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Cadmium Toxicity in Plants

Posted by: Scott Trimble
Nov. 19, 2019

Cadmium is a heavy, non-biodegradable metal that is toxic to plants, people, and animals. With increasing pollution and emission of cadmium, its levels in agricultural soils are increasing. The risk to humans is high due to consumption of cadmium-contaminated food. Modern applications of gas analysis and laser measurements are speeding up research in addressing the problem of cadmium stress.

Cadmium Sources

Cadmium occurs naturally in the environment but only in small amounts.

It is released into the air due to mining and through its use in industrial processes. It settles on the soil or is washed down by rain. It can be added to the soil directly when phosphate fertilizers are used. It is also added to soils through liquid pollutants such as sewage and industrial waste discharge from units manufacturing batteries, pigments, and plastics.

Cadmium moves easily in the soil because it is soluble in water. Slight acidity and presence of zinc or phosphates usually enhance plant uptake of this element. Cadmium uptake by the plant can be decreased through the application of supplements such as silicon and organic matter. Trace concentrations of cadmium in plants is normal, and cadmium levels of ≥ 0.1 mg/kg are considered safe.

However, due to increasing pollution, cadmium levels are rising in plants. High concentrations of cadmium have been reported in rice and tobacco. People are exposed to cadmium toxicity mainly by eating grains and vegetables that are grown in contaminated soils. Thus, it is necessary to decrease cadmium levels in plants to eliminate this health risk.

Since pollution is widespread and hard to control, research efforts are currently trying to establish how cadmium reacts with soils and plant growth, in a bid to control toxicity, by altering agricultural practices.

Cadmium Stress in Plants

Cadmium has no nutritional value for plants. On the contrary, its presence in soil has various deleterious effects by causing physiological, chemical, and structural changes in plants.

Of all the heavy metals, cadmium is considered to be the one that is most toxic for plants.

Hence, cadmium concentrations in the plants can be measured by the effects it has on plant growth. It inhibits the photosynthetic rate and production of chlorophyll and its activity. It also changes chloroplast structure and function, stomatal opening, transpiration, and antioxidant production. These effects, in turn, reduce water and nutrient uptake by plants. So, cadmium stress can inhibit plant growth and reduce crop yield.

Research efforts try to see which of these processes are affected by cadmium toxicity in different grains, vegetables, and other crops. The focus is also to identify varieties that can cope with cadmium stress.

Need For Field Instrumentation

Instrumentation that provides quick and accurate results, and is handy enough for large and repeated sampling, makes research easier. Chemical analysis that requires elaborate equipment can be expensive and require a laboratory. Sample collection, preparation, and a lengthy process are needed to analyze physiological and chemical changes in crops.

Funds are always scarce in research, and not every laboratory can afford to have all the necessary equipment.

Research results are only as good as the instrumentation and tools. The recent developments of small handheld devices that incorporate state-of-the-art technology and give rapid and precise results are providing solutions that can make research more affordable.

Small, affordable instruments can be used on the field to increase the number of studies that can be conducted, which is essential to deal with new problems, like cadmium stress, created by industrial pollution.

Some of the common techniques and new instrumentation used to study cadmium stress in crops are discussed here.

leaf area and cadmium

Fig. 1: Effect of type of organic matter application on leaf area of ties of rice in a) Bg300 and b) Suwandel. Effect of OM level on leaf area of rice grown on Cd contaminated sand is shown in c). (Image credits: International Journal of Chemical, Environmental & Biological Sciences (IJCEBS) Volume 4, Issue 1 (2016) ISSN 2320–4087)

Leaf Area Measurements Of OM Effect

A study in Sri Lanka used two varieties of rice, Bg300 and Suwandel, to check if the addition of organic matter reduced the intake of cadmium by plants. Scientists used two different types of organic matter: cattle manure and paddy straw.

When crops were one month old, their plant height, chlorophyll content, leaf area, number of leaves, and leave mortality was recorded.

Leaf area was measured using the CI-202 Portable Laser Area Meter. Produced by CID Inc.com, this is a hand-held device suitable for non-destructive measurements in the field and for use in the laboratory. Leaves are placed on the palette and flattened, and the laser scanner is slid over it. As the scanner moves over the leaf, it measures the width, length, area, and perimeter of tea leaves. The shape factor and ratio are calculated using pre-programmed formulae. It has a data logger that can store 8000 single measurements and communications software for easy transfer of data to computers.

The study was able to show that cattle manure succeeded in promoting large leaf areas in both varieties. With increasing amounts of organic matter, there was a significant increase in leaf area (See Figure 1). Though there was no difference in plant height between crops treated with the two manures and the control, there was an increase in root dry weight, showing the cumulative beneficial effects of organic matter. The scientists were able to show that cadmium stress was reduced by organic matter addition.

Leaf Area Measurements Reflect Amelioration by Phytohormones

In another study, radish seeds were treated with a phytohormone, 24- epibrassinolide (EBL). After twenty days, many of the vegetative parameters—such as photosynthetic rate, stomatal conductance, leaf area, and dry matter of roots—and many chemical concentrations were analyzed.

In this case, the CI-203 Handheld Laser Leaf Area Meter from CID Inc.com was used. As with CI-202, it can be used either for non-destructive measurements in the field or for collected specimens in the laboratory. It also has a laser scanner that can be used to rapidly measure leaf length, width, area, and perimeter. A built-in GPS tags location data to every reading for time series analysis. A conveyor attachment can be used to make quick measurements of detached leaves in the laboratory.

There was an improvement of leaf area, fresh, and dry mass by 18, 45, and 38%, respectively, due to the application of the phytohormones. Chlorophyll and carotenoid levels that had decreased by 48% and 41%, respectively, due to cadmium were avoided in treated seeds. The CI-710 Miniature Leaf Spectrometer is a simple, hand-held tool that enables users to measure carotenoids, along with many other metrics.

leaf area measurement and carotenoid

Figure 2. Visible symptoms in leaves of Cucumis sativus exposed to cadmium (Cd) for 20 days (A – control, B –500 μM Cd); ( Image credits: DOI 10.13080/z-a.2015.102.025)

Measurement of Photosynthesis and Leaf Gas Exchanges

In China, the effect of cadmium on cucumber, an economically important and popular vegetable, was studied. The experiment was designed to see how cadmium affects the absorption of other minerals and on vital physiological processes, such as photosynthesis, transpiration, and respiration.

After twenty days, seedlings were harvested, and the scientists measured concentrations of cadmium, iron, manganese, and antioxidants. They used the Handheld Photosynthesis System CI-340 by CID Inc.com to record leaf gas exchange of transpiration, stomatal conductance, carbon dioxide levels, and photosynthetic rates.

The CI-340 has ten customized chambers for use with leaves of different sizes, control modules to regulate light, temperature, carbon dioxide, water vapor, and chlorophyll flourescence.

It can be used for open and closed measurements, and readings are available within a few seconds.

There was a distinct difference in cucumber leaves’ color, with cadmium affected leaves showing signs of chlorosis, as can be seen in Figure 2.

Cadmium reduced the intake of manganese, a trace nutrient necessary for plant growth. Due to stress, there was also a reduction in photosynthesis, transpiration, and carbon dioxide at all the levels of cadmium added. Increasing cadmium concentrations in the soil showed more severe effects on all three processes.

Tackling Cadmium Stress in Plants and People

Cadmium causes kidney failure and damage to the skeletal system in humans, so it is necessary to eliminate or decrease cadmium in food production. Scientists are pin pointing the precise concentrations of cadmium in soil when the stress begins to affect various crops. Customized treatments and amendments that can restrict cadmium absorption by plants are also being identified. All of this research is made possible by easy, small instruments that are affordable and useful both in laboratories and the field.

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Vijayalaxmi Kinhal
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture

Sources

Agency for Toxic Substances and Diseases Registry. (2008, May 12). Cadmium Toxicity
Where is Cadmium Found? Retrieved from https://www.atsdr.cdc.gov/csem/csem.asp?csem=6&po=5.

Anuradha, S. & Rao, S.S.R. (2009). Effect of 24-epibrassinolide on the photosynthetic activity of radish plants under cadmium stress. Photosynthetica (2009) 47: 317. https://doi.org/10.1007/s11099-009-0050-3

Kirkham, M.B. (2006). Cadmium in plants on polluted soils: Effects of soil factors, hyperaccumulation, and amendments. Geoderma, 137:19-32. DOI:https://doi.org/10.1016/j.geoderma.2006.08.024

Nordic Council of Ministers Cadmium Review. (2003, Feb). Retrieved from https://www.who.int/ipcs/assessment/public_health/nmr_cadmium.pdf?ua=1

Sun, S., Zuo, J., Jiang, W., Liu, D., & Li, M. (2015). Cadmium effects on mineral accumulation, antioxidant defence system and gas exchange in cucumber. Zemdirbyste-Agriculture. 102: 193-200. DOI: 10.13080/z-a.2015.102.025

Wijayawardhana, Darshani & Weerasinghe, Aruni & Herath, Venura. (2016). Effect of organic matter application on growth of rice grown in cadmium contaminated sand. International Journal of Chemical, Environmental and Biological Sciences. 4. 67-70.


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Scott Trimble

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