Effects of water vapour pressure deficit and stomatal conductance on photosynthesis, internal pressurization and convective flow in three emergent wetland plants

Internal pressurization and convective gas flow in emergent wetland plants is a function of the water vapour pressure deficit (WPD) and stomatal conductance (G(s)) separating the external atmosphere from the internal aerenchyma. We have compared the effects of WPD and G(s) under a range of light intensities on static pressures and convective flows in Phragmites australis, Typha orientalis and Baumea articulata. The capacity of the three species to generate flows per unit leaf area differed, being greatest in P. australis and lowest in B. articulata. In all three species, decreasing light intensity from full sunlight (2200 mumol m(-2) s(-1) photosynthetically active photon flux density (PPFD)) to < 200 and < 10 mumol m(-2) s(-1)PPFD caused immediate decreases in photosynthetic assimilation, followed by more gradual decreases in transpiration and G(s). However, internal pressures and flows in the two low light intensities remained similar to values recorded in full sunlight. WPD was more significantly related to pressures and flows in P. australis and T. orientalis than G(s). In B. articulata, pressures increased at low G(s) values but flow rates were unaffected, as predicted by earlier models describing pore size effects on pressures and flows. The data suggest that emergent macrophytes can maintain significant internal convection even at low light intensities, and this may be beneficial for nocturnal aeration, particularly in arid climates where the atmospheric humidity at night is low.