## 1 Introduction  
Thinning is carried out regularly on stone fruit species such as peach and nectarine, but rarely on apricots, plums, sweet cherries, or sour cherries. The response to and effectiveness of thinning depend not only on the species and cultivar, but also on climatic and soil conditions and agricultural treatments, especially pruning. Yield as well as vegetative growth are related to the intensity and timing of thinning [1]. In recent times, interest in the use of thinning has increased due to increased market requirements regarding the size and quality of fruit [1]. Fruit trees produce numerous flowers that they cannot turn into fruit and maintain until harvest. The number of flowers on a tree is very large and, depending on the species and size of tree, there may be up to 50,000 flowers per sweet cherry tree and up to 20,000 flowers per peach tree [2]. In pome species, in order to obtain a good quality marketable crop, it is sufficient if only a few to about a dozen percent of flowers set fruit. For this reason, apple trees yield well when 7% of flowers have set fruit. Stone fruit species require a higher level of pollination. In peach, sufficient yield is obtained when 25% of flowers set fruit [2], whereas sweet cherries yield well with a fruit setting ratio of 25-40% [3,4]. Rootstock strongly affects the number of flowers. Fruit trees grown on dwarf and semi-dwarf rootstocks set more flower buds [5]. However, cherry trees on dwarf rootstocks often bear an excessive number of small fruit with low sugar content [6]. Therefore, flower thinning is necessary to prevent over-yielding and provide high-quality fruit [6,7] in terms of the basic fruit quality parameters, such as size, colour, total soluble solids, and firmness [2]. The fruiting of trees is affected by a number of factors, which include environmental factors, such as tree nutrition, light, and temperature. Often, however, endogenous factors are crucial, such as processes that regulate the initiation of flower buds; they take place during the summer and depend on the number of fruit on the tree. Fruit and developing seeds can, through the production of gibberellins, inhibit the initiation of flower buds and thus reduce the number of flowers in the next season [1]. Thinning is most often conducted with the use of chemicals. Chemical thinning reduces the workload compared to manual thinning and weakens the tendency to biannual bearing [2], a frequent feature of cross-pollinated cultivars. However, the number of chemical compounds approved for thinning and the protection of fruit plants is being constantly reduced in the EU. In addition, due to consumer pressure, retail chains are forcing the limitation of the number of permissible chemical treatments or the exclusion of certain active substances [8]. As manual thinning of cherries is very expensive due to the size of tree and number of fruit, it is necessary to develop a chemical thinning technology that would be especially suitable for the treatment of abundantly fruiting self-pollinating cultivars [9]. The need for thinning sweet cherries is driven by the market--a higher market price is paid for larger fruit. The size of fruit is primarily a cultivar-specific feature, but is largely influenced by the number of fruit on the tree, and this depends, among other things, on the level of pollination resulting from weather conditions prevailing in the period of pollination and fertilisation [1]. Cherry trees produce a very large number of fruit per shoot compared to peaches or apples, but as the weight of the fruit/cm\({}^{2}\) of the cross-sectional area of the trunk is low, thinning is not physiologically justified. Although cherry is a cross-pollinated species, which can sometimes cause poor pollination, the more commonly cultivated self-pollinating cultivars set much more fruit than the cross-pollinated ones and thus require thinning for the fruit to meet the quality requirements [9].