Several studies have shown that GO accelerates seed germination. Notably, the effects of GO vary by plant species. For example, in Festuca arundinacea seeds treated with 0.2 mg/L of GO, germination significantly increased [22]. A concentration of 50 mg/L of GO also significantly stimulated the seed germination of spinach and chive [23]. Low concentrations of GO (50, 100, and 150 mg/L) significantly promoted the seed germination of A. fruticosa. [24]. The mechanism by which seed germination is promoted may be that GO is able to penetrate seed husks, and the penetration may break the husks to facilitate water uptake, resulting in rapid seed germination and a higher percentage of germination rate. The oxygen-containing functional groups of GO collect water and the hydrophobic sp² domains transport water to the seeds to accelerate the germination of plants [19,23,36].

A small amount of GO was found to slightly promote the plant height and significantly increase the stem and leaf biomass of Medicago sativa (alfalfa) [25]. A concentration of 0.2 mg/L of GO could increase the plant height and biomass of Festuca arundinacea [22]. Guo et al. found that low concentrations of GO (50 and 100 mg/L) promoted the growth of mature tomato plants, and increasing the dose to 200 mg/L did not significantly affect the stem diameter and weight [20]. In particular, Park et al. pointed out that an appropriate amount of GO had a positive effect on the growth of A. thaliana L., as indicated by the increases in the length of roots, the area of leaves, the number of leaves, and the formation of flower buds [26]. In addition, Cao et al. found that 10–100 mg/L of GO promoted the growth rate of the aboveground parts of Populus alba L., cuttings in an approximately concentration-dependent manner. They speculated that GO promoted the growth of this plant by improving soil fertility [27]. Guo et al. believed that GO might effectively promote the growth of tomato plants by stimulating cell division in the shoots/stems in a concentration-dependent manner [20]. In contrast, the study by Zhang et al. revealed the role of GO in promoting Aloe vera growth by stimulating photosynthesis. They demonstrated that 10–100 mg/L of GO, with the best efficiency at 50 mg/L, could exhibit positive effects on the growth of Aloe vera L. by enhancing the photosynthetic capacity of leaves, increasing the yield and morphological characteristics of leaves, and improving the nutrient (protein and amino acid) contents of leaves [18]. Similarly, the authors also found that 50 mg/L of GO treatment could promote the growth rate of elm cut seedlings by increasing the stomatal density, the stomatal conductance, and the intercellular CO₂ concentration of leaves, thus improving the plant’s photosynthetic efficiency [28].

Root elongation is an important process in plant growth and development. In comparison to the aerial part, 10–100 mg/L of GO showed a greater influence on the root growth of Aloe vera. Its root fresh weight, total root length, total root surface area, and total root volume were all significantly elevated by different concentrations of GO treatment [18]. A concentration of 100 mg/L of GO could promote the root growth of wheat seedlings [29] and the rhizome elongation of rice [30]. A concentration of 50 mg/L of GO treatment remarkably increased the total root length, the root volume, and the number of root tips and forks of maize seedlings compared to those of the control group [21]. A concentration of 20 mg/L of GO promoted the number of adventitious roots in tobacco seedling [31], whereas a concentration as low as 0.1 mg/L of GO could achieve a similar effect on Gala apple plants [32]. These results suggest that the effects of GO in promoting root development vary by plant species and depend on the dosage of GO used.

Similar to most growth regulators, GO has a concentration-dependent effect on plant growth, and therefore, an optimal concentration exists for inducing such effect. For example, Zhang et al. treated maize with different concentrations (0, 25, 50, 100, and 200 mg/L) of GO in the soil. The growth state of the maize plants was analyzed after 14 days of the GO treatment to determine the optimal concentration (50 mg/L) [37]. With an increasing GO concentration, the root length, the root tip number, and the root specific surface area of the raspberry seedlings all showed a trend of first increasing and then decreasing; the optimal concentration of GO for promoting the growth of raspberry was 2 mg/L [33]. The formation and development of adventitious roots in raspberry were inhibited at 4 mg/L and higher concentrations of GO, probably because of its toxic effects. The results of this study demonstrated that only appropriate concentrations of GO could promote the growth of plants. Compared to the untreated control samples, 50 mg/L and 100 mg/L of GO significantly increased the surface area of the root tips and hairs of tomato roots. Specifically, GO increased the total surface area and the total projected area by 31% and 27%, respectively, compared to the control samples [20]. In addition, Guo et al. found that the root morphological indices of quinoa seedlings, which were grown in GO concentrations of 4 and 8 mg/L, were significantly higher than those of the control group, indicating that these specific concentrations of GO could promote the root growth and morphological development of quinoa [34]. The abovementioned research suggests that GO should be used at appropriate doses to improve the growth of plants and that it is a promising nano-carbon material for agricultural use.

Studies have shown that the positive effects of GO treatment are associated with an increase in antioxidant enzyme activity and indoleacetic acid (IAA) levels [20,38]. IAA is the most common natural auxin that regulates the root system architecture and growth [39]. Guo et al. found that auxin content increased markedly in the roots of GO-treated plants. They explained that the increase in the root surface area after GO treatment was due to the auxin-induced activation of quiescent pericycle cells and the initiation of cell division [20]. The results of Jiao et al. indicated that GO promoted root growth by affecting the IAA pathway in wide type tomato [38]. A concentration of 20 mg/L of GO resulted in increased transcript levels of the mRNA of various IAA, such as IAA3, IAA4, IAA7, ARF2, and ARF8, resulting in enhanced growth in tobacco seedling roots [31]. A concentration of 0.1 mg/L of GO increased the transcript levels of Adventitious Rooting Related Oxygenase 1 (ARRO1), Transparent Testa Glabra 1(TTG1), and Auxin Response Factor 19 (ARF19) in apples, which played various roles in the formation of adventitious roots, lateral roots, and root hairs [32]. Moreover, the study by Zhang et al. revealed that GO treatment induced changes in the expression of a large number of genes in response to stress in maize. Transcription factors BEARSKIN2 (BRN2), NAC domain-containing protein 2 (NAC2), and MYB domain protein 93 (MYB93), which are closely related to root growth, might be the candidate downstream genes of GO [37]. Gao et al. demonstrated that 50 mg/L of GO up-regulated the expressions of IQM3, ARF7, ARF19, ERFII-1, and IQM3, which promoted taproot elongation and an increase in lateral root number in A. thaliana. The authors proposed that the up-regulation of root-related gene expression was one of the main reasons that GO promoted the growth of the root system [40].

A GO concentration of 0.5–1.5% inhibited the germination rate of alfalfa [49]. Under the treatment of 10 mg/L, GO significantly inhibited the water absorption rate after soaking for 3–6 h, delayed the germination of rice seeds, and reduced the seed germination rate [50]. Gao et al. observed that stress due to a high concentration (>200 mg/L) of GO inhibited rice and wheat germination, which showed a dose–effect relationship [30]. However, GO inhibited the germination of wheat seeds at high concentrations (≥0.4 mg/mL) [51].

Compared to the control group, GO at 100 and 250 mg/L reduced the shoot biomass (25% and 34%, respectively) and the shoot elongation (17% and 43%, respectively) in rice [52]. Anjum et al. reported the negative impacts of GO concentrations (in decreasing order, 1600 > 200 > 100 mg/L), as indicated by the decreases in growth parameters [35]. High concentrations inhibited rice rhizome elongation, and different concentrations (100, 200, 300, 400, and 500 mg/L) of GO inhibited wheat rhizome elongation [30]. Consistent with the above study, 0.5–1.5% of GO concentration had a significant inhibitory effect on alfalfa seedling growth [49].

A concentration of 400–1000 mg/L of GO inhibited wheat seedling root growth [29]. A concentration of 200–800 mg/L of GO inhibited root elongation and reduced the number of lateral roots in wheat plants [53]. A concentration of 200 mg/L of GO treatment decreased the main root length and the root/shoot ratio of maize seedlings [54]. Consistently, 25–100 mg/L of GO treatment shortened the seminal root length of Brassica napus L., compared with the control samples. The fresh root weight decreased when being treated with 50–100 mg/L of GO [17]. In addition, Shen et al. demonstrated that GO significantly affected the development of rice roots, but the effects varied depending on the GO concentration and the rice variety. The highest concentration of used GO (50 mg/L) reduced the root length, the fresh weight, and the dry weight for all five rice species [55]. GO (4 mg/L) treatment could inhibit the growth and the development of adventitious roots in raspberry seedlings [33]. A concentration of 0.1–10 mg/L of GO could inhibit the adventitious root length, the moisture content, and the number of lateral roots in apple plants [32]. The root growth of ryegrass was not affected by 1–2% of GO, but it was inhibited by higher dosages of GO (3–5%). With an increase in GO dosage, the root volume and biomass of ryegrass decreased [56].

Studies investigating the negative effects of GO on plant physiology and biochemistry have focused on the activities of antioxidant enzymes and the MDA content. For example, Anjum et al. reported the negative impacts of GO, as indicated by the activities of H₂O₂-decomposing enzymes, such as CAT and ascorbate peroxidase (APX), and by the increases in the levels of EL, H₂O₂, and lipid and protein oxidation [35]. Similarly, treatment with 0.1–10 mg/L of GO increased the activities of oxidative stress enzymes, including CAT, POD, and SOD, in apple plants, relative to their controls. In addition, the MDA levels decreased at 10 mg/L of GO [32]. However, after treating maize seedlings with different concentrations of GO, there were no significant differences in the SOD or POD activity, while the CAT activity and the MDA content increased with an increase in GO concentration [54]. Additionally, it was found that treatment with 5–100 mg/L of GO had no significant effect on the MDA content, but it did affect the SOD, POD, and CAT enzyme activities [17]. Interestingly, although GO treatment raised the EL and the MDA content in Aloe vera, GO treatment did not increase the root antioxidant enzymes’ activities or decrease the root vigor [18]. These results suggest that high doses of GO induce oxidative stress in plants, leading to damage. These experiments indicate that the adverse effects of GO on plant growth and development are very complicated and depend on the plant genotype.