Cadmium is a naturally occurring heavy metal found in the Earth’s crust, but it can also become a significant environmental pollutant, particularly in soil and water. This element lacks any known biological function in plants or humans, yet plants readily absorb it from their environment. Its presence in agricultural systems is a growing concern due to its potential to enter the food chain, posing risks to human and animal health.
How Cadmium Enters Plants
Cadmium enters agricultural environments through both natural processes and human activities, though human activities contribute substantially more. Natural sources include volcanic eruptions, weathering of rocks, and forest fires, which release small amounts into the atmosphere and soil.
Industrial emissions from mining, smelting, and refining of non-ferrous metals are major contributors. Phosphate fertilizers, which can contain up to 300 mg of cadmium per kilogram, are another significant source in agricultural soils. Other sources include sewage sludge, industrial waste from manufacturing batteries, pigments, and plastics, and waste incineration.
Once in the soil, cadmium is highly mobile and soluble, making it readily available for plant uptake. Plants absorb it primarily through their roots as dissolved ions (Cd²⁺) from soil water. This absorption occurs via both apoplastic (through cell walls and intercellular spaces) and symplastic (through cell membranes and cytoplasm) pathways. Cadmium then transports from roots to other plant parts, including leaves, stems, and edible portions, through the xylem and phloem.
Impact on Plant Growth and Health
Elevated cadmium levels directly affect plant growth and overall health, causing various harmful physiological, biochemical, and structural changes. Common visible symptoms of cadmium toxicity include stunted growth, which inhibits plant development and reduces biomass. Chlorosis, or leaf yellowing, also occurs as cadmium interferes with chlorophyll production. This disruption decreases photosynthetic efficiency, impairing the plant’s ability to convert light energy into chemical energy.
Cadmium can also interfere with the uptake and transport of essential nutrients like iron, zinc, manganese, potassium, and calcium, further compromising plant health. At a cellular level, cadmium can cause oxidative stress by increasing the production of reactive oxygen species (ROS). This imbalance between oxidants and antioxidants damages cellular components such as lipids, proteins, and DNA, affecting cell structure and function. Cadmium also disrupts the plant’s water balance by altering membrane permeability, which can lead to reduced water content and overall physiological stress.
Cadmium’s Journey into the Food Chain
Once absorbed by plants, cadmium enters the human and animal food chain when consumed. This dietary exposure is a significant concern because cadmium accumulates in the body over time, particularly in the kidneys, with a biological half-life of approximately 10 years in humans.
Chronic exposure to cadmium, even at low levels, is linked to several serious health risks. The kidneys are the primary target organ for cadmium toxicity; long-term exposure can lead to kidney damage, including tubulus cell necrosis. Cadmium also causes bone demineralization, contributing to conditions like osteoporosis and increasing fracture risk, as seen in “Itai-Itai disease” in Japan. Furthermore, cadmium is classified as a human carcinogen, with potential links to lung cancer, and has been associated with cardiovascular diseases and reproductive organ impairment.
Many common food crops are prone to accumulating cadmium. Leafy greens such as lettuce and spinach, and root vegetables like carrots and radishes, often show higher cadmium levels in their edible parts compared to other crops. Grains, including rice, wheat, and oats, are also known to accumulate cadmium, making them significant dietary sources. Monitoring cadmium levels in these foods is therefore important to protect public health.
Reducing Cadmium in Agricultural Systems
Minimizing cadmium contamination in agricultural produce involves several strategies. Soil amendments reduce cadmium bioavailability, making it less accessible for plant uptake. Materials like lime, biochar, organic wastes, phosphate, and iron oxides can increase soil pH or form insoluble complexes, immobilizing it in the soil. Biochar application, for example, can reduce cadmium accumulation in plant tissues by up to 31.9%.
Another approach involves selecting plant varieties that naturally accumulate less cadmium in their edible parts. Genetic variation exists among crop species and cultivars in their ability to take up and translocate cadmium. For example, specific low-cadmium wheat cultivars have been developed to reduce cadmium levels in grains.
Phytoremediation techniques utilize plants to remove or stabilize cadmium from contaminated soil. Phytoextraction uses plants, particularly hyperaccumulators, to absorb and accumulate heavy metals in their tissues for safe disposal. Phytostabilization, conversely, uses plants to immobilize pollutants in the soil, reducing their mobility and preventing entry into groundwater and the food chain.
Careful fertilizer management, by selecting those with lower cadmium content and applying them at rates that meet crop needs, also helps prevent contamination. Managing irrigation water to avoid high chloride levels, which can increase cadmium solubility, further reduces cadmium uptake by crops.