Foliar Spraying Zinc–Carbon Dot Nanofertilizer Promotes Yield and Quality of Lettuce (<i>Lactuca sativa</i> L.) through Leaf–Root Regulation

Currently, the “invisible hunger” of micronutrient deficiencies is threatening human health worldwide. Herein, foliar spraying zinc–carbon dot nanofertilizers (Nano-ZCDs, 10 mg·L–1) improved lettuce yield (62.3%) and quality, especially Zn content (41.2%). Primarily, Nano-ZCDs could take advantage of CDs as an artificial antenna to enhance photosynthesis and up-regulate the expression of zinc-regulated and iron-regulated transporter-like protein (ZIP) genes to facilitate nutrient uptake, resulting in increased photosynthetic pigments, giving rise to an increase in chlorophyll a by 59.1%, chlorophyll b by 32.2%, and carotenoid by 85.9%, and enhance the photosynthetic efficiency by 106.7%. Additionally, the improvement of photosynthesis by Nano-ZCDs promoted the secretion of low molecular compounds (decenoic acid, proline, adipic acid, medronic acid, 4-aminosalicylic acid, etc.) in roots and then recruited rhizosphere beneficial microorganisms (e.g., Pseudomonas and Penicillium). These microorganisms could activate phosphorus in soil and make a 22.0% increase in available phosphorus (AP) in rhizosphere soil, thus augmenting the P content (4.2%) in the shoot, which mitigates the antagonistic effect between Zn and P. Consequently, the application of Nano-ZCDs can be considered a promising strategy for agricultural development, particularly in the aim of tackling the challenge of micronutrient deficiencies in crop products.