Background Because of their broad applications in our life, nanoparticles are expected to be present in the environment raising many concerns about their possible adverse effects on the ecosystem of plants. The aim of this study was to examine the effect of different sizes and concentrations of iron oxide nanoparticles [(Fe 3 O 4 ) NPs] on morphological, physiological, biochemical, and ultrastructural parameters in tobacco ( Nicotiana tabacum var.2 Turkish). Results Lengths of shoots and roots of 5 nm-treated plants were significantly decreased in all nanoparticle-treated plants compared to control plants or plants treated with any concentration of 10 or 20 nm nanoparticles. The photosynthetic rate and leaf area were drastically reduced in 5 nm (Fe 3 O 4 ) NP-treated plants of all concentrations compared to control plants and plants treated with 10 or 20 nm (Fe 3 O 4 ) NPs. Accumulation of sugars in leaves showed no significant differences between the control plants and plants treated with iron oxide of all sizes and concentrations. In contrast, protein accumulation in plants treated with 5 nm iron oxide dramatically increased compared to control plants. Moreover, light and transmission electron micrographs of roots and leaves revealed that roots and chloroplasts of 5 nm (Fe 3 O 4 ) NPs-treated plants of all concentrations were drastically affected. Conclusions The size and concentration of nanoparticles are key factors affecting plant growth and development. The results of this study demonstrated that the toxicity of (Fe 3 O 4 ) NPs was clearly influenced by size and concentration. Further investigations are needed to elucidate more about NP toxicity in plants, especially at the molecular level. Electronic supplementary material The online version of this article (10.1186/s12870-019-1864-1) contains supplementary material, which is available to authorized users.

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Caffeine, a purine alkaloid, is reported to act both as an inducer or inhibitor to plant growth in various species. The aim of this study was to examine the effect of exogenous caffeine on tobacco (Nicotiana tabacum) plants, a plant that does not naturally synthesise caffeine. A hydroponic experiment was carried out in a growth chamber for 14 d using Hoagland's solution supplemented with 0 (control), 25, 50, 100, 1,000; and 5,000 µM caffeine. None of the investigated caffeine concentrations significantly decreased the net photosynthetic rate except the highest concentrations of 1,000 and 5,000 µM. Light microscopy of thick-sectioned roots showed that 1,000 µM and 5,000 µM caffeine-treated plants possessed deformed epidermal cells, reduced number of cortical cells, and deformed vascular tissues with cells exhibiting thickened xylem walls as compared with control plants. Moreover, transmission electron micrographs of roots revealed that mitochondria and the plasma membrane were affected.

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