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Single-walled carbon nanotubes (SWCNTs) emerge as promising novel carbon-based nanoparticles for use in biomedicine, pharmacology and precision agriculture. They were shown to penetrate cell walls and membranes and to physically interact and exchange electrons with photosynthetic complexes in vitro. Here, for the first time, we studied the concentration-dependent effect of foliar application of copolymer-grafted SWCNTs on the structural and functional characteristics of intact pea plants. The lowest used concentration of 10 mg L−1 did not cause any harmful effects on the studied leaf characteristics, while abundant epicuticular wax generation on both leaf surfaces was observed after 300 mg L−1 treatment. Swelling of both the granal and the stromal regions of thylakoid membranes was detected after application of 100 mg L−1 and was most pronounced after 300 mg L−1. Higher SWCNT doses lead to impaired photosynthesis in terms of lower proton motive force generation, slower generation of non-photochemical quenching and reduced zeaxanthin content; however, the photosystem II function was largely preserved. Our results clearly indicate that SWCNTs affect the photosynthetic apparatus in a concentration-dependent manner. Low doses (10 mg L−1) of SWCNTs appear to be a safe suitable object for future development of nanocarriers for substances that are beneficial for plant growth.