TOCUYO DE LA COSTA (20m)
64 68 72 76 80 84 Years
8.2.3 Strategies of Adaptation of Plants in the Different Vegetation Units
18.104.22.168 Small Perennial Halophytes: Salt Inclusion and Stress Tolerance
The zone of small perennial halophytes surrounding the edges of vegetation islands and bordering the vegetation free salt flats is dominated by three species, namely Portulaca rubricaulis, Sesuviumportulacastrum and Batis maritima (Fig. 8.11). The creeping fruticose stems of the first two species contrast with B. maritima, which has a more upright growth habit. The three species are also primary colonizers of
the sand plain. They occupy the most extreme habitats in the sand plain, which range from being flooded with fresh water in the rainy season to dry, salt-encrusted soil in the dry season when the water table may drop to 1 m below ground (Luttge et al. 1989b; Figs. 8.5 and 8.8).
All of the three halophytes are salt includers and accumulate NaCl in their highly succulent leaves. However, P. rubricaulis combines this strategy of stress tolerance with that of stress avoidance in that it is deciduous and sheds its leaves during the dry season. Among the three species, it has the lowest salt concentrations in its leaf sap, viz. 230 mM Cl- and 60 mM Na+, while the other two species have much higher salt levels in their leaves, i.e. 260 -1, 080 mM Cl- and 370 - 720 mM Na+ in the wet season, and 540- 1,410mM Cl- and 920-1,590mM Na+ in the dry sea son, respectively. However, P. rubricaulis is a C4-plant while the other two species are C3-plants. Thus P. rubricaulis may use the higher instantaneous productivity of C4-photosynthesis (see Box 10.2) for production of enough perennial shoots to compete in the habitat effectively.
B. maritima also accumulated sulphate, with a twofold increase of leaf-sap concentrations in the dry season. In S. portulacastrum, Na+ accumulation exceeded Cl- accumulation by far and synthesis of the organic acid anion oxalate is found to serve in maintaining charge balance. Increased salt accumulation in the leaves of B. maritima and S. portulacastrum in the dry season is accompanied by a 1.5- to 2-fold increase in leaf succulence (see also Sect. 7.4). S. portulacastrum was also shown to use compatible solutes (see Sect. 7.4 and Box 7.1) such as proline and pinitol which augments the tolerance of salt inclusion (Fig. 8.12).
In leaves of the erect stems of B. maritima photosynthetic gas exchange, measured as net CO2 uptake and transpirational loss of water vapour, shows little response to the transitions between the rainy and the dry season (Fig. 8.13). In contrast, photosynthesis in the leaves of the prostrate stems of S. portulacastrum is severely impaired in the dry season, showing a pronounced midday depression of gas-exchange (see Sects. 22.214.171.124 and 10.1.2.3) and about 40% inhibition of light saturated rates of photosynthesis (Fig. 8.13). Thus, S. portulacastrum clearly suffers more under the stress of the dry season but it is also more of a pioneer coloniser of the sand plain as it occupies the outermost edges of the vegetation islands and larger areas of the flat plain (Figs. 8.7B and 8.11C).
In summary, the three species of halophytes, which are subject to very similar challenges by extreme environmental conditions, have different strategies of adaptation to stress. Notwithstanding the similarities in life-forms, with small fruticose stems and succulent leaves, the differences in ecophysiological comportment reflect ecological diversity, and once again prove to be a basis for species diversity (see Sect. 3.3.2).
Fig. 8.12 Concentrations of the compatible solutes proline and pinitol in the leaf sap of Sesuvium portulacastrum in the wet season (W) and the dry season (D). (Luttge et al. 1989b)
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