W D

Fig. 8.13 Net CO2 exchange ( Jco2) and water-vapour conductance of leaves (gH2o) of Sesuvium portulacastrum (•, o) and Batis maritima (A, A) in the sand plain of Chichiriviche, Venezuela, in the wet season (o, A) and the dry season (•, A). (Lüttge et al. 1989b)

8.2.3.2 Terrestrial CAM Plants: Salt Exclusion and Stress Avoidance

There are two different life forms of terrestrial CAM plants on the sand plain, which are stem succulent cacti and tank forming bromeliads. They are salt excluders. In this way they avoid the physiological stress of salinity, and their use of the flexibility of the CAM cycle (see Sect. 5.2.2.2 and Box 5.1) is important in this strategy in the salinas (Luttge 1993).

8.2.3.2.1 Columnar Ceroid Cacti

The dominating cactus of the salinas at the North coast of Venezuela is the columnar cactus Subpilosocereus ottonis with strongly branching individuals more than 6 m tall (Fig. 8.14A). Small seedlings are frequently found at the rim of vegetation islands (Fig. 8.14B) as well as among other vegetation and within the tanks

Fig. 8.14A-F Large plants (A), small seedling (B) and seedling rerooted after injury in the field (C) of Subpilosocereus ottonis in the alluvial plain of Chichiriviche, Venezuela. Experimental seedlings of Cereus validus (D), one rotting during an extended salt treatment (E), and another one dried out at the bottom and totally insulated from the substratum (F)

Fig. 8.14A-F Large plants (A), small seedling (B) and seedling rerooted after injury in the field (C) of Subpilosocereus ottonis in the alluvial plain of Chichiriviche, Venezuela. Experimental seedlings of Cereus validus (D), one rotting during an extended salt treatment (E), and another one dried out at the bottom and totally insulated from the substratum (F)

of bromeliads. A similar type of cactus occurs in the subtropical salinas, Salinas Grandes in Argentina, namely Cereus validus. Experiments with 10 cm tall seedlings of C. validus (Fig. 8.14D) and analyses of S. ottonis in the field show that these cacti are strong salt excluders (Fig. 8.15). When subject to salinity the roots rapidly accumulate large amounts of NaCl. However, there is no salt export from the roots to the shoots, so that the peripheral green stem chlorenchyma and the central water storage parenchyma of the stems receive very little additional salt during a salinity treatment of up to 14 days and 600 mM NaCl in the root medium (Fig. 8.15).

It is quite obvious that the fine absorptive roots of the cacti die under the stress of salinity and the rest of the cactus becomes quite isolated from the substratum. When the salinity treatment is extended for several months, in some cases salt solution may diffuse upwards killing the stem tissue so that eventually the whole cactus seedling rots away and dies (Fig. 8.14E). In other cases, however, the base of the shoot dries out and then finally becomes insulated very effectively from the ground (Fig. 8.14F). The major part of the green stem survives. But by what means and for how long?

Certainly, the cacti cannot survive indefinitely without water and nutrient supply from the substratum. In the strongly seasonal salinas, of course, they only need

Applied NaCl (mM)

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Renewable Energy 101

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