Carbon dots promote the growth and photosynthesis of mung bean sprouts
Graphical abstract
The impact of CDs on the growth of mung bean sprouts, including root, stem and weight.
Introduction
Nanomaterials have great potential applications in the field industry, agriculture and biology due to their small size and excellent properties [1]. Plant species and types of nanomaterials may cause different results (positive or negative) [[2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]]. Most nanomaterials, such as metal nanoparticles, show inhibition effect on plants [13]. Only a number of carbon and some metal-oxides based engineered nanomaterials have positive impact on the plant growth. In previous report, single-walled and multi-walled carbon nanotubes (SWCNTs and MWCNTs) could promote the growth of tomato, rice, gram and tobacco [[14], [15], [16], [17]]. However, these investigations are limited to the study of plant growth. TiO2 nanoparticles have been found to promote spinach growth as well as improve related enzyme activity [4,[18], [19], [20], [21], [22], [23]]. CdSe/ZnS quantum dot nanoparticles showed negative effect on the Arabidopsis thaliana [10]. CDs have attracted interest owing to their advantages in tunable fluorescent properties, easiness of surface functionalization, low toxicity in vivo and good biocompatibility [[24], [25], [26], [27]]. In addition, CDs possess excellent solubility in water compared to other carbonaceous nanomaterials, such as SWCNTs and MWCNTs [27]. Due to these advantages, CDs exhibited promising potential in bio-sensor [[28], [29], [30]], bio-imaging [[31], [32], [33], [34]], drug delivery [35] and gene transfection [36]. In recent report, CDs have showed positive effect on the growth of plants [[37], [38], [39]]. However, the research about other physiological effects of CDs on plants, such as photosynthesis, is rare to be reported.
Here, we studied the impact of CDs on the growth, root, water uptake of seeds and photosynthesis. Mung bean was served as the model plant, because it has short growth period and it is easy to study the physiological effects of CDs on plants in the short time. CDs were synthesized via hydrothermal method as previously reported [40,46]. We found that CDs could promote the growth of mung bean sprouts, including the root elongation, stem elongation and fresh biomass. Meanwhile, CDs at optimal concentration could significantly improve the root system of plants, especially the root vigor. Root is an essential part of plants, which can absorb water and nutrients to other sections of plants. Confocal images and TEM images demonstrated that CDs could penetrate the plant cells and be transported from root to leaf. In addition, CDs treatment could enhance the water absorbing capability of seeds, which is critical factor for the seed germination. It is possible that CDs improve the plant growth by enhancing the root vigor and water uptake of seeds. Further investigation exhibited that the carbohydrates content of mung bean sprouts exposed to CDs was obviously increased as compared to the control. The promotion of carbohydrates is related to the photosynthesis. To understand the role of CDs in photosynthesis, several significant factors in photosynthesis, such as photosystem activity, chlorophyll content and rubisco activity, were studied. The photosynthetic performance of photosystem (PS) were investigated firstly. Photosystem is the function unit in photosynthesis, which can capture light energy and transfer electron through a series of steps in light reaction [[41], [42], [43], [44], [45]]. If the activity of photosystem is increased, it is beneficial for plants to facilitate the light reaction and then accumulate more carbohydrates. Experimental results showed that CDs treatment could improve the activity of photosystem 1 (PS1) but not photosystem 2 (PS2). We speculated that CDs, as a good electron donor and electron acceptor, may accelerate the electron transfer rate of photosystem. Further investigation indicated that the addition of CDs improved the electron transfer rate of photosystem 1 but not photosystem 2. PS1 contains Fe-S cluster but PS2 not. CDs could combine with Fe in noncovalent bond, which was supported by the results of photoluminescence and lifetime. It is supposed that CDs with electron donating and electron accepting properties may combine with PS1 and accelerate the electron transfer in PS1. Chlorophyll, as an important photosynthetic pigment, can capture light energy in light reaction. The content of chlorophyll is related to the photosynthesis rate. The promotion of chlorophyll content is good for absorbing more light energy in photosynthesis. Rubisco (ribulose bisphosphate carboxylase oxygenase), as a key enzyme in photosynthesis, can catalyze CO2 fixation in dark reaction. The increase of rubisco activity is beneficial to the produce of carbohydrates. CDs treatment could enhance the production of chlorophyll and the activity of rubisco effectively. Based on these observations, we drew a conclusion that CDs treatment enhanced the formation of carbohydrates by influencing the photosystem activity, chlorophyll content and rubisco activity. In general, these experiments suggested that CDs could promote the growth and photosynthesis of mung bean sprouts. This study not only provides a simple model to understand the interaction between carbon dots and plants, but also indicates that carbon dots have becoming a promising star in the application of agriculture.
Section snippets
Materials and characterization methods
All the chemical reagents were purchased from USA Sigma-Aldrich and Adamas-bate, which were of analytical grade and used without further purification.
Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were collected by FEI-Tecnai F20 (200 kV). The UV–visible spectrum was measured with Perkin Elmer Lambda 750 UV–vis spectrophotometer, while the Fourier transform infrared (FTIR) spectrum were obtained with Varian Spectrum GX spectrometer.
Characterization of CDs
The CDs were characterized by TEM and HRTEM to investigate their morphology and structure. As shown in Fig. 1a, the TEM image of CDs exhibits the presence of well-separated and spherical morphology with average diameter of approximately 4–6 nm. The lattice spacing of CDs is 0.21 nm observed from the HRTEM image (the inset of Fig. 1a), which is close to the (100) diffraction facet of graphite [40]. The dynamic light scattering (DLS) of CDs shown in Fig. 1b displays that the particle size
Conclusions
We have studied the effect of CDs on the growth of mung bean srpouts, including the growth and photosynthesis. The uptake, transmission and distribution of CDs in mung bean sprouts were carried out by confocal and TEM images, revealing the CDs could penetrate plant cells and be transported from roots to the stems and leaves of plants. First, CDs show a dose-response on the root elongation, stem elongation and biomass. The reason for growth improvement were discussed in two aspects, the root
Acknowledgements
This work is supported by the Collaborative Innovation Center of Suzhou Nano Science and Technology, the National Natural Science Foundation of China (51725204, 21771132, 51572179, 21471106, 51422207, 21501126), the Natural Science Foundation of Jiangsu Province (BK20161216), China Postdoctoral Science Foundation (2017M611902) and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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