscission. If the applications of chemical thinners are followed by a few days of shading, much stronger abscission occurs comparing to thinner applications without shade. A photosynthesis inhibitor metamitron has gone into registration for fruitlet thinning purpose and is available in Europe last year. A good thinning results can be achieved up to 14 mm king fruit diameter, but additional studies should be made to adapt dosage for the present and predicted light intensity at the time just after metamitron application