9.1 Effects
The toxicity of PAHs to natural and cultivated plants is not well addressed in the literature. Much of the available data were gathered on excised sections of plants and germinated seeds and, therefore, were of limited use.
Deubert et al (1979) soaked corn and wheat seeds in a 0.5 - 20 µg/L B[a]P solution. The exposure to B[a]P at the low concentration (0.5 µg/L) appeared to stimulate corn root growth; shoot growth was unaffected for both corn and wheat. In another experiment, acenaphthene (15 400 µg/L) was observed to slow down the elongation rate in the young root tips from maize, while anthracene (17 800 µg/L) did not have an effect (OMOE, 1990).
There is abundant literature on accumulation of PAHs in higher plants grown on PAH-enriched soils. However, it suffers from a lack of detail on PAH accumulated simultaneously from other sources (e.g., air, volatilized fraction in soil, etc.); i.e., the uptake of soil PAH via plant roots could not be addressed adequately from the data.
Edward (1983) reported that PAH uptake rates by plants are dependent on: (a) PAH concentration in the environment, (b) plant species, (c) the nature of plant growth substrate (e.g., soil, water, etc.), (d) PAH solubility, (e) PAH phase (vapour or particulate), and (f) PAH molecular weight.
Edward et al (1982) noted that 14C-anthracene uptake by soybeans from nutrient solution was directly proportional to the anthracene concentration in the solution. However, anthracene in soil was quite unavailable for absorption by soybean roots, in comparison to its availability in hydroponic solution (Edward et al, 1982). The tendency of a soil to bind PAHs is related to its organic matter content and cation exchange capacity. Field experiments with several agricultural crops, where the soil was treated with fresh compost containing a number of PAHs, suggested little or no uptake of PAHs by plant roots (Ellwardt, 1977).
Concentrations of PAHs in vegetation are generally much less than concentrations in the soil. Wang and Meresz (1981) analyzed onions, beets, tomatoes, and soil for 17 PAHs, and found that the vegetation/soil concentration ratios ranged from 0.0001 to 0.085 for B[a]P and 0.001 to 0.183 for total PAHs. They also found that most of the PAH contamination was in the peels.
Negishi et al (1987) suggested that soybeans metabolized B[a]P using a mixed function oxidase similar to mammalian and eukaryotic12 systems. Their findings, however, were in contrast to Trenck and Sandermann (1980) who, using higher plant cell cultures, concluded that plants did not metabolize B[a]P similarly to mammalian systems.
In controlled environmental conditions, Grimmer and Duvel (1970) noted that vegetable crops were incapable of de novo (afresh) synthesis of PAH; however, opposite conclusions were drawn for algae by several other investigators (Knutzen and Sortland, 1982).
Wagner and Wagner-Hering (1971) reported phytotoxic effects for polycyclic aromatic hydrocarbons.. These investigators found that wheat (whole plant) and barley (straw) yields were reduced by about 10% when exposed to 3,4-benzfluoranthene in soils at the rate of 6.3 µg PAH/g dw(soil) and 9.4 µg PAH/g dw(soil), respectively. No other study related to phytotoxic effects of PAHs to terrestrial plants was found in the literature. Phytotoxic effects are apparently not severe in higher, terrestrial plants up to B[a]P soil concentrations of 18 µg PAH/g dw (Sims and Overcash, 1983).
9.2 Criteria from other jurisdictions
PAH criteria for irrigation waters were not found in the literature.
9.3 Recommended criteria
PAH criteria for irrigation waters are not recommended due to the lack of sufficient information.
12 Eukaryote or eucaryote: an organism composed of one or more cells with visibly evident nuclei.