Organs, pathways, and mechanisms involved in cadmium uptake and accumulation in wheat under above-ground exposure

Atmospherically deposited cadmium (Cd) can accumulate in crops through uptake by above-ground parts; however, the key organs, pathways, and mechanisms involved in Cd uptake and accumulation in plants under this mode remain unclear. To clarify these issues, this study carefully designed a pot experiment with wheat, exposing various above-ground organs for different durations (daytime and nighttime) using CdS nanoparticles to simulate atmospherically deposited Cd. Results showed that exposure of above-ground parts to CdS significantly increased Cd content in all tissues of wheat, indicating that Cd in wheat plants from above-ground uptake could be transferred upward to grains and downward to roots. Furthermore, spike and foliar uptake contributed 11-34% and 66-89%, respectively, to the increased Cd levels in wheat grains, while these two uptake modes contributed 1-49% and 51-99%, respectively, to the increased Cd levels in other tissues (including roots, stems, leaves, and husks). These findings suggest that, in addition to leaves, spikes were also important organs responsible for the uptake of simulated atmospherically deposited Cd. Regarding absorption pathways, the contributions of stomatal and cuticular penetration to Cd accumulation in grains were quantified as 42-66% and 34-58%, indicating that both pathways played key roles in the internalization of atmospheric Cd in plants. Subcellular distribution of Cd in above-ground tissues showed that the cell wall, particularly its pectin component, served as the primary barrier to simulated deposited Cd influx into the cytoplasm. This study highlights that controlling foliar and spiked Cd uptake-particularly by regulating stomatal opening and enhancing the cell wall's Cd-binding capacity-could be a promising strategy to prevent Cd accumulation in crops resulting from atmospheric deposition.