Maturity 1

Maturity 1
Measurements of accumulation of chemicals by plants are usually taken at a single time without
knowledge of whether or not vegetation may be in equilibrium with the soil with respect to chemical movement. However, longer exposure does not necessarily lead to higher plant concentrations. Both the age of the plant and seasonal processes apparently affect uptake. For example, for all leafy and root crops grown in a muck soil, heavy metal concentrations were greater in young crops in the early summer than in mature crops (Hutchinson et al. 1974). Moreover, the selenium content of birdsfoot trefoil exposed to natural levels of the element decreased with each cutting until midsummer, after which it remained constant (Lessard et al. 1968). On the other hand, selenium uptake by timothy increased until maturity.
In the sections below, the regressions of plant concentration on soil concentration (and pH) are
discussed. In addition, potential sources of variability in uptake of the chemicals by plants are discussed.
1 ARSENIC
As with most inorganic chemicals, the uptake of arsenic by crop plants has been observed to vary
with plant species and soil type (Otte et al. 1990). Additionally, phosphorus concentrations in soil have a large and complex effect on the uptake of arsenic by plants. The arsenic concentration in ryegrass (Jiang and Singh 1994) and that in the roots of Urtica dioica (Otte et al. 1990) were positively correlated with phosphorus in the soil, but in the latter case, negatively correlated with the concentration of arsenic in soil. In a second species, Phragmites australis, arsenic concentrations in the plant were measured at a level that was not correlated with concentrations of arsenic or phosphorus in soil (Otte et al. 1990). A better regression may have been obtained in this study if soil phosphorus were included as
a variable.
2 CADMIUM
The uptake of cadmium has been observed to vary with plant species (Haghiri 1973). Cadmium
uptake by plants has been shown in numerous studies to decrease with increasing pH (He and Singh 1994, Miller et al. 1976), so it is not surprising that the multiple regression with pH was significant in this study. Uptake by soybeans is also related to the sorptive capacity of soil (Miller et al. 1976). Lead has been widely observed to increase cadmium uptake; for example, the addition of both lead and cadmium increased the foliage content of each contaminant in American sycamore over the uptake values observed with a single metal added (Carlson and Bazzaz 1977). Lead has also increased the uptake of cadmium in rye and fescue (Carlson and Rolfe 1979) and in corn shoots (Miller et al. 1977). However, Miles and Parker (1979) found only low-level and inconsistent synergistic and antagonistic effects among cadmium, lead and other heavy metals in uptake by little bluestem and black-eyed Susan. A better regression may have been obtained in this study if soil lead were included as a variable.
3 COPPER
Prior to this study it was not known whether a significant regression of plant concentration on soil
concentration could be derived. Copper is a plant nutrient, and plants would be expected to exert control over uptake at certain ranges of soil c ncentration. As with other chemicals, in some previous investigations, no correlation was found between copper in plant foliage and underlying soil (Burton et al. 1984, Davies 1992). In contrast to the results in this study (in which pH did not contribute significantly to the multiple regression), pH has sometimes been shown to contribute to the variability in uptake of copper from different soils. Sims and Kline (1991) found a significant regression model between copper in wheat and soybean and soil copper and pH, but not with the copper concentration in soil alone.
4 LEAD
Lime has been observed to reduce the uptake of lead by lettuce and oats (John and Laerhoven
1972), suggesting that pH is a variable which controls the uptake of the element from soil. In contrast, Davies (1992) found that lead uptake by radish was best predicted by total lead in soil, and the regression of plant lead on soil lead concentration in that study was not improved by adding other soil characteristics. Similarly, in this study, pH did not contribute significantly to the multiple regression. The uptake of lead by plants has been found to be increased (Carlson and Bazzaz 1977), unaffected (Carlson and Rolfe 1977), and decreased (Miller et al. 1977) by increased concentrations of cadmium. Additional contributors to the variability in uptake of lead are: exposure time (Nilsson 1972) and plant taxon. While the attempt was made to exclude aerial exposure of lead, the use of lead in gasoline may have contributed to aerial exposure of plants to lead in some studies.