Atmospheric CO₂ Enrichment of Water Hyacinths: Effects on Transpiration and Water Use Efficiency

Open‐top clear plastic wall chambers enclosing pairs of sunken metal stock tanks, one of each pair of which contained a full cover of water hyacinths, were maintained out‐of‐doors at Phoenix, Arizona for several weeks during the summer of 1984. One of these chambers represented ambient conditions, while the other three were continuously enriched with carbon dioxide to approximate target concentrations of 500, 650, and 900 ppm. During a 4‐week period when plant growth was at its maximum, water hyacinth biomass production increased by 36% for a 300–600 ppm doubling of the atmospheric CO 2 content, while water use efficiency, or the biomass produced per unit of water transpired, actually doubled. These results are similar to what has been observed in several terrestrial plants and they indicate the general trend which may be expected to occur as atmospheric CO 2 continues to rise in the years ahead.