Gas exchange responses of the wetland plant Schoenoplectus hallii to irradiance and vapor pressure deficit

Abstract Schoenoplectus hallii had a maximum rate of assimilation ( A max ) of 13.3 ± 1.3 μmol CO 2  m −2  s −1 , a light compensation point (LCP) of 24 μmol photons m −2  s −1 , a light saturation (LS) of ca. 1200 μmol m −2  s −1 and a dark respiration rate (DR) of 1.3 ± 0.6 μmol CO 2  m −2  s −1 . Stomata of S. hallii did not respond to vapor pressure deficit (VPD): increases in VPD explained only 7% of the variation in assimilation ( A ) and 24% of the observed variation in conductance ( g s ) but accounted for 96% of the variation in transpiration ( E ). The relationship between photosynthesis and transpiration resulted in a decrease in instantaneous water-use-efficiency ( A / E ) with an increase in VPD. This “drought escape” strategy has been reported for a variety of desert ephemerals, but this is the first evidence of non-responsive stomata in a species from a transient wetland habitat. It was concluded that the species is adapted to its high light habitat, but that its existence is threatened by a reduction in available soil moisture. In the development of conservation and reintroduction strategies for S. hallii , its dependence upon high light and abundant soil moisture should be considered of critical importance to the success of any management plan.