the same time leaf conductance for water vapour decreases hyperbolically providing a trade-off between enhancement of salt excretion and control of water loss (Sobrado 2002, 2004).
Measurements of gas exchange of mangrove trees in relation to the degree of substratum salinity are summarized in Fig. 7.14, where for comparison the units of reference of different authors were unified so that salinity is roughly indicated as that of 1/10, 1/2 and 1/1 of sea water. (This also applies to Sect. 7.5.2 below.) An effect of salinity on net CO2-uptake (JCo2) is not very pronounced up to 1 /2-strength of sea water; only Avicennia corniculatum seems to be more sensitive than Avicennia marina and the 19 species averaged. Full strength (1/1) sea water then reduces JCO2 as stomatal conductance of the leaves (gH2O) is also declining, but these effects are not dramatic. Internal CO2-partial pressure (pCO2) remains between 150 and 250Pa/MPa. The increase of pCo i® A- corniculatum, while stomata partially closed (reduced gH2O) and JCO2 strongly decreased at 1/1-strength sea water can be explained by photoinhibition (see Sect. 7.5.3) preventing fixation of internal CO2. In any of the cases shown in Fig. 7.14 comparatively high rates of photosynthesis are still maintained at full strength sea water. These rates compare well with rates of glycophytic C3-plants and even C4-plants in the absence of salinity (Table 7.2), which underlines the strong capacity of mangroves to perform effectively under high salinity.
Comparative studies of Conocarpus erectus and Avicennia germinans during the rainy season and the dry season offer additional insights into the success of man-
Table 7.2 Maximum rates of net-CO2-uptake (JcO2) and water-use-efficiency ratios (WUEratio) for mangrove trees at various salinities (as summarized in Figs. 7.14 and 7.19) in comparison to C3-, C4- and CAM-plants (Black 1973)
Maximum Jco2 WUEj.atio x 103 (|imolm-2s-1)
Mangroves 1 /10 sea water 1 /2 sea water 1/1 sea water
C3-plants C4-plants CAM-plants Darkness Light
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