In this study, we researched the effects of different concentrations of nZnO on the rice growth and As accumulation under hydroponic conditions, to test whether nZnO can be used as an environmentally friendly agrichemical to reduce As toxicity in rice. This experiment shows that the application of nZnO can promote the rice biomass under As stress, especially for the treatment of As + nZnO50 (Fig. 1). Our early germination experiments also showed that low concentration of nZnO (10, 20 mg/L) attendment increased the shoot length by 13.1–14.1% and shoot weight by 15.1–41.2%, respectely[29]. It’s reported that kinds of NPs increase the growth of plants under metal stress. For example, Liu et al. found that nCuO treatment promoted rice growth and reduced absorption of As[30]. Similarly, it’s depicted that nZnO application improved the height, number of leaves, shoot and roots dry biomass of maize plants grown on a Cd-contaminated soil[19]. Studies have shown that Zinc NPs and Zn positively influenced the growth of sorghum and nutritional status of the plants[31, 32]. Besides, the promoted biomass with nZnO application might be associated with the decreased metal stress in rice.
Chlorophyll is an important substance for plants to absorb sunlight for photosynthesis. The amount of chlorophyll affects the growth rate of plants, so it can be used as an important indicator to measure the degree of stress of heavy metals on plants[33]. Research by Rahman et al. showed that the chlorophyll content in rice is positively correlated with shoots growth[34]. It is consistent with our results. Arsenic interfere with the activity of chlorophyll synthase and hinder the synthesis of chlorophyll[35]. When the photosynthesis of the leaves is weakened, the carbohydrates produced are reduced so that the biomass of the shoots is decreased. The present study also indicated that with the application of nZnO (10–100 mg/L), the chlorophyll concentration gradually increased, and the shoots biomass showed the same trend(Figs. 1 and 3). It’s assumed that Zn plays a great role in photosynthesis induced the increase of chlorophyll concentration. Recent studies found that the application of nZnO increased the chlorophyll content of plants under Cd and Pb stress[17, 20].
Oxidative stress is generally considered to be possible mechanisms of phytotoxicity caused by heavy metals. The photosynthesis of chloroplasts is disturbed under arsenic, which causes oxygen to become its electron acceptor, and the metabolite ROS is also produced[36]. Subsequently, in order to prevent oxidative stress, plants can actively activate various enzyme and non-enzyme defense systems[37]. The SOD is the first defense enzyme that catalyzes the more toxic O2− to less toxic H2O2[38]. However, our results showed that 2 mg/L As treatment decreased SOD activity compared with the control significantly (Fig. 5A). The reason might be that the high concentration of arsenic destroys the antioxidant reaction mechanism of rice. SOD activity increased with the addition of nano-zinc oxide, indicating that nZnO promoted the response of rice antioxidant mechanism. Due to the strong adsorption capacity of nano zinc oxide, arsenic is adsorbed before it enters the rice plant, thus protecting the antioxidant mechanism of rice[39]. Wang et al. found that SOD activity was up-regulated by nZnO in tomato plants, supported by enhancing transcription of Cu/Zn2-SOD and Fe-SOD genes[40]. On the other hand, previous studies have reported that nZnO can promote the generation of ROS in rice plants[41]. This is consistent with the results that SOD activity decreased again when high concentration of nZnO (20–100 mg/L) was added in our experiment. The following electrolyte leakage data also show that the addition nZnO of aggravated membrane lipid peroxidation. Differently, in our early germination experiments, CAT activity had the same trend with SOD activity, but it was not significantly affected in the current experiment.
The concentration of As in rice shoots decreased with the increase of nZnO concentration (Fig. 6A), while the As concentration in rice roots depended on the nZnO concentration. Low concentration nZnO (10–20 mg/L) treatments increased the content of As in roots, but high concentration nZnO (50–100 mg/L) treatments significantly reduced the concentration of As in the roots (Fig. 6B). To investigate the cause of this phenomenon in the roots, we studied the permeability of the root cell plasma membrane. The plasma membrane is selectively permeable, which controls the transport and exchange of substances inside and outside the cell. Electrolyte leakage was further used to detect the membrane permeability. The results of this study found that As treatment significantly disrupted the integrity of the root cell membrane and the application of nZnO aggravate the destruction of the root cell membrane (Fig. 4). This may be caused by high Zn concentration in the nutrient solution released by nZnO (Figure S2). As mentioned above, studies have found that excessive Zn can cause peroxidation of plant root membrane esters, leading to further damage to cell membranes[42]. Therefore, the application of low concentration nZnO (10–20 mg/L) cannot reduce the absorption of As by roots. Although 50–100 mg/L nZnO application also aggravates the destruction of the root cell membrane, As concentration in the roots were significantly decreased because the As concentration in the nutrient solution was significantly decreased by the strong adsorption of nZnO (50–100 mg/L) (Figure S1). Unlike the roots, As concentration in the shoots continued to decline with the nZnO increasing application. What happened when As transported from root to shoot? Transport factor (TF) was calculated, which is used to calculate the transport capacity of As in plants[26, 43]. It’s found that the nZnO application (10–100 mg/L) decreased As TF when rice was exposed to As stress. As speciation in rice tissues and PCs concentration in roots explained the phenomenon. We found that As speciation in rice roots and shoots was dominated by arsenite (95.01–99.58%) as reported by other reports[44, 45]. There is no significant difference of the As speciation in the roots when rice plants treated with nZnO, but arsenite percentages in the shoots were significantly decreased by nZnO application. Besides, we found that PCs concentration in rice roots were significantly up-regulated by the nZnO application. So it’s assumed that nZnO application up-regulates the PCs concentration in rice roots, causing more As(III) stored in the root cell vacuole in combination with PCs[46]. Then the As(III) transport to the shoot was suppressed, causing the arsenic concentration and the arsenite percentage also decreased in the shoots. Another possible reason is that the increase of Zn promoted the growth of plant shoots (Fig. 1), then the concentration of As in the shoots was diluted by the increasing biomass. We found that Zn concentration had a negative correlation with As concentration in rice shoots, and the Pearson correlation coefficient was − 0.973.
On the other hand, the application of nZnO increases the concentration of Zn in rice shoots and roots (Fig. 6). Zinc concentration in shoots and roots of rice showed the increasing trend with nZnO applied. nZnO have been used as a source of fertilizer in many studies to improve plant growth[15, 47]. It has been shown that application of 200 mg/L nZnO enhanced plant heights and plant weights approximately 105–113% and 122–160%, respectively[48]. ZnO nanoparticles not only increase the bioaugmentation of Zn but also improve the nutritional quality of plants. For example, compared with the control, the addition of nZnO to tomatoes increases the lycopene content[49].
In summary, in this study, we found a significant effect of nZnO on rice seedling growth, biochemical reactions, and arsenic uptake. The results showed that nZnO can be used as a fertilizer to promote plant growth and decrease the accumulation of As in rice. According to the plant growth and As accumulation, the optimal concentration of nZnO is 100 mg/L. A whole life cycle study in the soil system will be conducted to further determine the interaction of nZnO and As in plants.