Salt Tolerance in plants
Salinity is known to affect at least 20% of all arable land and the amount of land affected is doubled in areas that require irrigation. This has a major affect upon the plants that can grow in these regions, and if salinity becomes to high it is often the case that fields need to be abandoned and no crops are able to grow.
Although plants require salts for development and growth, the vast majority of plants suffer from ionic and osmotic stresses when grown in areas of high salt, such as arid regions with poor drainage, coastal areas, and areas where irrigation has left a residual deposit of salts. Even though most plants suffer badly in areas of excessive salts, some have developed to become salt tolerant.
Classifications of Plants with Regards to Salt
There are two main classes of plants with regards to salt; those that can grow in salty conditions and those that cannot.
1. Halophytes - These plants have evolved so as to tolerate high salt levels.
2. Glycophytes - These plants cannot tolerate high salt levels.
Unfortunately most crop species belong to the glycophyte class of plants. Some of the common symptoms associated with glycophyte plants when grown in areas of high salinity include:
1. Osmotic stress
2. Increased ionic stress
3. Oxidative stress
4. Nutritional disorders
5. Sodium toxicity
6. Chloride sensitivity
Even though halophytes are able to tolerate and grow in soil that has a high salinity their growth can still be affected by salt levels; this is usually as a result in a decreased osmotic potential in salty conditions which limits the ability of plants to uptake water.
Research into Salt Tolerance in Plants
As mentioned above over 20% of the worlds arable land is affected by salt. It is thought that by breeding plants that are resistance to high saline levels may result in this land becoming available or more productive for crop species. With this in mind much research is being carried out into salt tolerance in plants. This research can be broke down into the four main areas:
1. Salt toxicity and tolerance physiology - the metabolic and cellular response to salt
2. Analysis of mutant plants that have altered responses to salt tolerance
3. Salt transport mechanisms - ion transporters involved in salt uptake, compartmentalisation, and control mechanisms
4. Genes involved in salt regulation
Salt tolerant plants - Salt entry into root cells
As over 20% of all arable land is affected by high salt levels much research work has been carried out into understanding the mechanisms involved in how plants deal with salt and increases in salinity. Plants can be categorized into two main types, salt intolerant glycophyte and salt tolerant halophytes.
Arabidopsis thaliana is one of the most used plants in research and although it is a glycophyte it has helped to further the understanding of salt tolerance immensely as salinity stresses and mutations that can overcome these stresses can be investigated.
Mechanisms Involved in Root Cell Salt Entry
Although plants to not have mechanisms specifically developed for the uptake of salt ions there are many ways in which salt is able to enter the plant cell passively. It therefore follows that In areas of high salinity a greater amount of salt will be took up by the plant.
One of the main ways in which salt is passively took up by plant root cells is via cation channels, these have two main classifications:
1. Voltage dependent cation channels
2. Voltage independent cation channels
With regards to voltage dependent cation channels it is thought that the potassium transporters may act to import sodium ions into the root cells with a low affinity, whilst this is not usually of importance in areas of high salinity, this can result in an high uptake of salt into the plant via the root cells.
Despite the role of potassium channels in salt uptake, it is known that the voltage independent cation channels play a much larger role in salt entry to root cells.
Mechanisms that Control Salt Uptake in Root Cells
Although there is a gene known as 'salt overly sensitive' that has been shown to be involved as a part of a salt regulation pathway in arabidopsis many other factors act to control the regulation of salt uptake and removal in plants these include:
1. Soil acidity
2. Membrane potential
3. Salt concentration and form
4. the action of plant hormones
Plants and Salt Ion Toxicity
As much as 20% of all arable land is affected by high salt levels; this has a major impact upon the yield and type of plants that can be grown in these areas. Much research has been carried out into salt tolerance in plants with the hope that one day crop plants can be bred that have a higher tolerance to saline toxicity and will be able to grow or/and produce larger yields.
Much of the salt uptake into plants acts through passive networks such as those associated with cation transportation such as potassium ion channels. This section takes a look into the mechanisms that plants have developed to deal with ion toxicity.
Methods Used by Plants to Deal with Ion Toxicity
There are many mechanisms that plants are able to employ in combating salt stress, retain homoeostasis and overcome ion toxicity. Some of these mechanisms include:
1. Restriction the mechanisms involved in salt uptake
2. control of long distance transport of salt
3. compartmentalisation of salt
4. extrusion of salt from the plant
5. Prioritising the maintenance of potassium to sodium ratio levels in the cytosol
Effects and Control of Ion Toxicity in Plants
The presence of increased salt ions can lead to many toxic affects. An excess of sodium at the root surface can seriously impact the uptake of other cations such as potassium; this can lead to disruption of membrane potential, cell turgor and the disruption of enzyme function. Plants are able to overcome this to a degree by operating selective high affinity potassium channels, however even with these mechanisms in place the presence of large quantities of salt in the soil means that the plant can not take up potassium at will, resulting in slower growth and a lack of vigour.
The presence of sodium ions in the cytoplasm can have a dramatic inhibitory effect to many enzymes. Interestingly even in salt tolerant halophytes increases in salt in the cytoplasm is toxic and the ratio of potassium to sodium ions in the cytoplasm is maintained similar to that seen glycophyte plants. To help to reduce the level of sodium in the cytoplasm halophyte plants compartmentalise excess salt in their vacuoles.
Calcium and the Control of Salt Tolerance
One of the essential elements that is involved in the control of salt tolerance is calcium. If calcium is readily available plants are better able to maintain homoeostasis in saltier environments. It is the ratio of cytosolic to internal calcium that is important to the regulation of salt sensitivity. Calcium ions are also able to help with ion toxicity by maintaining potassium transport mechanisms and the selectivity of potassium to sodium uptake in areas of high salinity.
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