## 3 Results
### Ultrastructured Changes after Treatment with (-) AuNP  
At first, the root cap cells and rhizodermis came into contact with the AuNP. Observations at the cellular level showed some changes between the control and treated roots in these root parts. Therefore, we decided to investigate whether there were any differences in the ultrastructure of the columella root cap cells (Figure 4A-Q) and the rhizodermis (Figure 5A-L). Firstly, it was determined whether the (-) AuNP had penetrated into the root cells. Regardless of the concentration, (-) AuNP did not cross through the cell wall of the peripheral root cap cells (Figure 4F,L) or the outer periclinal wall of the rhizodermal cells (Figure 5E,I). In both cases, the (-) AuNP were retained on the cell wall surface, especially in the fibrous material structures that are characteristic of the outer cell wall of the peripheral root cap cells (Figure 4A) and the rhizodermis (Figure 5A). The ultrastructure of the columella cells was investigated along the first three layers in the root cap. In the control, each cell contained a cytoplasm with numerous ribosomes, a rough endoplasmic reticulum (ER), dictoyosomes, mitochondria, plastids and a large spherical nucleus with electron-dense heterochromatin and electron-lucent euchromatin, which is specific for the interphase nuclei (Figure 4B-E and inset on E). The cell wall, which is a junction of the proximal meristem and distal columella cells of the distal meristem, was not altered in the roots that were treated with the low concentration of (-) AuNP compared to the control and was characterised by the presence of plasmodesmata (Figure 4B,G; insets D,I). However, at the high concentration of (-) AuNP, there was a periplasmic space between the cell wall and the plasmalemma of the cells from the proximal meristem (Figure 4L). There were no changes in the plastid, nucleus or ER ultrastructure of the columella cells in the roots that were treated with the lower dose of (-) AuNP (Figure 4G-J). However, alterations were observed in the structure of the mitochondria, which had a dense matrix and dilated cristae (Figure 4B,D,G,I,N). Moreover, at the lower concentration of (-) AuNP, there was an increased vacuolisation (Figure 4J,K). At the higher concentration of (-) AuNP, a very dense cytoplasm was observed in the columella cells (Figure 4L,M). In these cells, the nucleus structure was changed compared to the control (Figure 4M). Moreover, numerous vesicles and multivesicular bodies (Figure 4M,P), as well as a periplasmic space with vesicles, were also visible (Figure 4Q). Furthermore, in these cells, electron-dense spots were visible in the cytoplasm compartments and in the vicinity of the cell membrane (Figure 4Q,P). There were plasmodesmata in all of the analysed variants (Figure 4D, inset I). Figure 4: Ultrastruct of the root cap outer cell wall of the control (**A**), those that were treated with 25 \(\mathrm{\SIUnitSymbolMicro}\)g/mL (-) AuNP (**F**) or 50 \(\mathrm{\SIUnitSymbolMicro}\)g/mL (-) AuNP (**K**) roots. The outer cell wall had a fibrous character and was a barrier to the entry of (-) AuNP into the root cells regardless of the concentration (**F**,**K**). The ultrastructure of the columella root cap cells from control (**A**–**E**), the barley roots that were treated with 25 \(\mathrm{\SIUnitSymbolMicro}\)g/mL (-) AuNP (**F**–**J**) or 50 \(\mathrm{\SIUnitSymbolMicro}\)g/mL (-) AuNP (**K**–**Q**). Red arrows indicate electron-dense spots inside the cytoplasm; yellow asterisks indicate the periplasmic space; CW—cell wall; ER—endoplasmic reticulum; M—mitochondrion; Mb—multivesicular body; N—nucleus; NU—nucleus; P—plastic; Pò—paramural body; V—vacuole; Ve—vesicles.