DIURNAL GAS-EXCHANGE CHARACTERISTICS AND WATER-USE EFFICIENCY OF 3 SALT-SECRETING MANGROVES AT LOW AND HIGH SALINITIES

Continuous measurements of gas exchange characteristics were made on t wo to nine year old hydroponically grown Avicennia germinans (L.) Stea m, Aegialitis annulata R. Br. and Aegiceras corniculatum (L.) Blanco m aintained at 50 or 500 mol m(-3) NaCl. In Avicennia germinans and Aegi alitis annulata, CO2 assimilation rates were initially higher at 500 m ol m(-3) NaCl and decreased gradually towards the end of the photoperi od when rates were similar to those at the lower salinity. In Aegicera s corniculatum, assimilation rates were higher at 50 mol m(-3) NaCl an d about 55% lower at the higher salinity. In all three species, leaf c onductance and transpiration exhibited trends similar to those for CO2 assimilation. Intercellular CO2 concentrations were similar at both s alinities in Avicennia germinans and Aegialitis annulata, but consider ably higher at the lower salinity in Aegiceras corniculatum. Water use efficiencies (WUE), although similar between salinity treatments in A vicennia germinans and Aegialitis annulata, were greater at the higher salinity in Aegiceras corniculatum. Data obtained from CO2 response c urves indicated that assimilation at high salinity in Aegiceras cornic ulatum was limited by conductance, and to a lesser extent, by photosyn thetic capacity. In Avicennia germinans and Aegialitis annulata, assim ilation was greater at the higher salinity as indicated by increase in both the initial slope and the upper plateau of the CO2 response data . Greater assimilation at high salinity in Avicennia germinans and Aeg ialitis annulata may be attributed to lower carbon losses via photores piration and to efficient salt excretion and sequestration.