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Background and Objectives:
Cadmium content and protein level are two major quality considerations in durum wheat.
Much of the Canadian durum crop contains levels of Cd above the 0.1 ppm level that has
been proposed by Codex Alimentarus as a limit for grains traded on the international
market. Excess Cd levels could jeopardize many of our export markets for durum if the 0.1
ppm limit is adopted. However, if Canadian durum could be produced containing low Cd
concentrations, it would give our product a comparative advantage as compared to our
American or Australian competitors on the world market. While breeding efforts are
successfully reducing the Cd content of durum wheat, even the low Cd cultivars can
frequently have higher than acceptable Cd content, depending on the soil characteristics
and environmental conditions.
Low protein content is also a major problem in durum wheat, leading to a reduction in
the market value of the crop. Protein premiums are paid for high protein durum, leading to
increased crop receipts. Both Cd content and protein content in durum wheat are greatly
influenced by crop management and by environmental conditions. Determination of the
influence of management factors and environmental conditions can help to develop
production practices for producers that will reduce Cd content and increase protein level
in the crop, increasing both the value and the market competitiveness of Canadian durum
wheat. Both Cd content and protein level are influenced by N management and vary
substantially from year to year. Year to year variation in Cd and protein content are
related to differences in environmental factors such as temperature and moisture levels,
however, detailed work identifying influence of environmental stresses on Cd or protein
content in durum wheat is limited.
Procedure and Project Activities:
Field experiments were conducted on a fine sandy loam and a silty clay soil in
southwestern Manitoba from 1997 to 1999. Two cultivars of durum wheat (Arcola and AC
Melita), varying in genetic potential for Cd accumulation, were planted at three different
seeding dates in early, mid and late May, with 0 or 80 kg N ha-1 and
treatments repeated four times at each location. Full soil sampling was conducted in the
spring and fall of each growing season at each site. A weather station at each location
recorded rainfall, air temperature and surface soil temperature. Additional temperature
recorders located in the soil in the various plots provided data on temperature profiles
within the soil. Crop samples were taken at five physiological crop stages during the
growing season, yield measured and tissue analyzed for Cd and N concentration. At the same
time, soil samples were taken and soil moisture content determined. At maturity, grain and
straw yield were measured and grain analyzed for Cd and protein content.
Additional growth chamber studies were conducted to more critically evaluate the impact
of temperature and moisture on Cd and protein content in durum wheat. The growth chamber
studies evaluated the interactive effects of temperature and moisture under controlled
conditions using root temperature control chambers available at Brandon Research Centre.
The soil selected for study was the A horizon of a loam soil from southwestern Manitoba. A
factorial treatment design (three soil temperature x three
moisture regimes) was sown in a randomized block experimental design. The cultivar sown
was AC Melita (a high Cd accumulator). Soil was amended with 200 µg g-1 N as urea, 20 µg g-1 P as MAP (which also includes an additional 15 µg g-1 N), and 50 µg g-1
K plus 20 µg g-1 S as K2SO4 and placed into one kg plant pots and seeded with four seeds of
durum wheat. An additional 100 µg g-1 N as
urea and 20 µg g-1 K (plus 10 µg g-1
S) as K2SO4 was added later as required for plant growth. The
treatments were applied by placing particular pots in water baths of different temperature
and using different watering patterns (dry down to 75, 50 or 25% of f.c.). The plants were
thinned by 50% after emergence and allowed to reach maturity in a growth chamber, at which
time the plant material was harvested and analyzed for Cd and nutrient status in both
grain and straw.
Results and Discussion:
Grain Yield: Arcola and AC Melita generally produced similar grain yields. Nitrogen
fertilization increased grain yield in three out of the six site-years. Late seeding
reduced grain yield while highest yield was produced with either early or mid-seeded
plots, depending on the site year.
Protein Content: Arcola had higher grain protein levels than AC Melita in the three
site-years where differences occurred. Addition of N fertilizer increased grain protein,
with the effects being significant in five of six site-years. The effect of seeding date
on protein content was variable. Grain protein concentration increased with added urea and
with later seeding at both 1997 sites. The increase with later seeding may be related to
the lower grain yield that was produced with late seeding. In addition, as the grain fill
is delayed until later in the growing season, the rate of mineralization of N may increase
due to higher soil temperatures as long as sufficient moisture is present to support
microbial activity. The late release of N may contribute to enhanced protein
concentration. Late seeded wheat did not produce higher concentration of grain protein in
1998 and 1999.
Cd Concentration: In 1997 and 1998, grain Cd concentration in both cultivars was below
the 100 ppb limit being proposed by Codex Alimentarus, with levels in Arcola being
approximately half of those in AC Melita. Grain Cd concentration was increased by N
application in five site years. In 1999, the Cd concentration of AC Melita exceeded 100
ppb on the loam soil if fertilized, or if unfertilized with early seeding. On the clay
loam soil, the 100 ppb level was exceeded only with early seeding and fertilizer
application.
The effect of seeding date on grain cadmium concentration was variable on the loam
soil, but Cd concentration tended to be highest in the mid season seeding date on the clay
loam soil and highest in the early season seeding date on the loam soil. Further
interpretation of weather information is required to determine which factors are
influencing Cd concentration.
Other Nutrients: In the two site-years where differences occurred between cultivars in
grain P concentration, Arcola had higher concentrations of grain P than AC Melita. Grain P
concentration was not affected by the urea treatment in any site-year. In half of the site
years, early seeded wheat contained the highest grain P concentrations. Grain Zn content
was only measured on the last two site-years. Cultivar had an effect on one site only,
with AC Melita having a higher grain Zn concentration than Arcola. In both cases, the
addition of urea decreased the amount of Zn in the grain, while the effect of seeding date
was not consistent.
Decreases of P and Zn were presumably due to dilution from the increased dry matter
yield produced in response to urea application. If straw or grain yield is increased, and
Cd or nutrient uptake is not increased accordingly, the result is a decrease in
concentration. This effect was not apparent for Cd, which increased with urea application,
indicating that the mechanism for changes in Cd concentration in grain is not the same as
the mechanism for Zn concentration.
Growth Chamber Studies: Straw and grain yield decreased and grain protein content
increased with increasing moisture stress and increasing soil temperature. Similarly, Cd
concentration in both the straw and grain increased with increasing moisture stress and
generally increased with increasing soil temperature. Grain P concentration also increased
slightly with increasing temperature and increasing levels of moisture stress, while grain
Zn concentration was unaffected by changes in soil temperature, but increased with
increasing moisture stress.
The consistent increase in nutrient concentration as available moisture decreased
likely reflected dilution/concentration effects, where the nutrients taken up by the crop
were distributed through a smaller amount of dry matter as the yield decreased.
The impact of temperature on Cd and protein was similar. Concentration of Cd in the
straw and grain, and grain protein increased with increasing soil temperature. Changes in
straw concentration may have been a function of changes in biomass production, since straw
yield decreased with increasing soil temperature. Concentration may also have been the
driving factor for effects on grain Cd and protein content, since grain yield decreased,
and grain Cd and grain protein concentration increased with increasing soil temperature.
Increasing soil temperature may have also increased the ability of the root to extract Cd
and N from the soil, or the ability of the soil to supply Cd and N.
In contrast to the effects on Cd and N concentration, concentration of P and Zn in the
straw, and Zn in the grain did not change or decrease with increasing soil temperature.
Grain P increased slightly with increases in soil temperature. The increase in P
concentration in the grain coupled with no change in the concentration in the straw may
reflect enhanced translocation of P from the straw to the grain as the soil temperature
increased, or concentration of P in the grain due to the lower yield. Similarly, the
decreased concentration of Zn in the straw coupled with unchanged Zn concentration in the
grain may also reflect enhanced translocation.
As expected, AC Melita absorbed higher amounts of Cd from the soil than Arcola. Cadmium
concentration in straw and grain was also consistently increased by urea application.
There was a wide variation in Cd concentrations between site-years. Grain protein was
higher in Arcola at half of the sites studied and there was no difference between Arcola
and AC Melita at the other sites. Grain protein content was consistently increased by the
addition of urea to the soil. Again there was considerable variation between site-years.
In a controlled environment, Cd concentration of durum wheat increased with increasing
soil temperature and increasing moisture stress. Grain protein also increased with
increasing soil temperature and increasing moisture stress. A possible cause of this is
yield reduction as these experimental variables were increased, resulting in concentration
of Cd and protein.
There are complicated environmental interactions that need to be studied carefully
before any conclusions can be drawn about the effects of microclimate on Cd and nutrient
uptake in durum wheat. Analysis of climatic data is currently underway in attempt to
establish relationships between microclimate and Cd and protein content of durum wheat.
Cadmium concentration of durum wheat is frequently above the limit of 100 ppb proposed
by Codex Alimentarus. This study clearly shows that different cultivars accumulate
different levels of Cd in the grain and that seeding date, nitrogen fertilizer management
and environmental conditions can influence grain Cd concentration. Further evaluation of
the meteorological information and the growth chamber studies may help to determine some
of the environmental factors driving the differences in Cd in the grain, so that we can
predict which locations will be likely to produce low Cd grain. We may also be able to
manipulate management practices to minimize Cd concentration in durum, which could help us
to retain and expand our international markets. We should also be able to target specific
locations that produce very low Cd durum and possibly segregate the durum produced in
those areas to market them separately where Cd content of the durum is of concern. It may
also be possible to develop a "Healthy Choice" pasta, with enhanced nutritional
quality and reduced content of Cd.
Acknowledgements:
This project was made possible through funding from the governments of Manitoba and
Canada through the Canada-Manitoba Agri-Food Research and Development Initiative (ARDI),
support from the Natural Sciences and Engineering Research Council and a contribution from
Simplot Canada, Ltd.
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