## 1 Introduction  
Due to the serious ecological and environmental problems caused by the extensive use of pesticides, the world faces a serious pressure and challenge of reducing application and increasing effective utilization of pesticide. Improving the transport performance of pesticides in target crops is one of the important methods to improve pesticide utilization. Recently, the use of nanotechnology in crops is increasing, one of the applications is the study of controlled release of pesticides and another important is to promote uptake and transport [1, 2]. Particle size is one of the most important factors that endow nanomaterials with special physical, chemical, as well as biological properties, bringing a wide range of promising applications [3]. In most cases, pesticides can be transported across biological membranes through passive diffusion or ion-trapping mechanisms, therefore, the primary way to improve the transmission performance is to increase membrane permeability by optimizing the physical and chemical properties of the active ingredients. The permeability can be changed by changing the lipophilic property, and particle size of the pesticide by loading on the carriers [4, 5, 6]. To take advantage of unique properties of nanoscale materials and structures, the size of nanoparticles need to be optimized, achieving their best control effect. Nanoparticlesof extraordinarily small sizes are able to penetrate through tissues and be translocated from one pot to other organs in plant. In biological system, the penetration and translocation of foreign substances in vivo is highly dependent on the particle size [7,8]. Many studies focus on the size effect of nanoparticles on medicine translocation and distribution in marine mammals [9] or on cancer cells [10,11]. However, there are limited studies on the size effect of nanoparticles on pesticide loading, release, and delivery in plant. Various nanomaterials are employed for pesticide delivery, including inorganic nanoporous materials [12], such as calcium carbonate [13], TiO\({}_{2}\)[14], silica [15,16], metal oxide [17], and biodegradable polymer nano-materials such as chitosan [18] and nano-hydroxyapatite [19]. Among them, mesoporous silica nanoparticles (MSN) appears to be the most attractive [20, 21, 22, 23]. Due to the large specific surface area and high loading rate, little toxicity to plants and the characteristics of promoting plant absorption and transmission, MSN is increasingly used as pesticide carriers [24,25]. Some studies indicated that only particles with a diameter less than or equal to 50 nm can pass through the cells walls in the root systems of most plants [26,27]. [PERSON] et al. found that MSN with a diameter of 20 nm can penetrate the cells walls, enter the endodermis, the intercellular spaces, and reach the vascular tissue, to be transported to the aerial parts of the plants [28]. In our previous study, it was indicated that MSN with particle sizes between 200-300 nm could also be absorbed by cucumber leaves [29]. [PERSON] et al. suggested that plant cells under the influence of calcium, silicon, proteins, viruses, and environmental stresses might appropriately change the size limits of the absorbed substances [30]. To date, the size effect of MSN on pesticide delivery in plant was not studied in detail. Therefore, it is important to understand the nanoparticle size effects on pesticide uptake and translocation in plants, when the pesticide delivery system is designed. Pesticides in traditional formulations have some disadvantages such as coarse particles, poor water-solubility, and dispersibility, which make it difficult to give full play to their efficacy [31]. For example, Pyr, a novel strobilurins fungicide, was independently developed by the Shenyang Research Institute of Chemical Industry in China [32]. The compound exhibits fungicidal activities by binding at the coenzyme Qo site of cytochrome b, which blocks the electron transfer between cytochrome b and cytochrome c1.