The newly absorbed atmospheric lead by wheat spike during filling stage is the primary reason for grain lead pollution.

Wheat spikes could directly absorb lead (Pb) from atmospheric depositions. However, the mechanism by which the spikes contribute to Pb accumulation in the grain remains unclear. To investigate this mechanism, a field experiment was conducted using three comparative spikes shading treatments: 1) exposed to atmospheric deposition and light (CK), 2) non-exposed to atmospheric deposition and light (T1), and 3) non-exposed to atmospheric deposition but light-transmitting (T2). Spikes shading treatments reduced the average rate and peak value of the accumulation of Pb in grains, which significantly decreased the grain Pb concentration by 57.44 % and 50.26 % in T1 and T2 treatments, respectively. Moreover, Pb isotopic analysis shows that the Pb in spike and grain was mainly from atmospheric deposition, and the percentage of the grain Pb originated from atmospheric Pb decreased from 85.98 % in CK to 72.87 % and 79.59 % in T1 and T2, respectively. In addition, the spikes, rather than the leaves/roots, were the largest wheat tissue source of Pb in grains, and the relative contribution of spikes to grain Pb accumulation increased to 65.57 % at the maturity stage, of which the stored Pb re-translocation of spikes and the newly absorbed Pb by spikes during the filling stage contributed 13.37 % and 52.20 % to the grain Pb, respectively. Thus, the contribution of the spike to the grain Pb was mainly from the newly absorbed Pb from the atmospheric deposition during the grain filling phase, and grain filling phase is the key stage for the absorption of Pb by the grain. In brief, the newly absorbed atmospheric Pb by wheat spike during filling stage is the primary cause of grain Pb contamination, which provided a new insight for effective control of wheat Pb pollution.