## 4 Discussion  
The fact that the in vitro cultures of the three cultivars, which were derived from photoplasma-infected plants, tested negative shows the potential of the in vitro explant excision and transfer for photoplasma elimination. At the beginning of sprouting, the pathogen may not yet have been present in the actively dividing, intensively growing apical parts of the plant shoots, which were used for in vitro culture establishment. We hypothesize that the efficiency of this procedure may depend on the cultivar, the position of the initial scions and buds on the plant, or the time of the in vitro transfer. The appearance of the initially infected Golden Delicious and Taborita explants suggests that in vitro cultures of different apple cultivars differ in their ability to cope with the infections studied. The poor growth of Taborita could have been caused by the susceptibility of this genotype to the pathogens present in high concentrations in the initial plant material. The relatively healthy and vigorous state of the infected Golden Delicious is rather contradictory to reports by previous researchers that AP photoplasma causes the stunted growth of cultures derived from this cultivar [17]. The key mechanism of cryotherapy is the selective survival of unique meristematic cells with small vacuoles in the shoot tips, which are more resistant to dehydration and subsequent damage caused by ice crystal formation during freezing due to their distinct morphological structure compared to differentiated cells [13]. In most cases, systemic infections are unable to penetrate the intensively dividing meristematic tissue at the apical tip of the plant. Cryotherapy then cuts off/destroys infected differentiated cells on the principle of a cryo-knife, leaving only healthy cells in the pathogen-free meristematic area, which are capable of further division, differentiation, and regeneration of whole plant organs [4]. In agreement with [PERSON] et al. [10], we observed different efficiencies of virus cryotherapy for the two genotypes studied. However, the fact that the initial plant material of the Taborita cultivar was obtained from a long-term-maintained in vitro gene bank must also be taken into account. Long-term maintenance of less intensively growing infected material in the form of explant cultures may have led to pathogen penetration into the meristem. In the case of Taborita, this phenomenon would disrupt the above-mentioned sanitation principle of cryotherapy. On the other hand, the vigorous growth of the Golden Delicious explants may have ensured that the meristematic zone remained virus-free. The results of the PCR tests were indirectly supported by the observed morphology of the explants: in Golden Delicious, a significant enlargement of the leaves was visually observed, which could be related to an improvement of the health status due to the elimination of the pathogens studied, whereas the growth vigor of Taborita did not improve and even worsened after cryopreservation. We hypothesize that even a fraction of the meristematic cells may have been damaged during cryopreservation, resulting in a phytohormone imbalance and decreased differentiation potential. This fact, together with the effect of persistent systemic pathogens, may have caused the subsequent growth deterioration. We confirmed the possibility of using cryotherapy for photoplasma eradication, proposed by [PERSON] et al. [13; 14], for apple cultures. However, the results suggest that sanitation does not work equally for different cultivars, and further studies are needed. The possibility of specific interaction between the systemic pathogen and the particular apple genotype as a host may explain the difference in the success rate of the sanitation procedure. The possible uneven distribution of photoplasma or virus particles in the plant explants must also be taken into account when performing diagnostics after sanitation procedures, subsequent regeneration, and in vitro cultivation.