Adaptation Reactions to Restore Ion Homeostasis
Glycophytes and halophytes basically make use of the same mechanisms to deal with salinity stress. Halophytes are able to adapt faster and to tolerate extreme salinity, whereas glycophytes adapt stepwise to develop tolerance to a moderate degree of salinity. In both constitutive types salt tolerance requires the expression of certain, similar genes. However, the transition from extremely salt-sensitive plants to extremely salttolerant plants is not clear cut, and differentiation between the two types is difficult because of the plant's ability to adapt.
Considering the many examples of salt-stressed glycophytes and halophytes having been investigated, it becomes clear that adaptation serves to regain ion homeostasis and the original membrane potentials and pH values. The better the plant is able to achieve this, the better it becomes tolerant of osmotic stress. However, regaining ion homeostasis does not signify a "return" to the original ion concentrations. Rather, the plant attempts to achieve homeostasis with the entire ionic load, including the NaCI that has been taken up (see Fig, 1.6.2 C). Depletion of the aggressive NaCI ions in the cyto plasm during adaptation requires considerable energy, which cannot be supplied with the normal provision of the cellular transport systems. Therefore, more of those proteins are synthe-sised which contribute either directly or indirectly to the removal of NaCl from the cytoplasm. Both the apoplast and the vacuole can be compartments for the "final" deposition.
Relief of the cytosol by removal of the Na+ ions from the cytoplasm and their transport into the vacuole and the apoplast is effected mainly by means of Na+/H+ antiporters, both in the plasmalemma (e.g. in Atriplex nummularia; Has-sidim et al. 1990; see also Chap 18.104.22.168) and in the tonoplast (NHE, Fig. 1.6.3 B), e.g. in Beta vulgaris (Barkla and Blumwald 1991). These antiporters are induced by elevated cytosolic Na+ (NaCl) concentrations and require high proton concentrations in the external medium and in the vacuole, i.e. effective H+ pumps. These pumps are synthesised to a greater extent upon salt stress. If ion homeostasis is established, the increased expression of, e.g., the plasma membrane H+-ATPase decreases once more. Adapted cells no longer exhibit enhanced pump activity. Figure 1.6.2 C shows the result of salt adaptation with regard to the distribution of ions. The Na+ concentration in the cytosol has returned to the original value (Fig. 1.6.2 A), while that in the vacuole has increased many-fold. The original membrane potential and pH value have nevertheless been reestablished. Over-expression of a vacuolar Na+/H+ antiporter gene in Arabidopsis increased salt tolerance considerably (Apse et al. 1999). The K+ status is the weak point in the cellular ion budget subsequent to an adaptation process, irrespective of ion homeostasis. The reduced growth of halophytes and glycophytes under salt stress is possibly related to the problematical provision of the cell with K+ (see Box 1.6.1).
The effectiveness of salt elimination into the apoplast is supported, in many halophytes, by salt glands (Fig. 1.6.7), which lead to the deposition of NaCl onto the surface of the leaf, where it then crystallises. This applies particularly to some mangrove species, but also to desert plants (e.g. the genus Reaumuria and other Chenopo-diaceae).
Other halophytes can enhance the sequestration of salt in the vacuole by developing large salt-storing mesophyll cells (salt succulence, Fig. 1.6.8 A) or by loading large bladder hairs with NaCl (Fig. 1.6.8 B). Both these excretion mechanisms are also termed recretion. Leaves which have accumulated high concentrations of salt are shed.
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