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A series of field trials were conducted to investigate the potential of Noccaea caerulescens F.K. Mey [syn. Thlaspi caerulescens J &C Presl. (see Koch and Al-Shehbaz 2004)] populations (genotypes) derived from southern France to phytoextract localized Cd/Zn contamination in Thailand. Soil treatments included pH variation and fertilization level and application of fungicide. N. caerulescens populations were transplanted to the field plots three months after germination and harvested in May, prior to the onset of seasonal rains. During this period growth was rapid with shoot biomass ranging from 0.93-2.2 g plant(-1) (280-650 kg ha(-1)) DW. Shoot Cd and Zn concentrations for the four populations evaluated ranged from 460-600 and 2600-2900 mg kg(-1) DW respectively. Cadmium and Zn Translocation Factors (shoot/root) for the populations tested ranged from 0.91-1.0 and 1.7-2.1 and Bioaccumulation Factors ranged from 12-15 and 1.2-1.3. We conclude that optimizing the use of fungicidal sprays, acidic soil pH, planting density and increasing the effective cropping period will increase rates of Cd and Zn removal enough to facilitate practical Cd phytoextraction from rice paddy soils in Thailand.

Past applications of biosolids to soils at some locations added higher Cd levels than presently permitted. Cadmium phytoextraction would alleviate current land use constraints. Unamended farm soil, and biosolids amended farm and mine soils were obtained from a Fulton Co., IL biosolids management facility. Soils contained 0.16, 22.8, 45.3 mg Cd kg(-1) and 43.1, 482, 812 mg Zn kg(-1) respectively with initial pH 6.0, 6.1, 6.4. In greenhouse studies, Swiss chard (Beta vulgaris var. cicla), a Cd-accumulator maize (inbred B37 Zea mays) and a southern France Cd-hyperaccumulator genotype of Noccaea caerulescens were tested for Cd accumulation and phytoextraction. Soil pH was adjusted from ∼5.5-7.0. Additionally 100 rice (Oryza sativa) genotypes and the Ni-hyperaccumulator Alyssum murale were screened for potential phytoextraction use. Chard suffered phytotoxicity at low pH and accumulated up to 90 mg Cd kg(-1) on the biosolids amended mine soil. The maize inbred accumulated up to 45 mg Cd kg(-1) with only mild phytotoxicity symptoms during early growth at pH>6.0. N. caerulescens did not exhibit phytotoxicity symptoms at any pH, and accumulated up to 235 mg Cd kg(-1) in 3 months. Reharvested N. caerulescens accumulated up to 900 mg Cd kg(-1) after 10 months. Neither Alyssum nor 90% of rice genotypes survived acceptably. Both N. caerulescens and B37 maize show promise for Cd phytoextraction in IL and require field evaluation; both plants could be utilized for nearly continuous Cd removal. Other maize inbreds may offer higher Cd phytoextraction at lower pH, and mono-cross hybrids higher shoot biomass yields. Further, maize grown only for biomass Cd maximum removal could be double-cropped.

There are very few practical demonstrations of the phytoextraction of metals and metalloids from soils and sediments beyond small-scale and short-term trials. The two approaches used have been based on using 1) hyperaccumulator species, such as Thlaspi caerulescens (Pb, Zn, Cd, Ni), Alyssum spp. (Ni, Co), and Pteris vittata (As) or 2) fast-growing plants, such as Salix and Populus spp. that accumulate above-average concentrations of only a smaller number of the more mobile trace elements (Cd, Zn, B). Until we have advanced much more along the pathway of genetic isolation and transfer of hyperaccumulator traits into productive plants, there is a high risk in marketing either approach as a technology or stand-alone solution to clean up contaminated land. There are particular uncertainties over the longer-term effectiveness of phytoextraction and associated environmental issues. Marginally contaminated agricultural soils provide the most likely land use where phytoextraction can be used as a polishing technology. An alternative and more useful practical approach in many situations currently would be to give more attention to crops selected for phytoexclusion: selecting crops that do not translocate high concentrations of metals to edible parts. Soils of brownfield, urban, and industrial areas provide a large-scale opportunity to use phytoremediation, but the focus here should be on the more realistic possibilities of risk-managed phytostabilization and monitored natural attenuation. We argue that the wider practical applications of phytoremediation are too often overlooked. There is huge scope for cross-cutting other environmental agenda, with synergies that involve the recovery and provision of services from degraded landscapes and contaminated soils. An additional focus on biomass energy, improved biodiversity, watershed management, soil protection, carbon sequestration, and improved soil health is required for the justification and advancement of phytotechnologies.

Thlaspi caerulescens is known to hyperaccumulate high quantities of Cd with Cd concentrations up to 3000 mg kg(-1) in some populations from south of France. However, within these populations, the Cd concentrations can vary widely from plant to plant in a way that appears to be not entirely due to variations in soil Cd. The aim of this work was to investigate the variability in the Cd uptake ability of individual plants within a population and among seedlings grown from seeds from a single plant. Ten populations of T. caerulescens plants were selected from four locations (V: Viviez; SF: Saint Félix-de-Pallières; LB: Le Bleymard; CMA: Col du Mas de l'Air) depending of the extent and soil homogeneity of the site. One population from CMA consisted of the progeny of a single maternal plant. Hundred plants of each population were grown for three months in the same homogeneous and lightly Cd-polluted soil (about 20 mg total Cd kg(-1) dry soil). Cadmium uptake behavior of the plants was monitored by labeling the soil with 109Cd. To allow partial plant destruction, radioanalysis was performed on the largest leaf of each plant as an indicator of the total Cd concentration in plant shoots. Results showed significant differences in biomass production and Cd uptake by T. caerulescens between sites and between populations within sites. We observed a wide intra-population variation in biomass, Cd concentration and total Cd uptake. For these properties, 1 to 5 percents of the plants in each population varied by more than a factor of two from the mean values. The mean Cd uptake by the single-plant population from CMA was more than 40% higher than for the population at large. T. caerulescens would respond to traditional selection methods, which would significantly improve the phytoextraction of Cd.

Leptoplax emarginata and Bornmuellera tymphaea are nickel hyperaccumulators of the Brassicaceae family endemic to serpentine soils in Greece. The aims of this work were to compare the growth and uptake behavior of these plants with the Ni hyperaccumulator species Thlaspi caerulescens and Alyssum murale, and to evaluate their effect on soil Ni availability. Plants were grown for 3 mo on three soils that differ in Ni availability. Ni availability in soils was measuredby isotopic exchange kinetics and DTPA-TEA extractions. Results showed that L. emarginata produced significantly more biomass than other plants. On the serpentine soil, B. tymphaea showed the highest Ni concentration in shoots. However, Niphytoextraction on the three soils was maximal with L. emarginata. The high initial Ni availability of soil Serp (470.5 mg kg(-1)) was the main explanation for the high Ni concentrations measured in plant shoots grown on this soil, compared to those grown on soils Calc and Silt A. murale was the least efficient in reducing Ni availability on the serpentine soil L. emarginata appeared as the most efficient species for Ni phytoextraction and decrease of the Ni available pool.