From root entry to harvested commodity: Transport checkpoints governing toxic metal(loid) accumulation in major cash crops

Toxic metal(loid) contamination of cash-crop commodities is determined not only by root uptake but by successive transport and retention processes that govern delivery to the harvested organ. This review synthesizes evidence for Cd, As, Pb, Ni and Cr transport in six major cash crops: tea, tobacco, cacao, coffee, cotton and sugarcane, representing leaf, bean/seed, fibre and stem commodities. We organize these processes within a conceptual Root-to-Commodity Transport Continuum comprising six operational components: rhizosphere availability, root entry, root retention, xylem loading, phloem redistribution and harvest-organ loading. Mechanistic resolution was strongly metal- and crop-dependent: direct transporter and isotope evidence was concentrated in Cd studies of tobacco, tea and cacao, whereas As, Pb, Ni and Cr were supported mainly by crop-level partitioning, model-crop inference or unresolved mechanisms. To distinguish evidence strength, transporter and checkpoint claims were classified as direct crop-native causal or directional-flux evidence, strong crop-specific mechanistic evidence, crop-specific associative or descriptive evidence, or model-crop or homology-based inference. The synthesis shows that similar whole-plant metal burdens can produce contrasting commodity contamination because the dominant transport bottleneck differs with crop architecture and metal chemistry. The proposed framework is therefore conceptual and experimentally operational rather than presently predictive. It identifies the measurements required for future quantitative models, including stable-isotope tracing, vascular flux analysis, spatial elemental imaging and crop-native functional genetics, with the long-term objective of reducing commodity metal(loid) loading while preserving essential micronutrient homeostasis, yield and product quality.