The main pathways for carbon dots (CDs) to enter plants include root absorption from soil/water and leaf absorption. Studies have shown that CDs can penetrate plant cells, and then be transported along with water and minerals from the roots to the stems and leaves; they are absorbed through cell walls and plasmodesmata via extracellular pathways in intercellular spaces and extracellular spaces, and then pass through the cortex and enter the xylem through the plastid pathway ^[1][35]. CDs have stable and unique fluorescence signals, providing a good pathway for tracking in plants. The process of CDs being absorbed by plants can be demonstrated using methods such as fluorescence imaging, transmission electron microscopy, or Raman spectroscopy measurement ^{[2][3][4][5][6]}[25,30,36,37,38]. Using a CD aqueous solution to cultivate mung beans, concentration-dependent red orange fluorescence enhancement can be clearly observed in mung bean seedlings under 365 nm ultraviolet light. The roots, stems, and leaves of young seedlings were observed using confocal laser scanning microscopy, and it was found that CDs mainly exist in the vascular system. Transmission electron microscopy was used to observe the transverse sections of roots, stems, and leaves, and large aggregation clusters of CDs were observed in the intercellular spaces. Therefore, CDs are absorbed by the root and enter the root vascular bundle, which is then transported to the stem and leaf vascular bundles, and then enter the intercellular space for aggregation ^[7][39]. In addition, studies on crops such as rice and corn have shown that CDs can be absorbed and used by plants through foliar spraying, thereby increasing the grain weight and yield ^[2][8][25,31].

The growth and yield of crops depend on effective photosynthesis, which is the fundamental material source for plant growth and biomass accumulation, contributing over 90% of crop biomass. One important physiological function of CDs is to enhance plant photosynthesis. Photosynthesis includes two energy conversion processes from light energy to electric energy and from electric energy to chemical energy, involving light absorption, electron transfer, photophosphorylation, carbon assimilation, and other important reaction steps. CDs typically exhibit strong absorption in the ultraviolet region (200–400 nm), but their light absorption can be extended to the visible light range due to the type and content of surface groups, as well as changes in the oxygen/nitrogen content in carbon nuclei. In the 500–800 nm range, CDs exhibit longwave absorption, converting UV light that cannot be used by plants into visible light ^[9][10][11][18,40,41]. CDs are both excellent electron donors and electron acceptors ^[8][12][13][31,42,43]. Amine functionalized CDs are strongly conjugated on the surface of chloroplasts and assist in absorbing photons to transfer electrons to the chloroplasts, accelerating the all-electron transfer chain pathway in photosynthetic reactions, thereby enhancing photosynthesis ^[14][44]. Five mg L⁻¹ nitrogen-doped CDs significantly increased the net photosynthetic rate of maize (21.51%). Further studies have shown that there is an increase in the light conversion rate, electron supply, chlorophyll content, ATP synthase (adenosine triphosphate synthase) activity, and NADPH (nicotinamide adenine dinucleotide phosphate) synthesis during photosynthesis, with a significant increase of 122.80% in the electron transfer chain rate ^[8][15][31,33]. In addition, CDs significantly increase the expression of chlorophyll synthase and chlorophyll enzyme genes in rice, which helps to improve chlorophyll synthesis and CO₂ assimilation ^[16][17][27,29]. CO₂ assimilation is a physiological process responsible for the conversion of electrical energy to chemical energy in photosynthesis, and rubisco is a key enzyme in this process ^[18][45]. Rubisco enzyme activity directly affects the photosynthetic rate and carbohydrate accumulation. Wang et al. ^[5][37] found that the rubisco enzyme activity of mung bean seedlings treated with CDs was 30.9% higher than that of the control. Similar increased effects of CDs on rubisco enzyme activity were found in plants such as rice and arabidopsis ^[2][19][25,46]. In terms of enhancing photosynthesis, CDs have more advantages in monocotyledonous plants than dicotyledonous plants do. The structural differences in the vascular system and root structure of monocotyledonous and dicotyledonous plants are the reasons for the excessive photosynthesis ^[20][10]. CDs also have different effects on the photosynthesis of C3 plants (rice) and C4 plants (corn), and their effect on the CO₂ assimilation of rice is larger than that of corn, which is because corn is a C4 plant that has an internal way to reduce photorespiration and improve the CO₂ fixation rate; CDs also significantly improve the stomatal conductance of rice. The greater the stomatal conductance is, the higher the CO₂ absorption rate in the stomata is; therefore, CDs can enhance the gas exchange capacity of plant leaves ^[21][22][28,47]. In addition, CDs can be degraded in plants to form plant hormone analogues and release CO₂ and hormone analogues to promote plant growth, and CO₂ released is further assimilated through the Calvin cycle, thus increasing carbohydrate accumulation ^[2][19][23][25,46,48]. The above research indicates that CDs have great potential to improve crop growth and photosynthesis in agricultural production. In conclusion, CDs provide artificial photosynthesis support for crops, which increases the photosynthesis rate and, consequently, increases the grain yield. The morphological characteristics of plants, such as plant height, biomass, and leaf area, and the physiological characteristics, such as stomata conductance, rubisco activity, ATP and NADP formation, PSI and PSII rates, and electron transfer chain, are all improved by CDs ^[24][49], which raises the carbohydrate levels and, finally, increases grain yield.

Research has shown that CDs can be applied as nanofertilizers, improving crop photosynthesis, while also increasing crop yield. Spraying 560 mg L⁻¹ of CD aqueous solution on leaves can increase the yield of dicotyledonous plants, such as soybeans, tomatoes, and eggplants, by 20% ^[19][46]. Wang et al. ^[5][37] found that 0.02 mg mL⁻¹ CDs increased the root length, stem length, root activity, and fresh weight of mung beans, resulting in a 17.5% increase in bean sprout yield compared with that of the control.

Photosynthesis is the main source of assimilation for crop yield formation, and the level of yield is determined by the accumulation and distribution of photosynthetic products. The continuous spraying of CDs (50 mg L⁻¹) during the vegetative growth stage of maize can increase carbohydrate accumulation during the reproductive growth stage, resulting in an increase in the 1000-grain weight and a final yield increase of 24.50%. One possible reason is that the expression of sucrose transporter (SUT) genes in leaves increased 1.61 times after treating with CDs, and the upregulation of SUT expression enhanced the transportation capacity of sucrose in the phloem. Therefore, more carbohydrates are transported from leaves to grains, thereby promoting grain filling and increasing yield ^[8][31]. Under drought conditions, spraying nitrogen-doped CDs on maize leaves promotes the synthesis of carbohydrates by enhancing photosynthesis, resulting in a 30% reduction of maize yield loss; at the same time, the starch, soluble sugar, protein, linoleic acid, and α-linolenic acid contents of grains significantly increased by 7.0%, 9.8%, 49.7%, 10.5%, and 12.3%, respectively, thus improving the grain quality ^[15][33]. An appropriate concentration of nitrogen-doped CDs can significantly promote the accumulation of lettuce biomass, significantly improving the soluble sugar and other nutritional quality indicators of lettuce ^[25][50].

Fertilizer, as the main source of crop nutrients in modern agricultural production, directly participates in or regulates crop nutrient metabolism and cycling and is closely related to crop yield and quality; adding CDs as fertilizer enhancers to different types of fertilizers can accelerate chemical reactions, accelerate nutrient decomposition, improve fertilizer release characteristics, increase fertilizer use efficiency, and promote crop growth and development ^[26][51]. Adding nanocarbon to slow-release fertilizers can promote the formation of rice tillers, increase the chlorophyll content during booting, promote dry matter accumulation, and increase the number of effective panicles and grains per panicle, ultimately leading to an increased grain yield and nitrogen fertilizer use efficiency in rice ^[27][52]. Therefore, CDs have great potential in improving crop yield and quality.

Seed germination is the first and most crucial step in plant growth, and good seed germination can help plants develop better. It has been found for both rice and wheat that seeds treated with a CD aqueous solution can promote seed germination ^[2][28][25,26]. Water and nutrient absorption and assimilation are important factors affecting seed germination, crop growth and development, and yield formation. One of the reasons that CDs promote seed germination and enhance seed vitality is that they can penetrate the hard seed coat, promote water infiltration, and facilitate seed water absorption and germination ^[20][10]. Seed germination, root development, seed moisture content, and seedling length are related to the surface hydrophilic groups (–OH and –COOH) of CDs. The hydrophilic groups on the surface of CDs provide rich binding sites for water molecules, and they are absorbed by the plant; adequate water absorption promotes seed germination and accelerates seedling growth ^[2][19][25,46]. In addition to serving as adsorption sites for water, CDs upregulate the expression of seed aquaporin genes, activate aquaporins, and reduce the rhizosphere pH, which promotes water and nutrient absorption, improves the rhizosphere microbial environment, and is conducive to seed germination and growth ^[29][30][53,54].

The absorption of water by crops is accompanied by the absorption of nutrients. Hydroxyl and carboxyl groups also endow CDs with the ability to adsorb various nutrient ions (K⁺, Ca²⁺, Mg²⁺, Cu²⁺, Zn²⁺, Mn²⁺, and Fe³⁺), which are important nutrient elements for crop growth. They interact with hydrophilic groups on the surface of CDs through hydrogen bonding and electrostatic interactions, and adsorb on the surface of CDs; when CDs enter the plant, the concentration of nutrient ions increases with CDs in the plant, and this is also the reason for the sustained and slow release of nutrients from the xylem ^[31][55]. The nutrient ion content in arabidopsis treated with CDs is higher than that in a control, indicating that nutrient ions can enter the plant with CDs ^[19][46]. Studies on coriander showed that spraying 40 mg L⁻¹ of CDs increased the content of K, Ca, Mg, P, Mn, and Fe by 64.3%, 21.0%, 26.2%, 12.8%, 56.0%, and 125%, respectively ^[32][56]. When 0.02 mg mL⁻¹ of CDs was used to treat lettuce, the N, P, and K contents of the plants increased by 4.4%, 10.8%, and 16.5%, respectively ^[33][57]. On the other hand, after treating them with CDs, the expression of genes related to aquaporins (membrane proteins used for transporting water, nutrients, and gases) is upregulated, thereby increasing water and mineral ion absorption and promoting crop growth ^[34][35][58,59]. Carbon is widely present in soil, and CDs are more environmentally friendly than general nanomaterials are. The CDs entering the soil increase the EC value of soil. The increase in EC value is the direct reason for the formation of a large amount of bicarbonate ions in soil, promoting the absorption and utilization of water and nutrients, such as nitrogen, phosphorus, and potassium, via crop roots ^[36][60]. In addition, CDs can improve the activity of nitrogenase and the nitrogen fixation efficiency of nitrogen-fixing bacteria by affecting the secondary structure of nitrogenase and improving electron transfer in the biocatalytic process. This provides an economic and environmentally friendly way to improve the biological nitrogen fixation ability of nitrogen-fixing bacteria and provide a nitrogen source for crop growth when nitrogen is insufficient ^[5][37][2,37].

In actual production, crops are constantly challenged by adverse abiotic environmental conditions. According to a report of the Food and Agriculture Organization of the United Nations (FAO), in 2007, more than 96% of the world’s rural land was affected by various abiotic stresses, including droughts, high temperatures, low temperatures, nutrient deficiency, and excessive salt or heavy metals in the soil. These abiotic stresses have a negative impact on crop productivity, leading to serious yield losses ^[38][39][61,62]. For example, drought stresses on rice and wheat after flowering can lead to premature plant senescence, reduced material production, a shortened filling period, and a reduced grain weight ^[40][41][42][63,64,65]. CDs can promote plant growth, and a large number of studies have also proved that they can improve plant resistance to abiotic stress, thereby improving crop yield. The increase in reactive oxygen species (ROS) is the main factor affecting crop growth under abiotic stress. The accumulation of ROS in cells usually leads to oxidative damage to proteins, lipids, carbohydrates, and DNA ^[43][44][66,67]. The mechanism of CDs improving crop resistance to abiotic stress is that CDs have the effect of scavenging free radicals, and the surface of CDs fused with carboxyl and amino groups can transform DPPH free radicals into stable DPPH-H through a hydrogen transfer mechanism ^[45][46][68,69]. On the other hand, CDs can increase the activity levels of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) and reduce the contents of ROS and malondialdehyde (MDA) ^[47][70]. CDs combine the free radical scavenging characteristics and the ability to improve the activity of antioxidant enzymes to protect crops from abiotic stress, providing a theoretical basis for their application in crop-stress-resistant cultivation. Research has shown that under a drought stress, the application of CDs in maize reduces the accumulation of ROS within the plant, weakens the oxidative stress caused by the drought, and promotes the synthesis of proline and abscisic acid in the leaves and long-distance transportation to the roots, thereby upregulating the expression of aquaporin (AQP) genes from 2.3 to 7.6 times. This increases the proportion of K⁺/Na⁺ (by 47.7%) and promotes crop water absorption under drought conditions (by 49.0%); in addition, it increases the transport rate of carbohydrates to the roots, increases the content of root exudates (organic acids, amino acids, etc.), and increases the abundance of rhizosphere microorganisms (proteobacteria, actinomyces, ascomycetes, etc.), promoting the absorption of nitrogen and phosphorus in maize, thereby improving the drought resistance of these crops ^[15][33]. CDs also have a positive impact on the growth of soybeans under drought conditions; applying 5 mg L⁻¹ CDs on the leaves can eliminate the accumulation of reactive oxygen species in soybean leaves under a drought stress, enhancing photosynthesis and carbohydrate transport. On the other hand, CDs stimulate the root to secrete amino acids, organic acids, auxin, and other organic substances; recruit beneficial microorganisms for the rhizosphere (such as actinomyces, ascomycetes, acidobacteria, and mycobacterium). and promote the activation of soil nitrogen. At the same time, the expression of the GmNRT, GmAMT, and GmAQP genes in the root was upregulated, promoting nitrogen absorption, utilization, and metabolism, significantly improving the nitrogen content and water absorption capacity of soybeans. In addition, it also increased the protein, fatty acid, and amino acid contents in soybean seeds by 3.4%, 6.9%, and 17.3%, respectively ^[48][49]. Wang et al. ^[50] explored the relieving effect of CDs on heat stress in Italian lettuce. The study showed that CDs enhanced the antioxidant enzyme activity and osmoregulation (manifested by a decrease in proline content) and reduced the damage caused by lipid peroxidation in plant cells (manifested by a decrease in MDA levels), thereby improving the plant’s heat tolerance. Soil salt stress is also an important factor affecting plant growth. Salt stress leads to crop nutrient deficiency, osmotic stress, and ion toxicity. Li et al. ^[51] showed that CDs exhibit Ca genes in the root was upregulated, promoting nitrogen absorption, utilization, and metabolism, significantly improving the nitrogen content and water absorption capacity of soybeans. In addition, it also increased the protein, fatty acid, and amino acid contents in soybean seeds by 3.4%, 6.9%, and 17.3%, respectively [71,72]. Wang et al. [73] explored the relieving effect of CDs on heat stress in Italian lettuce. The study showed that CDs enhanced the antioxidant enzyme activity and osmoregulation (manifested by a decrease in proline content) and reduced the damage caused by lipid peroxidation in plant cells (manifested by a decrease in MDA levels), thereby improving the plant’s heat tolerance. Soil salt stress is also an important factor affecting plant growth. Salt stress leads to crop nutrient deficiency, osmotic stress, and ion toxicity. Li et al. [74] showed that CDs exhibit Ca²⁺ mobilization characteristics and can alleviate salinity stress by enhancing Ca²⁺ signaling and ROS scavenging activity. In addition, Gohari et al. ^[52] found that putrescine-functionalized CDs can increase the K, photosynthetic pigment, proline, and phenolic substance contents and antioxidant enzyme activity levels in grape leaves, while reducing the Na signaling and ROS scavenging activity. In addition, Gohari et al. [75] found that putrescine-functionalized CDs can increase the K, photosynthetic pigment, proline, and phenolic substance contents and antioxidant enzyme activity levels in grape leaves, while reducing the Na⁺, MDA, and H₂O₂ contents, thereby alleviating salt stress and increasing the leaf fresh and dry weights. The above research provides a theoretical basis for CDs as abiotic stress modifiers to improve crop yield and plant protection. Crop diseases are important limiting factors that affect crop growth and yield formation. Currently, prevention and control measures mainly rely on the widespread use of pesticides/fungicides, and the inefficient use of pesticides seriously threatens ecosystems’ biodiversity and functions. A previous study found that CDs have broad-spectrum antibacterial activity against bacteria and fungi, laying the foundation for their application in crop disease control and improving crop disease resistance ^[53][76]. CDs can destroy the secondary structure of DNA/RNA in bacterial walls and bacteria and fungi, thus showing broad-spectrum antibacterial/antifungal activity against Gram-positive (Staphylococcus aureus and Bacillus subtilis) and Gram-negative (Bacillus sp. WL-6 and Escherichia coli) bacteria ^[2][37][2,25]. Luo et al. ^[54][77] found that foliar spraying 10 mg L⁻¹ nitrogen-doped CDs activated the acquired resistance dependent on jasmonic acid—and salicylic acid—in tomatoes, resulting in the stagnation of pathogen growth in vivo, effectively reducing the symptom severity of tomato green wilt syndrome caused by Ralstonia solanacearum by 71.19%. Therefore, CDs can be used as green and efficient antibacterial agents for preventing and controlling crop diseases.

Through the above review, rwesearchers compared the differences between carbon dots and traditional cultivation techniques in agricultural applications, mainly manifested in the following aspects. First, carbon dots can improve crop nutrient absorption and utilization efficiency, promote crop growth and development, and thereby increase crop yield by regulating plant physiological processes and metabolic pathways ^[37][2]. Traditional technologies mainly increase crop yield by improving soil fertility, fertilization, and pest control. Second, CDs functionalized with different functional groups can serve as carriers to accurately control and release agricultural inputs, such as pesticides, fertilizers, and water; improve the utilization efficiency of pesticides and fertilizers; reduce dosage; and reduce waste and environmental pollution ^[20][55][10,11]. However, traditional technologies are relatively extensive in the use of agricultural input materials, which can easily lead to resource waste and environmental pollution. Third, carbon dots can reduce the impact of environmental stress on crops by enhancing their resistance to stress and pests ^[2][37][2,25]. In contrast, traditional technologies mainly rely on chemical pesticides and biological control methods for disease and pest control, with relatively limited effectiveness. Fourth, carbon dots reduce the use of pesticides and fertilizers in crop production, reducing environmental pollution while reducing agricultural production costs. CDs come from abundant sources, have low costs, and are low in toxicity to the environment ^[56][14]. In contrast, the extensive use of pesticides and fertilizers in traditional technologies requires a significant amount of energy consumption and high costs, and low fertilizer utilization rates can easily cause environmental pollution, reducing agricultural economic benefits. In summary, the application of carbon dots has advantages in crop production compared with traditional technologies. However, the application of CDs also requires relevant environmental risk assessment and management ^[4][20][10,36]. Further strengthening the application research of carbon dots in agriculture in the future is of great significance for sustainable agricultural development.