Contamination of Durum Wheat by Cadmium : Lessons from the Field

Contamination of food crops by cadmium (Cd) is a major concern, particularly for durum wheat, which is widely consumed and which accumulates this toxic metal to a greater extent than bread wheat. Following the assessment of the risks to human health related to the presence of cadmium in foodstuffs, the European Commission intended to reduce the maximum content of Cd in wheat from 0.2 to 0.1 mg/kg grain. France is in the top 5 of European countries for the production of durum wheat and the project of a lower regulation limit for Cd stressed the urgent need i) to characterize the risk that harvests could exceed the new limit, ii) to identify, in the field, the major factors controlling the level of contamination. For that purpose, a specific project 'Cadur' was conducted between 2011 and 2014 in France. Samples of grains were collected in 2011, 2012 and 2013 among farmers of the different areas of production. The median value of Cd in durum wheat grain was 0.07 mg/kg with less than 5% of samples above the 0.2 mg/kg limit but 25% above 0.1 mg/kg (n=244). However, there were strong variations depending on the geographical areas and for some of them, up to 60% of the samples exceeded 0.1 mg Cd /kg. In order to understand the source of these variations, trials crossing cultivars and the areas of production were investigated. The cultivars were estimated to be responsible of 10-25% of the variation of the grain Cd concentration against 60-70% for the location of the culture. The effect of the location was mainly due to the concentration of Cd in the soil and to the soil pH. A statistical model with these two explanatory variables was able to explain 57% of the variability of the Cd concentration in the grains. Some of the durum wheat cultivars commonly grown in France were assessed with respect to their ability to accumulate Cd. It was found that the high:low ratio for the Cd content in grains was around 1.5 when cultivars were grown on the same soils. Contrary to Canadian cultivars, in France, no specific genetic selection has yet been performed to lower the contamination by Cd. Nitrogen fertilization was shown to have a significant effect, both by the dose and by the timing of application, presumably by modifying the mobility of Cd in the soil. Increasing N supply from 200 to 250 kg N/ha increased the Cd in grains by 20% and by 25% if the total N reached 300 kg N/ha. However, if the additional N was applied as two applications including a late one instead of a single dose, the increase in the grain contamination was not significant for a total of 250 kg N/ha. These studies confirm that both the soil conditions (Cd content, pH) and the cultivars are key points for controlling the contamination of durum wheat by Cd. French cultivars have the potential to be genetically selected to improve their ability to exclude Cd from the grain. They also have to be carefully chosen depending on the local bioavailability of Cd in soil, which can be in part, managed by controlling the pH and the nitrogen fertilisation. Funded by the projects 'Cadur', Arvalis-Inra and ANR-15-CE21-0001-04 'Cadon'

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