Quantifying effects of atmospheric CO2 enrichment on stomatal conductance and evapotranspiration of water hyacinth via infrared thermometry

Measurements of stomatal conductance and evaporative water loss from two tanks of water hyacinths growing at Phoenix, AZ, one under ambient conditions and one considerably enriched in atmospheric CO₂, are reported. Stomatal conductances of plants in the CO₂-enriched treatment were reduced to values half as great as those of plants in the ambient treatment at a mean mid-day CO₂ concentration of 550 ppm, which resulted in a 22% decrease in total evaporative water loss; while in going from an ambient CO₂ concentration of 310 ppm to a doubled concentration of 620 ppm there was a 27% decrease in evaporative water loss. Both of these physiological responses were well characterized by the Idso—Jackson plant water stress index. Additionally, it was found that the stomatal response to increasing atmospheric CO₂ was identical to that induced by removing water from the plant roots, and that the reduction in evaporative water loss with increasing atmospheric CO₂ was an inverse linear function of the plant water stress index — both of which phenomena had previously been theorized but never before experimentally verified.