Practice question

Phytovolatilization constitutes specialized phytoremediation pathway where selected plants absorb contaminants that are volatile in their native form or can be enzymatically transformed into volatile derivatives, translocate upward via xylem driven by transpiration pull generating negative pressure minus 1 to 2 MPa, and release through stomatal conductance into atmosphere as less toxic gaseous species. Classic illustrations include selenium taken up as selenate via sulfate transporters Sultr in Brassica juncea, metabolized through cysteine synthase to selenocysteine, methylated by selenocysteine methyltransferase to methyl selenocysteine and further converted to dimethyl selenide, dimethyl diselenide volatile compounds emitted 10 to 100 micrograms per square meter per day, elemental mercury taken up as Hg2+ and reduced by bacterial merA mercuric reductase expressed in transgenic poplar to elemental Hg0 vapor with high Henry's constant, and trichloroethylene oxidized by cytochrome P450 2E1 then transpired as chlorinated metabolites. High transpiration rate 10 to 100 liters per day per mature phreatophyte tree generates large water flux facilitating contaminant mass flow. Although atmospheric dilution reduces immediate risk near ground, regulatory acceptance controversial because pollutant transferred rather than destroyed, necessitating atmospheric dispersion modeling, risk assessment and air monitoring to avoid local hotspots. Distinguished from accumulation in roots or rhizosphere microbial degradation.