## 2 Thinning Methods
### Chemical Thinning  
Chemical thinning is much cheaper and faster than manual thinning not only because it involves significantly less workload compared to manual thinning [2], but also because standard plant protection equipment can be used to carry it out [63]. Chemical thinning involves the use of blossom desicants, growth regulators, and photosynthesis inhibitors. Dessicant agents, include e.g., fertilisers (ammonium thiosulfate (ATS), urea), and some sulphur compounds [17; 18; 28; 31]. They cause damage to the stigma and anthers, which prevents pollination [19]. Growth regulators, such as hydrogen cyanamide, interrupt dormancy, stimulate uniform development of flower buds, but also cause flower thinning if applied to flowering plants [28]. Other substances that have been found to be effective chemical thinners include herbicides or desiccants used in agricultural production [80; 81] or oils of vegetable and animal origin. In the United States, research has been conducted on the use of cyanoamide, monocarbamide dihydrosulphate, pelargonic acid, endothalic acid, and Tergitol TMN-6 as chemical thinning agents. In Europe, studies are carried out on the use of ammonium thiosulfate (ATS) and plant growth regulator 2-chloroethylphosphonic acid (ethephon), which are commonly applied to apple and pear trees during flowering [19; 30; 82; 83; 84; 85; 86; 87; 88; 89; 90; 91; 92; 93; 94; 95; 96; 97; 98; 99; 100; 101; 102; 103; 104; 105; 106; 107; 108; 109; 110; 111; 112; 113; 114; 115; 116; 117; 118; 119; 120; 121; 122; 123; 12  
in the cultivation of apples and pears are to improve fruit quality and prevent alternating bearing [40,84]. Chemical thinning can be applied to stone fruits to a limited extent only [85]. The period during which chemical thinning can be carried out on cherries is shorter than for apples or pears, i.e., only during flowering or shortly thereafter [86]. The advantage is that the removal of flowers and buds in the initial period of growth increases assimilation, thereby reducing competition between the vegetative and generative organs of the tree. The result is stronger growth in the year of treatment, better fruit quality, increased yield and greater differentiation of flower buds responsible for flowering in the following year [9,19]. Gibberellic acid inhibits the formation of flower buds in many pome species but also in stone species such as peaches, nectarines, cherries, apricots, plums, and almonds [87,88]. Its use during the initiation of flower buds allows reducing the total number of flowers in the next growing season. The timing of application is important, since the development of buds can be influenced from full flowering to mid-season [89,84,88,86,89,90,88,91,92]. One or two applications of gibberellic acid (GA\({}_{3}\)) can be carried out during endocarp lignification (the end of first phase of fruit development) or when fruit colour changes from green to red (the end of the second phase of fruit development), whereas double GA\({}_{3}\) application has been found to more effectively reduce flowering density and yield [25]. Trees treated with two applications (50 and 100 mg\(\cdot\)L\({}^{-1}\)) yield fruit with higher soluble solids, higher firmness, and greater weight. Just like double spraying, a single application of two different gibberellic acids (GA\({}_{3}\) and GA\({}_{4+7}\)) at a dose of 100 mg\(\cdot\)L\({}^{-1}\) also reduces flowering density. GA\({}_{3}\) alone inhibits flowering more than GA\({}_{4+7}\) even if both are applied at an equal dose (200 mg\(\cdot\)L\({}^{-1}\)). The simultaneous use of both acids additionally delays the flowering date by 4-5 days and improves fruit firmness, but does not affect fruit weight and total soluble solids [25]. The application of growth hormones gives satisfactory results if it is correlated with the period of induction and differentiation of flowers specific to a given species and sometimes a cultivar [25,91,92,93]. Another hormone, cytokinin, can be used for fruitful thinning. Cytokinin treatment is carried out after petal fall.