## 3 Results
### Histological Changes after Treatment with (-) AuNP  
An investigation of the histological changes under the influence of (-) AuNP (two concentrations: 25 \(\upmu\)g/mL or 50 \(\upmu\)g/mL compared to the control roots was conducted in the root apex (RA; Figure 2A-N) and DZ (Figure 3A-L). The RA is composed of the distal meristem (lateral root cap and columnella), proximal meristem (according to [33]) and a central organising centre, which is known as the quiescent centre (Figure 2A-C). In the meristematic zone, the rhizodermis, cortex and the central cylinder can be distinguished (Figure 2A,B). The root cap cells consist of the columnella cells, peripheral cells and border cells (Figure 2C). Figure 1: Morphology of the barley roots that were growing in the control conditions (**A–D**), 25 \(\upmu\)g/mL (-) AuNP (**E–J**) or 50 \(\upmu\)g/mL (-) AuNP (**K–O**). The red dotted lines indicate the boundary level of the (-) AuNP solution in which the roots were growing; blue dotted lines indicate the local radial thickening of the roots. Scale bars: (**A**,**E**,**K**) = 1 mm; (**B–D**,**F–J**,**L–O**) = 0.5 mm. Figure 2: Longitudinal section through the barley RA (root apex) from the control roots (**A**–**C**), the roots growing in the 25 \(\upmu\)g/mL (-) AuNP (**D**–**H**) or 50 \(\upmu\)g/mL (-) AuNP (**I**–**N**) conditions. Black arrows—large intercellular spaces between the cortical cells (**E**); red arrow—atypical division of the central metaxylem vessel in the distal meristem (**F**); red open arrows—the amyloplast in the outer cortical cells (**G**); black open arrow—rearrangement of the proximal meristem organisation (**H**). TBO staining (apart from the G inset—[PERSON]’s staining that indicates the presence of amyloplasts in the cortical cell). Scale bars: (**A**,**D**,**J**) = 50 \(\upmu\)m; (**B**,**C**,**H**,**L**,**M**,**N**) = 20 \(\upmu\)m; (**E**,**F**,**G**,**G** **Inset**,**J**,**K**) = 10 \(\upmu\)m.  
In the roots that were treated with the lower concentration of (-) AuNP (25 \(\mathrm{\SIUnitSymbolMicro g}\)/mL), disturbances were detected in the rhizodermis. The rhizodermal cells were squashed, had an unregular shape and were smaller compared to the control and had a very dense cytoplasm (Figure 2E). In the cortical cells, increased vacuoilsation was another visible alteration at the lower concentration of (-) AuNP (Figure 2E,G). A detachment of the cell files, especially between the cortical cells, was also observed (Figure 2E). In some cases, there was a visible accumulation of amyloplasts in the outer cortical cells (Figure 2G, G inset). Another disorder in the root histology that was caused by the lower concentration of (-) AuNP was the atypical divisions of the cells as was visible in the central metaxylem (Figure 2F) and in the disrupted pattern of the meristem organisation (Figure 2H). At the higher concentration of (-) AuNP, there was a disrupted morphology in distal meristem and columella mother cells (Figure 2K). Moreover, increased vacuoilsation of rhizodermal and cortical cells was observed (Figure 2J,L). In some cases, there were prominent alterations in the arrangement of the cortical cells (Figure 2M,N). The histology of the control roots of Hordeum vulgare from the DZ was previously presented and described in detail [31]. In brief, it consists of a one-layer rhizodermis  
Figure 3: Cross-sections through the DZ (differentiation zone) of the roots growing in 25 \(\mathrm{\SIUnitSymbolMicro g}\)/mL (-) AuNP (**D–H**) or 50 \(\mathrm{\SIUnitSymbolMicro g}\)/mL (-) AuNP (**I–L**). Red open arrows—multiplicated metaxylem vessels; black open arrow—**collapse** of the endodermis; black arrow—atypical divisions of the pericycle cells; red arrow—ticked inner periclinal wall of the endodermis; yellow asterisks—central metaxylem. TBO staining. Scale bars: (**A–I**) = 10 \(\mathrm{\SIUnitSymbolMicro m}\); (**E–G**) = 20 \(\mathrm{\SIUnitSymbolMicro m}\).