Foliar Uptake and Translocation of Heavy Metals from Atmospheric Particulate Matter: A Critical Review

Intensified industrialization, urbanization, and agricultural activities have led to excessive heavy-metal accumulation in agricultural products, threatening food safety. Recent studies suggest that foliar exposure under atmospheric deposition is the dominant route for controlling metal accumulation in edible tissues. However, their specific mechanisms remain poorly understood. This Review synthesizes the foliar uptake and translocation of atmospheric heavy metals, focusing on the interactions among particle size, metal speciation, and leaf morphology. Key debates include the form of entry (ionic vs particulate), the mechanism of uptake (passive vs active), and the transporters involved. Emerging evidence shows that micrometer-sized particles enter leaves through stomata via passive diffusion, although intracellular transport may involve active mechanisms. In contrast, nanosized particles can penetrate both cuticular and stomatal pathways. Notably, leaves preferentially take up ionic heavy metals over particulate forms. The trichome pathway emerged as an additional route. However, whether it governs direct uptake or enhanced retention and if the chemical form taken up is ionic or particulate remains elusive. Specific transporters that mediate translocation to edible tissues have not yet been identified. This review represents a paradigm shift in crop safety, moving beyond the traditional root uptake perspective to foliar uptake of atmospheric matter.