Agrochemical application practices are inefficient and require more sustainable approaches. Foliar application of engineered nano-carriers could provide highly efficient and controlled delivery of agrochemicals into plants, but the physical chemical properties controlling leaf uptake and translocation in plants are unknown. Here, we systematically evaluated the effect of nano-carrier size, charge, and hydrophobicity on their uptake, phloem loading, and distribution in tomato (Solanum lycopersicum) plants. Poly(acrylic acid)-block-poly(2-(methylsulfinyl)ethyl acrylate) (PAA-b-PMSEA) and poly(acrylic acid)-block-poly((2-(methylsulfinyl)ethyl acrylate)-co-(2-(methylthio)ethyl acrylate)) (PAA-b-P(MSEA-co-MTEA)) star polymers were synthesized using controlled polymerization methods to provide carriers with different sizes (6-35 nm), negative charge content (17-83% PAA) and hydrophobicity (varying MSEA/MTEA ratio). All of the foliar applied star polymers displayed similar rapid and high rates of phloem loading (~30 wt%). However, the property differences affected their distribution to other plant compartments including younger leaves, older leaves, stems and roots. The smallest ~6 nm star polymers translocate most to younger leaves, while the largest ~35 nm star polymer moved preferentially to roots, likely due to different transport pathways in the leaf mesophyll tissue for the different sized materials. Charge content also affected the distribution of star polymers in the plants. The lower charge content star polymer moved more into the non-vascular tissues (younger leaf, older leaf and root), while the higher charge content star polymers mainly stayed in stem. This is likely due to charge dependent interactions with cell walls. Hydrophobicity did not affect star polymer translocation after entering the mesophyll but did affect their interaction with the leaf surface and the ability to penetrate the epidermis. These results can promote rational design polymer nano-carriers for more efficient and targeted agent delivery in plants, and more sustainable agricultural practices.

Robert Tilton

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