Photosynthetic gas exchange and water use in tropical and subtropical populations of the mangrove Aegiceras corniculatum

Photosynthetic gas exchange and stomatal behaviour in the tropical (Darwin: 12°25'S) and subtropical (Ballina: 28°50'S) populations of the mangrove Aegiceras corniculatum (L.) Blanco were compared at elevated air temperatures (>32°C) and leaf-to-air vapour pressure deficit (vpd >25 mbar) with a quantum flux at, or above, their light saturation capacity (>600 μmol m^-2 s^-1). At the lower end of the tested aridity range, the tropical population showed a less conservative water use than the subtropical population. As aridity increased, both populations showed a reduction in stomatal conductance. However, transpiration rates remained higher in the subtropical population at all times, reducing its water-use efficiency from that under less arid conditions. At extreme aridity (temperature >37°C and vpd >35 mbar), the efficiency of the evaporative cooling of fully exposed leaves was evident in the leaf-to-air temperature differential which remained minimal in the tropical population while it increased significantly in the subtropical population. Aridity tolerance was more pronounced in individuals from the tropical site than from the subtropical site, as evidenced by a tighter stomatal control on water use in the tropical population. These data suggest that the tropical and subtropical populations of A. corniculatum have different physiological responses to aridity.