Photosynthetic gas exchange of the mangrove, Rhizophora stylosa Griff., in its natural environment

Photosynthetic gas exchange properties of leaves of the mangrove, Rhizophora stylosa Griff., were investigated in order to assess its productivity and gain some insight into the constraints set upon it by the saline habitat. Mature trees of this dominant species were studied in their natural, tidal-forest environment at Hinchinbrook Is., North Queensland for two periods during the dry season. Individual leaves were enclosed in a chamber wherein environmental conditions were varied. CO₂ assimilation, transpiration and environmental parameters were monitored during daylight hours by instrumentation housed in a mobile laboratory mounted on a barge. Analysis of the daily course of leaf gas exchange revealed a CO₂ assimilation capacity comparable with that of many glycophytic trees. Photosynthesis was strongly influenced by leaf temperature as well as photon flux density. There was a strong and steadily increasing inhibition of gas exchange as leaf temperatures and, consequently, the leaf to air VPD increased. CO₂ assimilation rates and leaf conductances to water vapour diffusion were strongly correlated, resulting in nearly constant internal CO₂ concentrations in the leaves under the full range of conditions. The effect of leaf orientation in minimizing the leaf-to-air temperature difference was striking. The close coordination between stomatal conductance and CO₂ assimilation rate in this mangrove results in high water use efficiency. This sparing use of water may be an important factor underlying the high salinity tolerance of mangroves.