• Reduce text

    Reduce text
  • Restore text size

    Restore text size
  • Increase the text

    Increase the text
  • Print

    Print

Long-straw durum wheat to reduce cadmium levels in grain?

Cadmium is a metallic element, traces of which are found in agricultural soils. Its accumulation in cultivated plants is the subject of increasing monitoring. Collaborative efforts by INRA and Arvalis have revealed the potential impact of straw length on the accumulation of cadmium in durum wheat grain.

L'Inra reveals the potential impact of straw height on the accumulation of cadmium in grain in durum wheat. © Inra, Jean Yves Cornu
Updated on 11/02/2017
Published on 08/29/2017

Cadmium: an element under surveillance in the cereals sector

The sources of cadmium (Cd) in soils are both natural (changes to rock formations, volcanic emissions) and man-made (inputs and emissions from industry). Mineral fertilisers (notably phosphates) and to a lesser degree livestock waste are the principal means by which Cd is added to agricultural soils. Humans mainly absorb this element via their diet, and notably through cereal products. In order to reduce the risks of dietary exposure, the European Union is planning to lower the acceptable levels of Cd in harvested agricultural products. For wheat, the threshold may be reduced by half, from 0.2 to 0.1 µg/g of raw material, which would exclude from consumption a significant percentage (>20%) of the durum wheat currently produced. In order to encourage the use of durum wheat varieties that accumulate little Cd, INRA and Arvalis have evaluated the differences in Cd accumulation between different varieties of this species and sought to understand the underlying processes.

Eight French durum wheat varieties tested under experimental conditions

Cd accumulation capacity monitored in eight spring durum wheat varieties that are widely cultivated in France and display different straw heights. They were grown under hydroponic conditions in order to strictly control the bioavailability of Cd throughout plant growth.  Uptake and partitioning between different organs were evaluated at ripeness based on Cd concentrations measured in the roots, leaves, stems, ear structure (rachis, glumes and glumellas) and grains.

Cadmium accumulation in wheat grains is influenced by the partitioning of biomass between aboveground organs.

The scientists showed that Cd concentrations in grain could vary by a factor of 2 to 3, depending on the variety. They also observed that neither root uptake nor the sequestration of Cd in the roots could explain most of the variability in Cd accumulations in grain, but rather its partitioning between aboveground organs.

A modelling approach was adopted in order to test whether straw biomass was a parameter that was likely to modulate Cd allocation to grain. The underlying hypothesis was that the transpiring vegetative tissues found in straw (and notably leaves) behave like Cd “sinks”, competing with grains during their filling. Indeed, through transpiration, leaves mobilise some of the xylem flux containing Cd arising from the roots.

A model was therefore generated in line with this hypothesis, and based the variability in grain Cd levels, in particular, on the biomass of straw and grains. This model could easily be adjusted to the inter-cultivar variability of Cd accumulation observed, and highlighted the fact that straw biomass exerted a negative effect on grain Cd levels. This means that durum wheat varieties which allocate a high proportion of their aboveground biomass to straw (so potentially those with long straws) will accumulate less Cd.

These findings suggest that the allocation of Cd to grain is a two-stage process: control by the roots of Cd exports to aboveground organs and then the partitioning of Cd between the straw and grain as a function of their relative biomass.

Agronomic consequences of breeding long-straw wheat

Cd concentrations in durum wheat grain can vary by a factor of 2 to 3, depending on the variety. Durum wheat varieties that allocate a high proportion of their aboveground biomass to straw have been proved to accumulate less Cd. Breeding that tends to reduce straw height in order to improve resistance to lodging may, for this reason, favour the contamination of grain by Cd. Using long-straw durum wheat varieties in agricultural soils where the availability of Cd is relatively high – for example, because of a substrate rich in Cd, a poorly controlled pH (< 6.5) and/or a low level of organic matter – may help to produce grain that will comply with increasingly strict European regulations.
These experiments were carried out under hydroponic conditions where the high levels of water and nitrogen during the filling stage may have inhibited the remobilisation of Cd from straw to grain. It is now planned to determine whether this relationship between straw length and grain Cd accumulation will be confirmed in the field, where Cd reallocation processes are probably more important.

Contact(s)
Scientific contact(s):

  • Jean-Yves Cornu INRA-Bordeaux Sciences Agro Joint Research Unit for Soil-Plant-Atmosphere Interactions (UMR 1391; ISPA), INRA - Bordeaux Sciences Agro, 71 avenue Edouard Bourlaux, 33883 Villenave d’Ornon cedex
Associated Division(s):
Environment and Agronomy, Forest, Grassland and Freshwater Ecology
Associated Centre(s):
Nouvelle-Aquitaine-Bordeaux

Sources

  • Perrier F., Yan B., Candaudap F, Pokrovsky OS, Gourdain E, Meleard B, Bussière S, Coriou C, Robert T, Nguyen C, Cornu JY, (2016), Variability in grain cadmium concentration among durum wheat cultivars: impact of aboveground biomass partitioning. Plant Soil, 404, 307-320.

More information on the regulations

Since 2001, the maximum cadmium content in certain foodstuffs used for human and animal foods has been determined by the European Community (Regulations EC 1881/2006 and EC 87/2005, respectively). At present, the threshold value is fixed at 0.1 µg/g raw material for cereal grains, except for wheat and rice (threshold fixed at 0.2µg/g). Since publication in January 2009 of an opinion from the European Food Safety Agency (EFSA) on cadmium, suggesting a possible overexposure of certain populations (children, vegetarians) and an important contribution of cereals to cadmium exposure because of their high consumption, this threshold of 0.2µg/g for wheat has been a subject of debate. Although suggested, lowering the regulatory threshold for cadmium in wheat has since shifted towards a simple recommendation (2014/193/EU) on reducing the presence of cadmium in foodstuffs. This recommendation however stipulates that "Member States should ensure that available mitigation measures for the reduction of cadmium levels in food, in particular in cereals, vegetables and potatoes, are progressively implemented by farmers and food business operators".