I MATERIALS AND METHODS Gas exchange characteristics were studied in two mangrove species, Aegiceras cornicalatum (L.) Blanco and Avicennia marina (Forstk.) Vierh. var australasica (Walp.) Moldenke, grown under a variety of salinity and humidity conditions. The assimilation rate was measured as a function of the intercellular CO2 concentration IA(c,) curve]. The photosynthetic capacity decreased with increase in salinity from 50 to 500 millimolar NaC1, as shown by decline in both the initial linear slope and the upper plateau of the A(c;) curve, with A. corniculatum being the more sensitive species. The decline in photosynthetic capacity was enhanced by increase in the leaf to air vapor pressure difference from 6 to 24 millibars, but this treatment caused a decrease in only the upper plateau of the A(cp) curve. Stomatal conductance was such that the intercellular CO2 concentration obtaining under normal atmospheric conditions occurred near the transition between the lower linear and upper plateau portions of the A(c;) curves. Thus, stomatal conductance and photosynthetic capacity together co-limited the assimilation rate, which declined with increasing salinity and decreasing humidity. The marginal water cost of carbon assimilation was similar in most treatments, despite variation in the water loss/carbon gain ratio. Humidity has been reported to modify the response to salinity in several plant species, presumably because of interactive effects of these factors on carbon gain in relation to water use and hence also ion uptake. With increasing salinity, increasing humidity adversely affected the growth ofAtriplex halimus, a halophyte of arid regions (14), but ameliorated the reduction in growth in the glycophyte, Gossypium hirsutum (11), and in the mangroves, Aegiceras corniculatum and Avicennia marina (1). The decline in growth rates of the mangrove species with increasing salinity and decreasing humidity was attributed to decrease in both the leaf area/plant mass ratio and A2, with the latter being the major factor (1). The present study describes the influence of changes in stomatal conductance and photosynthetic metabolism on water use in relation to carbon gain by A. corniculatum and A. marina grown under different salinity and humidity regimes. 'This work has been submitted by M. C. B. in partial fulfillment of the requirement for the Ph.D. degree. 2Abbreviations: A, photosynthetic CO2 assimilation rate; c,, intercel-lular CO2 concentration; A(c,), assimilation rate as a function of the intercellular CO2 concentration; E, evaporation rate; g, leaf (primarily stomatal) conductance to water vapor, vpd, leaf to air vapor pressure difference. Plant Material. Propagules3 of Aegiceras corniculatum (L.) Blanco and Avicennia marina (Forstk.) Vierh. var australasica (Walp.) Moldenke were collected from trees growing along Cul-lendulla Creek, New South Wales, Australia (35°42'S, 150°1 2'E). These propagules were cultivated in sand beds, and subirrigated with 10% and 50% seawater, respectively, at which growth of the ...

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ABSTRACIMeasurements of ps exchange characteristics were made on intact, attached leaves of hydroponically grown seedlings of Avicennia marina (Forstk.) Vierh. var australasica (Walp.) Moldenke as the NaCI concentration of the culture solution was varied by step changes of 50 millimolar NaCI every 2nd day from 50 to 500 to 50 millimolar NaCl. The CO2 assimilation rate, stomatal conductance, intercellular CO2 concentration, and evaporation rate decreased at salinities above 250 millimolar NaCI and recovered substantially upon return to the original salinity.The give a vpd2 of 12 mbar, and a 12-h photoperiod with an average of 400 ,E m 2 s-' incident at leaf height. Upon reaching a postcotyledonary phase of development, the seedlings were transferred to 500-ml containers for hydroponic culture in Johnson's nutrient solution (8), which was made to approximate 10% sea water by addition of 50 mm NaCl. Four seedlings (three experimental seedlings plus a control) were then grown hydroponically until the next flush ofleaves was fully developed before beginning the experiment. Solutions were changed weekly and the levels were maintained by addition of distilled H20 every other day. The time required for leaf development under these conditions is approximately 6 weeks.Experimental. Gas exchange characteristics were determined on the same leaf of each plant over a period during which the NaCl concentration of the nutrient solution was increased from 50 to 500 mm NaCl and then returned to the original salinity by step changes of 50 mm NaCl. Solutions were changed at the beginning of the dark period with an interval of 48 h between changes. Measurements of gas exchange characteristics were made while the plants were experiencing concentrations of 50, 150,250, 350, 500, and 50 mM NaCl. The salinity ofthe nutrient solution ofthe control plant was maintained at 50 mm NaCl and the solution was changed at the same time as those ofthe salinitytreated plants. Dark respiration rates were measured before illumination and were followed by measurement of the assimilation rate as a function of the intercellular CO2 concentration, ci. Variation in the latter was obtained by changing the ambient CO2 concentration in the sequence 330, 400, 500, 200, 100, and 50 Ml 1-'. Other conditions were similar to those experienced in the growth chamber, i.e. leaf temperature of 25C, quantum flux density of 500 ME m-2 s-', and a vpd of 12 mbar. Atmospheric pressure usually was 950 mbar. Boundary layer conductance to diffusion of water vapor was 0.4 mol m 2 s-'. RESULTS Gas Exchange Characteristics under Normal AtmosphericConditions. The effects of short term changes in salinity on the 2Abbreviations: vpd, leafto air vapor pressure difference; A, photosynthetic CO2 assimilation rate; A(cd, assimilation rate as a function of the intercellular CO2 concentration; ci, intercellular CO2 concentration; E, evaporation rate; g, leaf(primarily stomatal) conductance to water vapor, g,, total (boundary layer plus leaf) conductance to CO2.

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