Uptake, metabolism, and volatilization of selenium by terrestrial plants

Authors: PILON-SMITH, E.A.H - Colorado State University; Banuelos, Gary; PARKER, D. R. - University Of California

Submitted to: Drainage and Salinity in the San Joaquin Valley: Science, Technology and Policy
Publication Type: Book / Chapter
Publication Acceptance Date: 1/31/2010
Publication Date: 12/1/2013
Citation: Pilon-Smith, E., Banuelos, G.S., Parker, D. 2013. Chapter 6. Uptake, Metabolism, and Volatilization of Selenium by Terrestrial Plants. In A. Chang and D.B. Silva (Eds.) Salinity and Drainage in the San Joaquin Valley: Science, Technology and Policy. New York, NY. Springer, p. 147-164.

Interpretive Summary: The use of green plants and associated microbes for environmental remediation has been called “phytoremediation”. This green technology is being developed for the management of metal(loid)-contaminated soils and waters via the processes of phytoextraction and phytovolatilization. The objective of this paper is to provide an overview of the most salient studies concerning plant uptake, metabolism, and volatilization of selenium (Se), framed within the context of phytoremediation approaches to the selenium (Se) problem in the Central Valley of California. Long term multi-year field studies were conducted with canola, mustard, and broccoli on field sites in central California. Water used for irrigation contained Se, predominately present as selenate salts. Generally, selenate is readily translocated to the shoot via the xylem. Plant uptake of selenate occurs due to the chemical similarity between selenate and sulfate. Moreover, selenate is readily assimilated by S-metabolizing enzymes of the plant and can be nonspecifically incorporated into any sulfur compound. Some of these organic-based compounds include volatile forms of Se, dimethylselenide. This release of gaseous Se by the plant represents an added value to Se phytoremediation strategies- phytovolatilization. Results from the multi-year field studies showed that Brassica crops were moderately more effective than typical agronomic crops, e.g. alfalfa, cotton, or tree crops such as poplar trees for managing soluble Se via plant extraction and biovolatilization. These on-going field studies show that the phyto-management of Se-contaminated soils by canola, mustard, and broccoli not only gradually removes soluble Se that has accumulated on the soils after irrigation with Se-laden drainage water, but harvesting the crops produces cash value Se-enriched by-products of economic value.

Technical Abstract: The green technology of phytoremediation has being developed for the management of metal(loid)-contaminated soils and waters via the processes of phytoextraction, and phytovolatilization. Based upon these processes a plant management remediation strategy for selenium (Se) has been developed for the Westside of central California. The objective of these long-term studies was to report on the uptake, metabolism, and volatilization of Se by terrestrial plants used for the phytoremediation of Se in central California soils. Long term multi-year field studies were conducted with Brassica napus var. Hyola (canola), B. juncea (mustard), and B. oleracea var. Marathon (broccoli), on field sites in central California. Water used for irrigation had a range of Se from 0.100-0.150 mg L-1, B from 4-7 mg L-1, and a sodium sulfate-dominated salinity (EC) of 5-8 dS m/1. The multi-year field studies showed that these Brassica crops were moderately more effective for managing soluble Se than irrigated unvegetative sites. Losses of Se occurred via accumulation, volatilization and leaching from the upper soil profile. Because Se is so chemically similar to sulfur (S), higher plants tend to readily take up and metabolize Se via S transporters and pathways. Higher plants as a group do not seem to require Se, but they nonetheless readily take it up from their environment. Typically selenateis transported to the leaf chloroplast, where it is first reduced to selenite, and then further reduced and assimilated into organic Se, which may include selenomethionine (SeMet). The SeMet can also be converted to volatile dimethyl selenide (DMSE), offering a release value for excess Se from the plant. This volatile characteristic is of added value to phytoremediation of Se because now selected plants, e.g., Brassica sp., can also extract and volatilize Se from soils irrigated with Se-laden waters. Since all plants live in association with adverse array of bacteria and fungi, and many microbes can also metabolize and volatilize Se, plant associated microbes may play a key role in plant Se accumulation and Se volatilization. Our field studies clearly show the plant extraction and volatilization of Se are key to effective phytomanagment of Se, however, this green process will require time (years), expecially under high sulfate salinity and boron field conditions. Under some conditions, the best we can do under field conditions is to phytomanage soluble Se by promoting plant uptake and biological volatilization. Coupling phytomanagement of Se with the creation of new byproducts, e.g., Se-enriched vegetables, animal feed, fertilizers, biofumigants, and even a byproduct like biofuel, may provide California growers unique opportunities to increase the environmental and economic sustainability of a plant Se-management system, while supporting the agriculture community in central California.