Pb Tolerance Research Articles

Mineral status of dicotyledonous crop plants in relation to their constitutional tolerance to lead

Four crop species—mustard, flax, tomato and sunflower—used in experiments, were grown in the growth chamber in solution culture. The plants were used in the following studies: (i) constitutional tolerance to Pb (index of tolerance was estimated using Wilkin's root test with 0, 5, 10 and 20 mg dm −3 Pb concentrations in Knop's medium deficient in PO 4 −3 ions); (ii) tolerance to deficit of P, Ca and Mg and to general nutrient stress (50 × diluted Knop's medium) as determined by dry matter yield; and (iii) effect of Pb and used nutrient regimes on mineral status of the plants, by estimation of internal ion concentration. The species most tolerant to Pb was tomato. Tomato was also the least tolerant to P deficit (although the test for Pb tolerance was conducted in medium without PO 4 −3 ions). The least Pb-tolerant species, mustard and sunflower, were moderately tolerant or non-tolerant to the mineral deficiencies used in the study. High constitutional tolerance to Pb in tomato was associated with the highest tissue of level of Ca during adminstration of Pb, and with the highest tolerance to Ca deficit. The magnitude of the Pb concentration in the media and its absolute amounts in the roots and shoots were not proportional to the degree of Pb tolerance. Two of the least Pb-tolerant species had the highest (mustard) and lowest (sunflower) root Pb concentrations. The absolute amount of Pb in the roots was highest in mustard. However, the amount of Pb which was transported to the shoot in relation to the total amount of Pb taken up by the plant was the highest for the least tolerant species, sunflower (43.4%).

Measurement of heavy metal tolerance of soil bacteria using thymidine incorporation into bacteria extracted after homogenization-centrifugation

Thymidine incorporation into soil bacteria DNA decreased with increasing Cu pollution in an artificially contaminated sandy loam (0–400 μg Cu as CuSO 4 g −1 soil) stored for 2 yr. The ATP content was affected to almost the same degree as thymidine incorporation. No effects on total or viable bacterial counts were found. To study tolerance of the bacterial community, bacteria were first extracted by homogenization-centrifugation from the differently polluted soils. To these bacterial solutions a range of Cu concentrations (as CuSO 4) was then added and the thymidine incorporation rate was measured. By comparing with a control (no Cu added to the solutions), IC 50 values (giving 50% reduction compared to the control) for each bacterial solution could be calculated. There were no differences in these IC 50 values for bacterial solutions from soils with 0 to 120 μg added Cu g −1 soil. With more severe pollution higher IC 50 values were found, indicating that a more Cu tolerant bacterial community had evolved. Bacteria extracted from the most polluted soils had IC 50 values 10 times higher than bacteria from nonpolluted soils. Using the same technique, an increased tolerance to Ag, Zn, and Cd was also evident in a Cu polluted soil compared to a nonpolluted one, while no effect on the Pb tolerance of the bacteria was found.

Differential experience following developmental lead exposure: Effects on brain and behavior

Long Evans hooded rat pups were exposed to lead (Pb) via the maternal milk supply from Postnatal Day 1 (PN 1) to PN 25. Mothers were fed diets containing either 4% Pb CO 3 (High Pb), 0.4% Pb CO 3 (Low Pb) or 2.2% Na 2 CO 3 (Controls) throughout this period. Pups were weaned at PN 30 and littermates randomly assigned to either an Enriched or Isolated environment for a period of 30 days. Increases in activity levels and decreases in passive avoidance latencies were observed in Pb exposed animals. However, there were minimal effects due to Pb on symmetrical maze performance. Experience in the enriched environment had no effect on open field activity levels but resulted in a marked reduction in symmetrical maze errors. While enrichment had no effect on passive avoidance performance in High Pb animals, it was capable of raising latencies in Low Pb animals to Control values. Thus, the therapeutic value of environmental enrichment in Pb exposed animals depends on both the task employed and the severity of the pre-enrichment brain damage. From both brain regional analysis and behavioral testing results, it appeared that the hippocampus was a major site of Pb action. From comparison of blood Pb levels of our animals and those reported in children, it became apparent that the rat may have a greater tolerance for Pb, and as such, caution must be used in making direct comparisons between the two species in terms of blood Pb levels.

Effects of lead on the soil bacterial microflora

The sensitivity of soil bacteria towards Pb was investigated. Soil suspensions from fourteen different soil types with a high or low Pb content were plated out on soil extract agar containing various concentrations of PbCl 2. In agar with a high Pb content, higher bacterial counts were found with suspensions from Pb-containing soils than with those of soils with a low Pb content. In the Pb-containing soils, proportionally more gram-negative rods were present while coryneform bacteria decreased. In an additional experiment, in which Pb was added to a sandy soil, more Pb-tolerant bacterial strains were found 3 years later than in the same soil without Pb. When pure cultures of the bacteria isolated from the soils were tested in liquid media for Pb tolerance, a higher proportion of tolerant strains was found in Pb-containing soils. Among strains of gram-negative bacteria isolated from these soils a higher proportion of tolerant strains was found than in corneform bacteria. It was concluded that as a consequence of Pb pollution of soil a selection of Pb-tolerant bacteria may take place.

Effects of Cu, Mg, Pb, and Sn on the corrosion of sand-cast Zn-12 pet al alloy in wet steam

The effects of individual additions of from 0 to 0.04 pet Pb and Sn on the corrosion resistance of sand cast Zn-12 pet Al alloy in saturated water vapor at 95°C were investigated to establish limits for these impurities and thus the feasibility of using secondary Zn to make this alloy. The effects of Cu (0.5, 1.0, and 2.0 pet) and Mg (0.02, 0.05, and 0.1 pet) on corrosion resistance were also studied and the optimum combination for tolerance to Pb and Sn was found to be 1.0 pet Cu and 0.02 pet Mg. In this alloy up to ∼0.01 pet Pb could be allowed. The harmful effect of Sn was much greater than that of Pb and was not progressive but showed a maximum at 0.0025 to 0.005 pet and a second minimum at 0.01 to 0.02 pet. The safest limit for Sn is considered to be ∼0.001 pet.

Experimental Ecological Genetics in Plantago II. Lead Tolerance in Plantago Lanceolata and Cynodon Dactylon from a Roadside

A population of Plantago lanceolata (ribwort plantain) from a roadside was found to have higher Pb tolerance than populations away from the roadside; this reflected the sharp differences in Pb content of the soil and the plants at these sites. Tolerance tests on seedlings showed that the Pb tolerance was transmitted to seed progeny. Cynodon dactylon (Bermuda grass) showed generally a higher Pb tolerance than P. lanceolata, but there was no evidence of a greater Pb tolerance of the roadside population when compared with populations from control sites. These results suggest that the Pb level found at the roadside was sufficiently high to impose selection pressure for the evolution of tolerance in a sensitive species, but no overt effect was seen in a species with a greater inherent tolerance. This provides another example of rapid and highly localized evolutionary change in plants.