Discussion Of The Adaptations Of Plants

The following sample essay on Discussion Of The Adaptations Of Plants. Salt marshes are intertidal ecosystems, surrounded by land and open to the sea by way of an estuary. As a result salt marshes are constrained, by silt and saline water, to the type of communities it can sustain. However because of the tidal nature of these ecosystems and the presence of many migratory birds we find some of the the most widley distributed plant species in the world. The habitat is typically high in phosphates but low in nitrogen.

Sulphur washed in from the sea collects in the soil and during dry periods lowers the soil pH.

Essentially the main problem for salt marsh plants is that salt marsh ecosystems are perminantly waterlogged by seawater. Land plants obtain oxygen for their roots by diffusion of oxygen found in air spaces in the soil. When soil is waterlogged, water displaces oxygen from the air spaces and reduces oxygen transport to as much as 1/300000 of its original value.

Most land plants are used to water that has an osmotis potential close to zero, however salt marsh plant communities have to exist in water conditions of much lower osmotic potential, approximately -2 Mpa. Most land plants subjected to such low osmotic potentials would loose water to its environment and die.

It is the aim of this paper to discuss some of the common mechanisms which plants colonising salt marsh habitats have evolved to make effiecient use of what oxygen is available and be able to exclude salt and absourb water or excrete any excess salt that is absourbed as a result, with reference to specific salt marsh species.

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Salt-marsh plants are halophytes (Gr halos:salt + phyton:plant), meaning they can tolerate excessive salinity levels, and have characteristics of both terrestrial and marine environments.

The salinity may vary and depends on the structure of the marsh, rainfall, and how often it floods. If rainfall is high the marsh is washed of some of its salinity and will be colonised by different species such as Limonium spp. (sea lavender) and Triglochin spp. Grasses and rushes, such as Spartina spp. & Juncus spp. retrospectively, dominate salt marsh communities, however in the lower, muddy levels of salt marshes pioneer species such as Salicornia europaea are more common.

Many of the plants are terrestrial species and in the upper parts of the marsh, where salinity concentrations are prodominantly affected by the amount of rainfall and not tidal influences, soils similar too more obvious terrestrial habitats are witnessed. Plants found in more northenly located salt marshes tend to be more tollerant of higher salinities, e.g. Spartina anglica can tolerate salinities up to twice that of sea water (Ranwell 1972). Salt marshes have a particularly low osmotic potential due to its high sodium chloride concentration. To prevent excess loss of water and to obtain water from its environment it is vital that plants maintain a lower internal osmotic potential than that of its external environment (Purvis 2001).

This is a problem for non-halophytes at concentrations >0.05 M (1/10 sea water). However halophytes, subject to sea water (0.5 M), can develop internal osmotic potentials greater than 20 bars. There is also the problem for plants living in saline environments of the high toxicity of Na and Cl both found in high concentrations in salt marshes. High external osmotic potential influences excess ion accumulation in the tissues of plants, resulting in irregular metabolism and for this reason plants living in salt marshes have to be highly selective in ion uptake. High external osmotic potential also has the effect of reducing plant growth, transpiration rate, water availability, and uptake of essential minerals. No other toxic substance, worldwide, restricts plant growth more than sodium chloride.

Salt marshes, like any other habitat have sub-habitats e.g. emergence marsh level, submergence marsh level, or tidal flat, which all present relatively different growing conditions for the species that occupy them. As a result we find plants that have preferences to these zones and hence have adapted a diverse array of methods to contend with the conditions the different zones subject them to. These adaptations and environmental preferences also affect limits of such things as growth, age, and clonal size.

Adaptation to saline environments has occurred in Salicornia so much so that not only can members of this species tolerate high NaCl concentrations but the ability to persist in fresh water environments has all but been lost. There are four different methods in which halophytes have adapted to various external osmotic potentials in order to maintain normal metabolic activities. They are ion selection, extrusion, accumulation, and dilution, of which more than one may be exhibited by any one plant.

The vast majority of salt marsh plant species are perennials with only few annuals present and confined to distinct salt marsh sub-habitats, such as Salicornia sp. and Atriplex sp. located in the pioneer and strandline zones respectively. The distinction that few annuals have adapted to a salt marsh environment led Chapman to describe it as a Hemicryptophyte (herbs with buds at soil level, protected by the soil itself or by dry dead portions of the plant) area.

One shared adaptation that most halophytes possess is they accumulate Na & Cl ions and transport them to their leaves. The ions are stored in leaf cell vacuoles increasing the salt concentration in the tissues of halophytes and hence lowering its osmotic potential. This brings us back to an earlier statement that it is vital for plants living in saline environments to maintain a lower internal osmotic potential than that of its external environment in order to prevent water loss and so water may be taken up more effectively.

An important scientific breakthrough in 1999 located a gene in the non-halophyte Arabidopsis which encodes for a Na/H ion antiport protein in the tonoplast and enables sodium uptake. Although this plant is not a common halophyte found in salt marsh environments it does help to understand how plants may evolve different methods to living in these environments. Another adaptation of halophytes which reduces the risk of poisoning by excessive accumulation of NaLC is the formation of salt glands in their leaves. Salt, extracted by the glands, collects on the leaf surface and is removed by wind or rain. Osmotic potential in the leaf will inevitable become more negative as salt is excreted by the salt glands; this generates an increase in the osmotic potential gradient thus enabling the leaf to obtain water from the root more readily.

The amino acid proline, is often stored in the vacuoles of halophytes which acts to lower the plants tissues osmotic potential. The saline nature of a salt marsh makes it particularly difficult for plants to obtain water. For this reason a common characteristic of halophytes is succulence, which acts as a water reserve. This water reserve can be used when NaCl concentrations are high, e.g. evaporation in the soil during low tide. Many succulent halophytes use crassulacean acid metabolism, a metabolic pathway which allows plants to store CO2 at night and then photosynthesis during the day with stomata closed. Reversed stomatal cycles also allow halophytes to conserve water by closing them during daylight periods.

Damage by wave action is a serious threat to plants living in certain zones of the salt marsh; therefore many species have morphological adaptations as a result. Salicornia for example presents minimum leaf appendages by reducing to a phylloclade form, however it still maintains adequate photosynthetic surface for the high light level habitat in which it inhabits. Water logging is a characteristic of most salt marshes, particularly in the lower zones and as a result oxygen diffusion rates are low. During spring and summer algal blooms e.g. Pleurosigma colonize high level tidal flats and produce millions of small oxygen bubbles on the surface of the water-logged mud. Salicornia take advantage of the better conditions with regards to oxygen availability by germinating in April & May with most of its growth occurring during the summer months. This is more obviously an ecological adaptation to saline environments; however this is just as important to understand as morphological adaptations are in plants existing in salt marsh habitats.

Pappus hairs found on the seeds of Aster tripolium aid dispersal. The seeds tend to stick together as a result and more often than not are dispersed by water with only few dispersed by the wind. This type of dispersal adaptation allows Aster to colonize open ground, within the salt marsh, relatively quickly. Spartina has many specific adaptations to existing in more seaward zones of the salt marsh, which allows it to out-compete most other species for these sub-habitats.

Spartina has been successful in these zones due to a type of polyploidy which promotes rapid growth, large size, and high fertility. High phenotypic plasticity also allows Spartina to take advantage of this zone by elongating its stems to penetrate aggregating mud in both pioneer and mature salt marshes. Other adaptations that enable Spartina to successfully colonize more seaward zones include large seeds with substantial food reserves, rapid shoot & root growth, deep anchor roots, and shoots well supplied with air spaces.

In conclusion Salt marsh habitats are intertidal ecosystems sustaining widley distributed plant communities which can exist in an environment which is typically high in phophates, low in nitrogen, has a low soil pH, suffers from waterlogging, and a low osmotic potential. The main problems for salt marsh plants is low oxygen diffusion rates and a low osmotic potential due to daily waterlogging by sea water. High osmotic potential results in reduced plant growth, transpiration rate, water availability, and uptake of essential minerals and due to the toxicity of Na & Cl, excessive accumilation can result in irregular metabolism.

All salt marsh plants are halophytes, tolerating excessive saline levels, with Spartina anglica for example capable of tolerateing salinities twice that of sea water. To prevent excessive water loss and to obtain water salt marsh plants maintain a lower internal osmotic potential than that of its external environment. Halophytes have evolved four different methods to maintain normal metabolic activities in various external osmotic potentials; ion selection, extrusion, accumulation, and dilution. Accumulation involves the plant transporting and storing Na & Cl ions to their leaf cell vacuoles.

Salt glands often found in the leaves of salt marsh plants extracts salt which collects on the leaf surface and is removed by wind or rain. Succulence is a common characteristic of halophytes acting as a water reserve and can dilute high NaCl concentrations for example at low tide. These methods are all effective in lowering internal osmotic potential and increasing the osmotic potential gradient thus enabling the leaf to obtain water from the root more readily. Crassulacean acid metabolism allows halophytes to store CO2 at night and conserve water during the daylight periods by photosynthesising with closed stomata.

Ecological adaptation include for example adaptation by Salicornia to low oxygen diffusion rates takes advantage of millions of oxygen bubbles produced on the surface of waterlogged mud in high level tidal flats by algal blooms. Morphological adaptations include for example adaptation by Salicornia to reduce damage by wave action by reducing to a phylloclade form thus presenting minimum leaf appendages. It is clear that most of these evolved adaptations are a result of plants attempting to survive in an environment which is subject to especially low oxygen diffusion rates and in particullay low osmotic potentials.

Reference

1. www.biome.ac.uk Chapman, L.J. & M.J. Reiss (2000) Ecology principals & applications, University press;
2. Cambridge Pomeroy, L.R. & R.G. Wiegert (1981) The ecology of a Salt Marsh, Springer-Verlag Inc;
3. New York Purvis, W.K., D Sadava, G.H. Orians, H.C. Heller (2001) Life: The science of biology, Sinauer associates; Massachusettes