Salinity Stress in Plants: Reactions and Mechanisms

In this article we will discuss about Salinity Stress in Plants. After reading this article you will learn about: 1. Reactions of Plants to Salinity 2. Mechanisms of Salt Resistance.  

Reactions of Plants to Salinity:

The reactions of plants to salinity depend on the specific degree of tolerance against soil salinity. Plants can be classified according to their biomass production under salt stress.

Four groups are distinguished:

i. Eu-halophytes, which show stimulation of productivity at moderate salinity (e.g., Salicornea europaea, Suaeda maritima).

ii. Facultative halophytes, showing slight growth enhancement at low salinity (e.g., Plantago maritima, Aster tripolium).

iii. Non-halophytes with low salt tolerance (e.g., Hordeum sp., Gossypium sp).

iv. Halophobicplants, e.g., Phaseolus vulgaris, Glycine max, which cannot tolerate salts.

From an ecological viewpoint, halophytes can be characterised as plants that survive to complete their life cycles at high salinities.

Mechanisms of Salt Resistance:

Salt resistance is the ability of plants to tolerate excess salt in their habitat without any significant impairment of their vital functions. It is a complex combination of various mechanisms, not a single process or adaptation, and therefore not controlled by a single gene. Plants can achieve resistance to salt stress either by tolerating the stress or by avoiding it.

i. Salt Tolerance:

Tolerance to salt stress is the ability to tolerate toxic as well as osmotic effects of salt ions, like Na⁺ and Cl^– ions in the cytoplasm. High concentrations of salt ions have been found in the cytosol of salt-exposed plants. Under such conditions, the cytoplasmic enzymes have to function in presence of salt ions.

This was investigated with the enzyme PEP carboxylase from halophytes Suaeda monoica and Chiorisgayana. It is the key enzyme for CO₂ fixation in plant leaves. Addition of substrate PEP to the extraction and storage medium helps to stabilize the enzyme. At low PEP levels of the assay medium, the enzyme is inhibited by NaCl but at high PEP levels the enzyme was activated by NaCl.

Salt ions changed the kinetic properties of the enzyme and were suggested to function as allosteric effectors. Also other agents like betaine, proline or glycerol could stabilize PEP carboxylase. Therefore, the salt tolerance of plants may depend riot only on salt exclusion from cytosol, but also changes in the microenvironment of the enzymes, for instance on whether substrate or protective agents have been increased.

A similar mechanism of enzyme protection was found with Rubisco of the woody halophyte Tamarix jordanis. It is the key enzyme in photosynthetic carbon reduction in plants. The carboxylating activity of the enzyme was inhibited by NaCl.

However, the addition of methylproline restored the activity. Salt tolerance of T. jordanis is based on two mechanisms — increase of Rubisco content and formation of compatible solutes. Such solutes enable the Rubisco to function at high rates in presence of salt ions in cytosol.

ii. Salt Avoidance or Regulation:

In many plants, resistance to salt stress involves efficient restriction of salt uptake.

iii. Restriction of Uptake or Transport:

The foremost strategy to limit salt accumulation is the inhibition of uptake of salt ions. This can be achieved by inhibition of root uptake, which is found in mangroves. However, in most species, such a mechanism is not sufficient. Strategies have evolved to restrict salt transport into sensitive organs or tissues.

The plants sequester salt ions, which move with the transpiration stream, and thus prevent them from reaching sensitive parts of the organism. Such a mechanism was found in various species of Fabaceae.

Differences in ion relations in various members of Chenopodiaceae were also observed in another study by Riemann and Breckl. Plants were grown in sand culture irrigated with 10 mM Na⁺ and K⁺. The halophilic Atriplex rosea showed highest Na⁺ in shoots, which indicated an effective Na⁺ transport from root to shoot.

This was confirmed by low K⁺/Na⁺ ratios in shoot of only 2.4. In contrast, Chenopodium pumilio had highest Na⁺ in hypocotyls and very little Na+ in shoots. Such relations indicated Na⁺ retention in roots and hypocotyls. A K⁺/Na⁺ ratio in shoots of C. pumilio of 42 also showed preferential transport of K⁺ and retention of Na⁺ in basal part of the shoots.

iv. Salt Elimination of Plants by Glands:

The elimination of excess salt ions from the plant into its environment is called recreation. The terms secretion and excretion are also used. Specialized epidermal structures have evolved in various plant groups, these are bladder hairs and salt glands.

The salt bladder hairs are characteristics for Chenopodiaceae. They are hair-like structures on leaf surfaces and consist of several stalk cells that support a large balloon-shaped cell. Stalk cells transport salt ions into vacuole of bladder cell. Subsequently, the bladders die and they are shed. In this way salt is removed from the leaves.

Salt glands are structures that can transport salt ions directly out of the plant. Salt glands of halophytic tree Tamarix aphylla have been investigated. They are found on young green branches, and on small scale-like leaves. The Tamarix glands consist of 8 cells. 6 outer secretory cells and 2 inner collecting cells.

v. Salt Dilution by Succulence:

Salinity can give rise to morphological and structural changes in some plants. Certain plants develop thick, fleshy, succulent organs under salt stress. This is observed only in dicots, but not in monocots. Succulence results from increased water uptake of the tissue and this may help to dilute absorbed salt ions.

Consequently, succulence may be a mechanism to avoid high salt concentration in plant organs. However, the dilution capacity of the tissue is limited, and this strategy can help plants to cope with low levels of salinity.

vi. Osmotic Adjustment:

In order to counterbalance low water potentials of saline soils, some plants use a controlled accumulation of salt ions. This is osmotic adjustment on the whole plant level. In cells, salt ions are compartmentalized and sequestered in vacuoles to avoid toxic effects in the cytosol.

At the same time, the osmotic balance between vacuole and cytosol is maintained by accumulation of compatible organic solutes in the cytoplasm. The accumulation of salt ions into the vacuoles results in low osmotic potentials in the vacuole.

To prevent dehydration of the cytosol, its osmotic potential must be adjusted to the level of the vacuole. This can be achieved by accumulation of osmotically active, organic solutes in the cytosol, which do not interfere with the physiological processes. Such substances are called compatible solutes.

This concept is known as Intracellular model of solute compartmentation. It describes osmotic adjustment on the cellular level.

vii. Chemical Nature of Compatible Solutes:

These include polyols (e.g., sorbitol or mannitol), amino acids and amides (proline), quaternary ammonium compounds (betaine) and soluble carbohydrates (sugars). Besides osmotic adjustment, some compatible solutes, like betaine or proline were shown to have stabilizing effects on enzymes under salt stress.

viii. Interaction of Calcium with NaCl:

Calcium is an essential plant nutrient with many functions in metabolism like stabilization of membranes, signal transduction as a second messenger, and control of enzyme activity. There are also reports stating that Ca²⁺ can help to remediate the adverse effects of salinity on plants.

Inhibition of root elongation of pea plants under salt stress were reversed by increased Ca²⁺ levels in the medium. Adverse effects of NaCl on water transport of maize root cells were partly compensated by additional Ca².

A hypothesis explaining the effects of Ca⁺²on salt responses of plants was put forward by Rengel (1992) He suggested that Na⁺ reduces the binding of Ca²⁺ to plasma membranes, inhibits influx and increases efflux of Ca⁺²and depletes the internal Ca⁺² stores in cell compartments. Changes in Ca⁺² levels of cells are primary responses to salt stress, which are perceived by root cells.

The supply of Ca⁺² to leaf cells is reduced. Therefore, the amelioration of adverse salt effects by Ca⁺² supply is probably due to preventing Na-induced changes in Ca⁺² levels of cells.

ix. Salt Induction of CAM in Some Plants:

A remarkable response to salt stress has been discovered in members of the family Aizoaceae, particularly in the genus Mesembryanthemum. Grown under non-saline conditions, such plants perform C₃ photosynthesis.

It was observed that salt-stressed plants of Mesembryanthemum crystalline absorbed CO₂ in the dark, and at the same time, malate levels of their leaves increased. These are typical features of CAM. Apparently, under salt stress, this plant can shift its photosynthetic pathway from C₃ to CAM.

Regulation of gene expression in salt-stressed Mesembryanthemum plants was investigated by Vernon et al (1993). After exposure to salinity, various mRNAs encoding proteins of different biochemical pathways accumulated in leaf tissues.

It was suggested that water stress triggered the coordinated induction of mRNAs involved in different aspects of adaptive stress response of plants. Varied stressors, like drought or salinity, caused different transcription levels of several genes.