Halophytes: Adaptations to the Saline Environment.
The halophytic plants are unique in their ability to tolerate salt, living in or close to the marine environment these plants have evolved to cope with high salinity which would otherwise exclude fully terrestrial plants. In many cases the halophytes are of particular importance to us humans, often forming coastal defences, trapping atmospheric CO2 and forming a diverse habitat which can support an array of commercially exploited species. We will look at the difficulties faced by halophytic plants and how their physiological adaptations have led to their success in saline environments.
Like any other lifeform on earth, plants require water to facilitate many biological processes. To acquire water plants will use an osmotic gradient to draw water into the cells across a semi-permeable membrane. To do this plants will move ions to the roots or another part of the anatomy to create an internal hypertonic environment, thus creating a gradient between that of the external hypotonic environment. The membrane between the internal environment and external environment is only permeable to water and other small molecular compounds. So that equilibrium of the concentration of ions and salts can only be achieved through the passage of water or small dissolved ions into the cell. However, if the water outside of the cell has a higher ionic concentration than the inside, this process will be reversed and water will begin to leave the cell. If a relatively large amount of water leaves the cell it will become plasmolyzed (Figure 1, A). In contrast, if too much water enters the cell it will burst (Figure 1, C). The cell must aim to maintain a turgid state, meaning that the cell is sufficiently filled with a solution so that is neither plasmolyzed nor about to burst, and the cell can function properly (Figure 1, B).
There are two types of response to salt seen in the halophytes to maintain the osmotic gradient between within their cells. There are exclusion adaptations – adaptations that act to stop salt entering the cell; and inclusion adaptations – those that act to accumulate and then store or discharge the salt that enters the cells. Let’s take a closer look at several actual examples of these adaptations in the real world.
Avicennia marina (Figure 2, Main image) is found in the mangrove forests of Southern Sinai, Egypt. Along with other mangrove trees, A. marina is highly adapted to saline conditions possessing a combination of exclusion and inclusion adaptations. At the roots, it is able to prevent up to 80% of the salt present in seawater entering the plant by maintaining a strongly hypotonic environment leading to a high gradient which in turn acts to power filtration of dissolved salt ions from water flowing into the cells. This filtration prevents most of the salts from entering the plant through the root. The salt that is able to enter the system is then pumped to the leaves within the xylem (tree sap) where more of the salts (around 40%) are excreted onto the leaf surface via glands (Figure 2, Sub image). The remaining salt left in the system is then transported, again in the xylem, to leaves that are reaching the end of their lives and will soon be shed.
Salicornia europaea (Figure 3) is one of the most salt tolerant plants and has been found to be facilitated by the presence of salt whereas in some halophytes, although tolerant of salt they grow at a better rate at lower salt levels. S. europaea absorbs salts dissolved in waters and moves those salts into the vacuole of the cell in a process called compartmentalisation. By compartmentalising the absorbed salts the cells of S.europaea are able to remain turgid and continue proper cell function because the ionic gradient is controlled, but salt is stored away from the cytoplasm and organelles of the cell.
Mesembryanthemum crystallinum uses organic compounds that are produced within the cell that acts to create ionic balance without allowing salt from the external environment from entering the cell. This mechanism for salt tolerance works in much the same way as S. eurpaea.
These examples represent some common physiological adaptations in halophytic plants that allow them to grow in environments that would otherwise be void of plant matter. In future scenarios of global food and energy shortages, these adaptations may be of great benefit to the human race. In areas close to the sea which would not be used for agriculture, halophytic crops show great promise in providing bio-fuels and even food in the case of the recently popularised Rock Samphire (Crithmum maritimum).