Abstract: Methods plants use to tolerate salts with special emphasis on Salt Cedar - Tamarix spp.
Soil salinity is a problem worldwide with over 5 x 108 hectares affected and 1 x 107 to 2 x 107 hectares of irrigated agricultural land deteriorating each year because of salt buildup. Plants use different methods to survive the increase in soil salinity. Following is an abbreviated listing of the methods of salt tolerance/avoidance used by higher plants.
1. Tissue tolerance - ways to avoid excess sodium (Na) and chloride (Cl) levels in living tissues
2. Mechanisms to avoid the buildup of Na and Cl in plant tissue
a. Na and Cl is sequestered into shoots, organs, and other tissues (older leaves)
b. Na and Cl is sequestered into vacuoles within the cell. This protects cytoplasmic enzymes and chloroplasts
c. Non-ionic solutes such as glycinebetaine or praline are produced to osmotically balance the Na and Cl ions stored in the vacuoles
d. Enhanced tolerance of membranes and cytoplasm
e. The use of Na ions to replace potassium (K+) ion in certain metabolic functions
a. Movement of Na or other salts from the shoot to roots. This involves the active removal of Na ion from the xylem sap to phloem by specialized cells (stella and transfer). This movement of salts to the roots may result in excessive rootzone levels of salts and the breakdown of the complete system. This also requires the use of high levels of energy.3. Avoidance of salt uptake by controlling internal water pressures
b. The restriction of salt uptake by a thicker casparian strip in the roots or a second endodermis in the inner root cortex. Some plants alter K ion and Ca ion root cell concentrations to limit Na ion uptake.
c. Some halophyes excrete salts through salt glands. In some species, salts are translocated to lower leaves that are then shed. Tamarix species (Saltcedar) excrete salts through salt glands.
a. Osmotic adjustment. This results in continued water uptake, maintenance of turgor pressure, cell expansion, favorable water and turgor pressures and continued photosynthesis and protein synthesis.
b. Greater succulence. By producing large cells and retaining higher concentration of cellular water, plants can dilute the salt concentration.
The tolerance mechanism used different from species to species. Even within a species substantial variation in salt tolerance may occur. This discussion will focus on the techniques Tamarix utilizes to survive in high salt soils and the methods it uses to increase the soil salt level in its vicinity thus restricting competition by other plant species.
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