Effects of different foliar application on Cd concentration in wheat grains
The results show that foliar application of EDTA, Se, Zn, ASA, Si and Mo can significantly reduce Cd concentration of wheat grains in different treatments, with 32.3%, 32.0%, 27.7%, 27.7%, 26.3% and 25.9% decreased respectively in the pot experiment (Fig. 1). Foliar application of EDTA, Se and SNP can significantly reduce Cd concentration of wheat grains in different treatments, with 49.2%, 29.6%, and 28.8% decreased respectively in the field experiment (Fig. 4). There was no significant difference in Fe, Mn, Pb and Cu concentrations of wheat grains following six treatments with control in the pot/field experiment (Fig. S1, S2, S3 and S4). Foliar spray of EDTA and Se exerted excellent effects in controlling the Cd accumulation in wheat grains for both pot and field experiments. Also, the treatments with the different combined inhibitor have different effects on the cadmium content in wheat grains. Foliar spray of Se+Zn and Si+EDTA combined treatments can significantly reduce the Cd content in wheat grains by 39.1% and 40.2%, respectively (Fig. 5). Also, foliar applications of pot experiment have no significant difference on thousand grains weight of wheat at different growth stages (Fig. S5).
Effects of different foliar applications on Cd concentrations in different organs of wheat
There were great differences of Cd concentrations in different organs of wheat plants. Among different organs of above ground, the wheat grains displayed the lowest Cd concentration by 0.19-0.28 mg kg−1. Cd concentration in grains was much lower than that in husks, and Cd in uppermost internodes was generally lower than that in other internodes, and Cd in flag leaves was generally lower than that in other leaves (Fig. 2). Cd concentrations in middle leaves (second and third leaves), lower part of leaves, middle internodes (second and third internodes) and lower part of stalks were not influenced by foliar application (Fig. 2). Foliar application reduced Cd concentrations in husks by 5.8-37.4%, and increased Cd concentrations of EDTA, Se, Zn, ASA, Si and Mo in flag leaves by 35.9%, 38.6%, 39.1%, 31.4%, 39.0% and 32.5%, respectively in pot experiment. Inhibition efficiency of EDTA and Se on Cd accumulation in grains was significantly greater than that of other treatments in pot and field experiments (Fig. 1 and 4).
Effects of different foliar application on translocation factors of Cd in different organs of wheat
Foliar application with different foliar application slightly decreased TF of Cd from the husks to grain (TFhusk-grain) (Fig. 3). The TF of Cd from flag leaves to the grain (TFleaf-grain) decreased, and the TF of ASA, Citric Acid (CA), EDTA, Mo, Humic acid (HA), Potassium dihydrogen phosphate (K), Melatonin (MT), Se, Si and Zn decreased significantly (Fig. 3). Foliar application with different foliar application slightly decreased TF of Cd from the uppermost internodes to grain (TFhusk-grain). The TF of Cd from the flag leaves to husks (TFleaf-husk) decreased, and the TF of ASA, CA, EDTA, Gibberellin (GAs), Glutathione (GSH), Mo, HA, Lanthanum (III) chloride (La), Se, Si, Quinine, SNP and Zn decreased significantly (Fig. 3). The relationship between Cd concentration of grains and middle leaves, uppermost internodes were significantly positive, while the relationship between Cd concentration of grains and flag leaves was significantly negative (Table. 3).
Cd content in wheat grain at different growth stages
Cd concentrations were different among different treatments of wheat plants. It can be seen that spraying Se, EDTA can significantly reduce the Cd content in wheat grains no matter in heading stage, grain filling stage or headin+grain-filling stage, and the effect on reducing Cd concentration in different growth stages is relatively consistent, in order: heading+grain-filling stage > grain filling stage > heading stage (Fig. 6). The results showed that spraying Se or EDTA at the grain filling stage and heading+grain-filling stage had the best effect on reducing Cd concentration in wheat grains (Fig. 6). Foliar application with Se and EDTA efficiently reduced Cd concentrations in grains by 13.2-24.7% and 15.0-24.6%, respectively. Foliar application of Se and EDTA had no significant effect on other elements content (Fig. S4). The relationship between Cd concentration and Zn or Fe concentration was significantly negative, while the relationship between Zn concentration and Mn concentrations were significantly positive (Table S1). Foliar application with 0.1 mM Se and 2 mM EDTA significantly reduced Cd concentrations in grains at the grain filling stage and heading+grain-filling stage (Fig. 6).
Cd concentrations in different organs of wheat at different growth stages
The Cd concentration in different organs of wheat had great differences. The highest Cd concentration was found in uppermost node, which was about 0.9-1.5 mg kg−1. Cd concentration in grains was much lower than that in rachises, Cd concentration in uppermost internodes was generally lower than that in flag leaves (Fig. 7). Foliar application with Se and EDTA efficiently reduced Cd concentrations in husks and rachises by 1.8-35.6% and 9.6-35.4%, respectively. At the heading stage foliar application with 2 mM EDTA significantly reduced Cd concentrations in husks (Fig. 7). At the grain filling stage, foliar application with 0.1 mM Se and 2 mM Se significantly reduced Cd concentrations in husks, and foliar application with 2 mM EDTA significantly reduced Cd concentrations in rachises (Fig. 7). At the heading+grain-filling stage, foliar application with 0.1 mM Se and 2 mM EDTA significantly reduced Cd concentrations in husks and rachises, and foliar application with 0.1 mM EDTA significantly reduced Cd concentrations in husks (Fig. 7). However, Cd concentrations in flag leaves, uppermost internodes, uppermost sheath, and uppermost nodes were not influenced by Se and EDTA (Fig. 7).
Translocation factors of Cd in different organs of wheat at different growth stages
Foliar application with 0.1 mM Se and EDTA except 0.1 mM Se at the heading stage significantly decreased TF from the flag leaves to rachises (TFleaf-rachis) (Fig. 8). Foliar application with Se and EDTA had no significant effect TF of Cd ions from rachises to grains (TFgrain-rachis), TF of Cd ions from flag leaves to rachises (TFleave-rachis) and TF of Cd ions from uppermost internodes to flag leaves (TFinternode-leaf) (Fig. 8). The relationship between Cd concentration of grains and rachises, husks were significantly positive, while the relationship between Cd concentration of grain and flag leaves was significantly negative (Table. 4).