## 4 The Impact of Thinning on Fruit Quality  
Size is one of the most important fruit characteristics that consumers pay attention to and that affects the fruit's market value [105; 106]. Fruit quality is directly related to the intensity of flower and fruit thinning. Studies show that an increase in yield is often accompanied by a smaller fruit size [17; 33; 40; 105]. The response to thinning depends on the age of the tree, the quality of flower buds, the competition between flowers in the inflorescence and the tree crown, and the timing of the procedure. In addition, the order in which flower buds open on shoots can indirectly affect protection against frost and, at the same time, the effectiveness of thinning. Buds on short shoots generally develop faster than those on long shoots, and therefore it is advisable to thin short shoots if there is a risk of frost. On the other hand, delayed thinning may cause that mainly buds on weak long shoots, which flower latest, will be removed. In addition, treatment delay affects vegetative growth and differentiation of flower buds for the next year [90]. Fruit on short shoots (less than 25 cm) reach smaller sizes, because those shoots have too many flower buds and too few leaves to produce fruit of the right size. Therefore, in order to improve the fruit size in cultivars that produce a large number of short shoots, shoots should be headed back so as to leave only one bud [35; 42; 62; 107]. Such thinning intensity improves the fruit size, coloration, firmness, and content of sugars and acids [12; 107]. The effect of thinning on quality parameters also depends on the cultivar. A high crop load and a thinning treatment carried out 6-8 weeks after flowering caused a significant decrease in fruit size of 'Sweetheart' sweet cherries, but there was no decrease in fruit diameter when thinning was conducted earlier. The date of thinning also affected firmness and TSS (Total Soluble Solids) concentration, for example, no reduction in fruit acidity was observed in fruit from trees thinned immediately after flowering [35]. The size of fruit is also associated with greater availability of assimilates during the period of cell division, which occurs up to 11 days after flowering. Later, the fruit grows only as a result of an increase in the volume of cells [108; 109]. Thinning after flowering increases the susceptibility of fruit to cracking at lower crop load levels [110]. As consumers prefer large cherries, new sweet cherry cultivars should be able to bear fruit with a minimum diameter of 25 mm. According to consumer expectations, new cherry cultivars introduced to the market should have a weight of 11-13g, a diameter of 29-30 mm, TSS of 17-19%, firmness of 70-75 UF, and juice pH of 3.8 [111]. Thinning of flowers and spurs increases the share of fruit with a diameter over 25.5 mm [40]. Fruit from two-year-old spurs has been found to have higher TSS compared to fruit from other tree fruiting parts [37]. Thinning significantly increases not only the fruit size (Table 2), but also the level of TSS and anthocyanins, and the total antioxidant capacity of seeds. Therefore, the opinion that thinning can increase the organoleptic and nutraceutical properties of cherries has gained acceptance [36]. A higher leaf area-to-fruit ratio results in a higher fruit mass, a darker fruit colour, a higher TSS, a higher ratio of sugars to acids, and earlier fruit ripening [112]. In addition, the content of glucose, fructose, and sorbitol; the sum of individual sugars; and the content of malic acid differed significantly depending on the thinning treatment. By contrast, a low leaf area-to-fruit ratio may prolong the ripening process [112]. Years with high rainfall are associated with the problem of fruit cracking, which is cultivar-specific. Studies of the relationship between crop load and the incidence of cracking have shown a negative correlation. The cracking tendency can be determined, but only after the end of cell division.