Sodium perchlorate

Toxicity to terrestrial plants

Additional information

At the moment, bibliographic analysis supports the idea that perchlorate is not toxic to plant.

The following information is extracted from Sellers K et al. (2007), Perchlorate. Environmental Problems and Solutions, Taylor & Franci eds, 224p:

Researchers have evaluated two types of effects of perchlorate on plants: vegetative stress, or inhibition of plant growth, and concentration of perchlorate in plants that may serve as a food source. For perchlorate there is no evidence that trace (µg/kg) soil concentrations will stress or inhibit the growth of plants. Researchers have used slightly higher concentrations (mg/kg) to evaluate the concentrations of perchlorate in soil or sand that will inhibit the growth of plants. Estimated NOAEL for cucumber seed germination is approximately 10 mg/kg, while the concentration considered a LOAEL for lettuce is 10 times lower (1 mg/kg).

Most of the laboratory studies to date have used clean sand or soil and then irrigated the plants with water containing nominal (and widely varying) concentrations of perchlorate. It is therefore difficult to know how actual perchlorate residuals in soil may affect wild plants. After long periods of weathering, which allows perchlorate leaching, there is less perchlorate in the surface soil and it is less available for plant uptake.

Laboratory studies have clearly demonstrated that perchlorate, being a dissociable salt, is readily taken up by most types of plants. The rate of translocation from the soil to the plant, however, varies markedly with the type of plant tested, the concentration of perchlorate in the soil, and, in some cases, the season the plant matter was sampled.

Perchlorate appears to concentrate in plants via a simple "salting out"mechanism. The perchlorate ion is taken up into the plant from soil pore water, translocated to the leaves and/or distal portions of the plant and, through evapotranspiration, is concentrated in the leaves (although seeds typically have a low concentration of perchlorate). This type of mechanism is important from the standpoint of both human and ecological exposures.

Bioconcentration factors, which are simply the ratio of the concentration of perchlorate in the plant to the concentration in the soil, generally range from 2 to 200 (a few citations report higher bioconcentration factors but those studies included more toward aquatic plants or hydroponic testing). Some plants, such as kelp, appear to bioconcentrate perchlorate from natural sources. Others, like tobacco fertilized with Chilean salt peter, appear to accumulate the salt to a much higher degree than other plants tested. The U. S. EPA Screening Ecological Risk Assessment adopted "a simple, conservative, screening-level" bioconcentration factor of 100 for use in estimating plant concentrations from available soil data. Although soil-to-plant bioconcentration factors tend to agree from study to study, it is important to note that some studies report concentrations as dry weight but others will report wet weight. Because of vast regional differences in both soil chemistry and weather, it is best to consider site-specific sampling of vegetation rather than relying on a "default" bioconcentration factor.

Wild or inedible plants have primarily been sampled from areas with naturally occurring elevated levels of perchlorate (e. g., in the arid areas of the southwestern U. S.), or impacted military sites. Therefore the analytical results reported are most likely biased high radier than representative of a broad range of conditions. With this caveat in mind, the available literature cites concentrations of perchlorate in samples of wild plants (inclusive of grasses, shrubs, succulents and Crees) ranging from below the limit of detection (ND) to 5,500 mg/kg. Most values measured in vegetation, however, are in the low-to-mid parts per billion range (trees, ND-220 µg/kg; succulent cactus, 66-3,200 µg/kg; desert scrub, 16-900 µg/kg; winter wheat, 720-8,600 µg/kg fresh weight (FW); garden vegetables, 40-1,650 µg/kgFW).