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<br />licvcd to be the primacy reason for Se tOx-
<br />iWy. i~~ non-accumulating planes
<br />o Olt!PQrOiIl Shrift 1982). In this regard,
<br />(Banuelos et al. 1997) have observed cor-
<br />relations between tissue Se concentrations
<br />and tocal proeein in secondary accumula-
<br />tors, Le'l Brauica species grown In Se~
<br />laden soils. However, documentation is
<br />not available on the mechanisms within
<br />Brassica spp. chac are involved with Incor~
<br />potating or partially incorporating ab-
<br />sorbed Se inco proteins. In COntrast, Se-
<br />accumulators, Le., Astragalus, limit the
<br />incorporadon of Se into their proeein by
<br />avoiding the synthesis of selenomethion-
<br />ine. Selenomethionine is a seIenoamino
<br />acid thac is moscly incorporated inca peoR
<br />cein, and/or modifying the selena-com-
<br />pound co a derivacive (i.e.. methyl seleno-
<br />cysteine) chac cannoe be incorporated inco
<br />protein. Very little selenomethionine is
<br />found in 5e-accumulators (Brown and
<br />Shrift 1982).
<br />For plam species in general, Se uptake
<br />and accumulation in plane tissues is most~
<br />Iy governed by the chemical species of the
<br />elcmenr in thc soil and by pH and rcdox
<br />potential in the root-soil environment
<br />(Blaylock and James 1994; Elrashidi el al.
<br />1989; Mikkelsen el al. 1989). Thc chemi-
<br />cal forms of Sc have been considcred [0 be
<br />the mosr important for uptake of Se in
<br />plants. These include selcnide (Se'-), ele-
<br />menral Se (Se') , selenire (Se"), selenate
<br />(Se"), and other organic Se compounds
<br />(Shrift 1973). Because of their high solu-
<br />biliry in water, the oxidized forms, SeOl
<br />and 5e031-, are the 'most available td
<br />planes. Under aerobic and alkaline soil
<br />conditions, 5(04)- is the most common
<br />form of Se absorbed by plants and its up-
<br />takc is generally faster rhan SeO!,-. Seleni-
<br />um concentradons in plants grown on
<br />Se03l--rich soils are generally an order of
<br />magnitude less than plants grown on
<br />SeO/--rich soils (Banuelos and Meek
<br />1990). However, Se04'- uptake is greatly
<br />affected by the presence of one other ion,
<br />namely 504'-. Competition for uprake
<br />between 50/- and SeO/' has been exam-
<br />ined in shorr and long.term studies (Bell
<br />eI al. 1992; Ferrari and Renosto 1972;
<br />Gissel-Nielsen 1973; Leggett and Epsrein
<br />1956; Wu and Huang 1991). For many
<br />plant species, an increase in soludon
<br />SOl. concentration inhibits the uptake of
<br />5042-. High concentrations of soil 5e are,
<br />however, found not only in high SO/-
<br />soils but also in soils containing other
<br />salts (Wan et al. 1988).
<br />
<br />Phytovolatilization
<br />
<br />Another aspect of phytoremediacion
<br />that likely occurs in conjunction with
<br />
<br />plant uprake of Se is phytovolatilization.
<br />Planes growing in a Se-laden medium can
<br />produce volacile Se such as dimethyl se-
<br />lenide (OMSe) and dimcthyldiselenide
<br />(OMOSc), which has a distinctive garlic-
<br />like odor (Bearh et al. 1935; Lewis 1976).
<br />The release of volatile Se gases inca the atp
<br />mosphere may explain the disappearance
<br />of substantial amounts of Se from the soil
<br />in greenhouse and field experiments conp
<br />ducted by Banuelos and his colleagucs
<br />(Banuelos and Mcek 1990; Baiiuelos et al.
<br />1993a), which was not recovered in plant
<br />tissue (Tables 1 and 2). Since leaching
<br />losses were held [0 a minimum and did
<br />noc account for more chan 10% of Se re-
<br />moved from the soil, the missing Se was
<br />presumably lost by plant volatilization
<br />and microbial volacilizacion of Se
<br />(Frankenberger, Jr., and Karlson 1990).
<br />Although it is difficult to distinguish be-
<br />tween plane and microbial volatilization
<br />of Se, che presence of plants, however, in
<br />a Se-laden soil considerably increase the
<br />total rate of Se volatilization over those
<br />rates attained in soil microbes, where no
<br />plants arc cultivated (Bigger and
<br />Jayaweera 1993). Biggar and Jayaweera
<br />(1993) found that soil planred in barley
<br />volatilized 20 times more Se than soil
<br />alone. Wu and Huang (1991) measurcd
<br />greater volatilization rates of 18011g Se m-2
<br />do' (cquivalent to 16.2 g Se acre-' day)
<br />from sal, grass compared to barc soil and
<br />concluded that the race of Se volatili;z:adon
<br />was related to the biomass production.
<br />Soil planred to crops may have plant and
<br />microbial volatilization of Se occurring si.
<br />multaneously.
<br />PhytovolaciIization has been observed
<br />by Terry and colleagues (Terry et al. 1992;
<br />Terry and Zayed 1994). They demon-
<br />strared that the rate of Se volatilization by
<br />plants was dependent on plant species. By
<br />comparing the rates of Se volacilization of
<br />15 agricultural crops under Standardized
<br />environmental condicions, they were able
<br />to idcntifY those plant species that ap-
<br />
<br />f'j
<br />
<br />pea red to be superior volatilizers of Se
<br />(Terry et al. 1992). Superior voladlizer
<br />species (i.e., rice, broccoli, and cabbage)
<br />were able to volatilize Se at rates of 1.5 to
<br />2.5 mg kg-' OM day' (ppm) from nutri-
<br />ent solution containing 1.6 mg Se L-I
<br />(ppm). Based on these results, chey esti-
<br />mated that rice may remove soil Se by
<br />volatilization at rates of 4 mg m.2 soil sur-
<br />face dayl under optimum conditions.
<br />Phytovolatilization begins initially with
<br />plant uptake of Se. Soluble Se absorbed
<br />by the plant roots undergoes several meta.
<br />bolic processes before it is released into
<br />the atmosphere as volatile gases in the
<br />fotm of dimethyl ,denide (OMSe). A
<br />complete biochemical volatilization path~
<br />way has been proposed by Zayed and
<br />Terry (Zayed and Terry 1994) based on
<br />an analogy with the known S metabolism.
<br />The release of vol.atile Se compounds
<br />from intact planes is influenced by several
<br />factors including plane species, chemical
<br />form of Se available co the plant, presence
<br />of ocher competitive ions, temperature,
<br />pH, and light (Terry and Zayed 1994).
<br />Their research demonstrated chat plant
<br />roots and not shoots are the main sites of
<br />S~ vo1adlization in plants (Tablc 3). They
<br />found the removal of shoots resulted in
<br />enhanced rates of Se volatilizacion by the
<br />remaining roOts, reaching 30 times the
<br />original rates over 72 hours after shooe r~-
<br />moval (Zayed and Tercy 1994). Their in-
<br />vesdgation inco this phenomena indicated
<br />chac higher volatilization rates in roots are
<br />due to either the direct involvement of
<br />rhizosphere bacteria in the volatilization
<br />process, or because of enhanced Se bio~
<br />chemical metabolism in rooes compared
<br />to shoots (e.g., higher activities of specific
<br />enzymes or increased production of key
<br />mecabolites, or both [currendy under in-
<br />vestigation by Terry and associates, Fared
<br />et aI. 1997]). Elucidadon of these mecha-
<br />nisms may enable development of new
<br />planes with improved capacities for Se reoO
<br />movaI that can one day be used for phy-
<br />
<br />Table 3. Rates of Se volatilization of shoot, root, and detopped root 01 different species
<br />grown In half-strengh Hoagland's solution enriched with 1.5 Se L.I
<br />Rata of Sa volatilization found In'
<br />
<br /> Intact shoot Intact root Oetopped root
<br />Plant Species (~g Se kg-' OM per day)
<br />Broccoli 125 " 59' 1782 . 825 5160 . 3912
<br />Cabbage 200' . 92 2930 . 14 15075 . 4792
<br />CauHtlower 149 . 13 2775 .1966 9388 . 4759
<br />Chinese mustard 199 . 62 2010 " 1756 9375 " 3772
<br />Indian mustard 885 " 92 2785 . 375 12092 " 4883
<br />Rice 450 " 36 1000 " 36 3696 . 373
<br />
<br />.Vol:uile Se was extraCted from alkaline peroxide craps described in Zayed and Terry (1994)
<br />rvalues represent the mean followed by the srandard deviation
<br />
<br />428 JOURNAL OF SOIL AND WATER CONSERVATION
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