A pot culture experiment was conducted in Open Top Chambers during 2007-08. The plantlets (ramets) of water hyacinth were grown in pots with four different media (M1- tap water, M2- distilled water, M3- hoagland solution and M4- hoagland solution with added heavy metals) in three replications and the pots were kept in open top chambers (OTCs), maintained at ambient (360±20ppm) and elevated CO2 (550±30 ppm), and in open field conditions. Pots in three replications from each media-without plant-were kept under the above three conditions as control to measure the evaporation for WUE estimation. The growth of the plants grown in M1 and M2 was severely affected. The plants grown under elevated CO2 and nutrient rich media (M3 and M4) maintained higher green-leaf area over the growth period and recorded higher net assimilation rate (NAR). CO2 enrichment resulted into reduction of water loss (increased WUE) from plants grown in hoagland (M3) and heavy metal (M4) solutions. When the comparison was made in between M3 and M4 treatments, there was tremendous increase in WUE (reduced transpirational loss of water per gram of dry matter produced) in plants grown in M4. the elevated CO2 enhanced the uptake of heavy metals like Cu, Fe, Mn and Zn in both the media but it was higher in M4 than in M3 due to increased availability.