## 7 The Effect of Thinning on Physiological Processes  
Thinning, especially chemical thinning, is a serious interference with plant physiological processes. The use of chemicals, especially desiccants, may lead to leaf damage [16], which forces the plant to regenerate and repair injuries, causing the consumption of stored reserves.  
However, at the initial stage of fruit growth, assimilates are supplied from reserves accumulated in wood, and not from developing leaves. Therefore, it is assumed that the use of desiccant agents, also those damaging the leaves, will not have a significant impact on fruit set [16,17]. Later, however, fruit is nurtured by compounds produced by photosynthesis in leaves, which is reflected in the level of carbohydrates--leaves of non-fruiting cherry trees were found to have a higher concentration of carbohydrates comparedto fruit-bearing ones [113]. The distribution of assimilates is also affected by the removal of a part of spurs in the dormancy period. Such pruning improves the distribution of assimilates among a smaller number of fruit and, thus, can ensure a better balance between growth and fruiting [44].  
Thinning has been found to influence the mineral composition of leaves. Hand thinning of flowers improves the absorption and content of nutrients. Thinning of sweet cherry flowers affects the seasonal changes in the absorption of micro- and macroelements in cherries. The content of nutrients in leaves differed significantly between treatments carried out before, during and after fruit ripening [41].  
Photosynthetic activity can be monitored by measuring leaf gas exchange. It has been observed that leaf gas exchange is reduced after thinning. This is because it takes some time for leaves to regenerate after treatment. Among trees thinned with desiccant agents, the leaves of trees thinned with ATS regenerated the fastest, and ATS reduced gas exchange only slightly [20]. However, note that phytotoxicity of desiccant agents, such as ATS, depends on the course of weather conditions during treatment [16].  
Oil-based preparations used for thinning may also have negative side effects, such as choking of stomata, which limits leaf gas exchange and increases intercellular carbon dioxide concentration thus resulting in plant stress [114]. Phytotoxic activity manifest in leaf burn was observed on trees treated with ATS, tergitol, and fish oil mixed with California liquid. Those substances probably damage the structure of photosystem 2 and the thylakoid membrane [21,115], and consequently reduce fluorescence (Fv), gas exchange (NCER), and stomatal conductance (gs) [21]. Young, developing leaves are more vulnerable to damage, so thinning agents hinder photosynthesis in those leaves to a larger extent than in mature leaves. This is likely due to the yet unfully developed ability to photosynthesize, unripe stomata [116], and higher absorption of thinning preparation by the tender epidermis [20].  
Some thinners damage the photosynthetic complex absorbing light and increase energy dispersion through non-photochemical processes [117,118]. A similar fluorescence reduction was observed in several tree species in response to foliar application of salt [117]. Treatments with fish oil combined with California liquid (FOLS) after flowering did not affect fluorescence but significantly lowered the efficiency of photosystem 2 (\(\Phi\)PSII) [21].  
Thinners may also slow down the formation of chlorophyll in leaf tissues. Chemical thinning was observed to reduce chlorophyll content by 6-19%. The increase in SPAD values (chlorophyll content) after chemical thinning is positively related with the increase in leaf NCER (CO\({}_{2}\) exchange rate) [20], which proves that lower chlorophyll content is a factor limiting photosynthesis in young leaves [119].