Synthesis Of Novel Selenium-Doped Cyanobacteria-Based Carbon Quantum Dots And Application In Selenium-Enriched Tomato Plants

Nanotechnology has created many new applications in the field of plant nutrition,facilitating plant growth and nutrient uptake.Nano-fertilizers can be applied to plants through leaves or roots,thus improving the efficiency of nutrient utilization and reducing the loss of nutrients.In recent years,selenium nanoparticles have shown broad application prospects in agricultural production.Novel selenium nano-fertilizer is one of the most effective strategies to improve selenium utilization and promote the growth of crops.Therefore,novel selenium-doped cyanobacteria-based carbon quantum dots（Se-CQDs）were prepared by two methods using cyanobacteria from Taihu Lake as the carbon source,and physicochemical properties were comparatively investigated.Moreover,this study suggested the effects of different Se-CQDs on growth and selenium bioavailability of tomato,and unveiled the molecular mechanisms of Se-CQDs to improve selenium utilization at the level of gene expression and rhizosphere bacterial community.Furthermore,the effects of leaf-applied Se-CQDs on photosynthesis and fruit quality were shown,and the uptake and translocation mechanisms were revealed.The main results are as follows:（1）The diameter of Se-CQDs synthesized via co-cracking method（Se-CQDs1）was smaller than those synthesized via adsorption-reduction method（Se-CQDs2）.The content of negative surface charge was in the order of Se-CQDs2（-26.76±2.07 m V）>Se-CQDs1（-13.56±3.69 m V）.The doped selenium was reduced to Se⁰ on the surface of Se-CQDs2,while more selenium was fused into the carbon skeleton of Se-CQDs1.Moreover,Se-CQDs have a large number of oxygen-containing functional groups,photoluminescent properties,low toxicity,good biocompatibility and the slow release of Se.（2）Soil application of Se-CQDs had significant growth-promoting and yield-increasing effects on tomato,especially Se-CQDs1（2.5 mg/kg）.Se-CQDs1（2.5mg/kg）significantly increased above-ground dry weight（98.9%）and fruit yield（59.3%）compared to Na₂Se O₃.Similarly,root length（103.9%）,root surface area（66.2%）,mean diameter（42.1%）and number of root tips（96.9%）were significantly increased.Se-CQDs1 up-regulates the expression of water channel protein genes（PIP）and growth hormone synthesis genes（NIT）（87.7%）,thus promoting the absorption and transport of nutrients in tomato and increasing the content of mineral nutrients in fruits.（3）Se-CQDs improved the crop utilization of Se and bioavailability of tomato fruit.Compared with Na₂Se O₃,Se-CQDs1 and Se-CQDs2（2.5 mg/kg）treatments significantly increased the fruit selenium content by 120.9%and 70.2%,respectively.The expression of the key genes of selenium metabolism,selenium methyltransferase gene（SMT）and methionine methyltransferase gene（MMT）,was up-regulated,indicating that the effect of Se-CQDs1 on tomato selenium metabolism was significant.Meanwhile,Se-CQDs1 increased the abundance and diversity of soil bacterial community,especially the increase of relative abundance of Actinomycetes phylum,which could stimulate the root development and promote the uptake of selenium and other mineral nutrients by plant mycorrhizae.（4）Foliar application of Se-CQDs1 promoted photosynthesis and improved the quality of tomato fruits.Se CQDs1（0.5 mg Se/L）significantly increased chlorophyll content（60.1%）and net photosynthetic rate（Pn）（41.0%）.Meanwhile,foliar application of Se CQDs1 increased the content of mineral nutrients and selenium in tomato fruits.Moreover,the content of vitamin C and soluble protein in the fruit has been increased,and the antioxidant enzyme activity of tomatoes has also been enhanced,improving the quality of tomato fruits.（5）The physicochemical properties of Se-CQD1 facilitated the uptake on the leaf surface,and promoted the transport and effective accumulation of selenium to the fruit.Se-CQDs1 has smaller hydrodynamic size（～106.5 nm）,which facilitates higher leaf penetration;less negative surface charge（-13.56±3.69 m V）contributes to cross the biofilm;and the stable structure of selenium and carbon skeleton conduces to the slow release of selenium and improves selenium utilization.