Physiological constraints of salt stress
Physiological Constraints Imposed by Salinity
1.Primary stress (a) Direct membrane damage (b) Metabloic aletrations
2.Secondary stress (a) Osmotic stress, ionic stress
- Primary effects of salinity stress: osmotic & nutrient imbalance and ionic homeostasis:
- An immediate response to osmotic stress is inhibition of cell expansion.
- Excessive concentration of Na+ around roots causes ionic imbalance and disturbs the K+ uptake in cells.
- Increase in Na+ concentration inhibits the entry of K+ and disrupts the K+/Na+ ratio, which eventually proves detrimental for proper functioning of vital enzymes.
- Besides these, salt stress also interferes with the uptake of essential nutrients and thus, inhibits seed germination as well.
- Secondary effects of salinity stress: oxidative stress and generation of reactive oxygen species:
- During the later phases of salinity stress, generation of ROS leads to membrane damage and ion leakage, which is secondary in nature and deleterious for plants.
- Sodium toxicity acts at the whole plant level affecting the plant at cellular and tissue level, bringing about
- Metabolic toxicity
- Nutritional deficiencies,
- Reducing primary root growth, eventually resulting in inhibition of growth and reduction of crop yields
Generalized view of sources and effects of salt toxicity in plants.
- Increase in these salt limits (NaCl, Na2SO4, NaNO3, MgSO4,
- MgCl2, K2SO4, CaCO3) leads to two major secondary stresses for the plants:
- Osmotic stress
- Ionic stress.
- The osmotic stress firstly comes in plants when salt concentrations increase outside the roots, which leads to reduction in water uptake and subsequently plant development.
- The ionic stress develops when Na+ accumulation increases in plants particularly in leaves over threshold level which caused chlorosis in leaves and reduced photosynthesis and other metabolic activities
Plant Physiology Under Salt Stress
- In saline soil, water potential decreased in surrounding the root, and plants suffer from the osmotic stress and ionic effect of Na+ and Cl−.
- Accumulation of Na+ plays a central role in reduction of plant growth and senescence during salinity. Therefore, cytoplasmic Na+ concentration is regulated by the plants to tolerate salt stress.
1. Osmotic stress
- A salt concentration in the soil of 4 dS m−1 or 40 mM NaCl has an osmotic pressure of about 0.2 MPa, which affects the ability of plants to take up water.
- This osmotic effect has a flow-on effect, via internal signals, to reduce the rate of cell expansion in growing tissues, and the degree of stomatal aperture in leaves.
- The reduction in stomatal conductance of CO2 limits the rate of photosynthesis, which together with the slower formation of photosynthetic leaf area, reduces the flow of assimilates to the meristematic and growing tissues of the plant, both leaves and roots, although leaves are often more affected than roots.
- The decreased rate of leaf growth after an increase in soil salinity is primarily due to the osmotic effect of the salt around the roots.
- A sudden increase in soil salinity causes leaf cells to lose water, but this loss of cell volume and turgor is transient.
- Within hours, cells regain their original volume and turgor owing to osmotic adjustment, but despite this, cell elongation rates are reduced.
- Over days, reductions in cell elongation and also cell division lead to slower leaf appearance and smaller final size.
- Cell dimensions change, with more reduction in area than depth, so leaves are smaller and thicker.
- For a moderate salinity stress, an inhibition of lateral shoot development becomes apparent over weeks, and over months there are effects on reproductive development, such as early flowering or a reduced number of florets.
- During this time, a number of older leaves may die. However, production of younger leaves continues.
- All these changes in plant growth are responses to the osmotic effect of the salt, and are similar to drought responses.
Plant Responses Can Occur in Two Distinct Phases Through Time
- In the simplest analysis of the response of a plant to salinity stress, the reduction in shoot growth occurs in two phases:
- a rapid response to the increase in external osmotic pressure
- a slower response due to the accumulation of Na+ in leaves
2. Ionic imbalance
- The second major constraint imposed by salinity is Na+ toxicity and ionic imbalance in the cell cytosol.
- Due to the similarity in physicochemical properties between Na+ and K+, the former competes with K+ for major binding sites in key metabolic processes in the cytoplasm, such as
- Enzymatic reactions,
- Protein synthesis
- Ribosome functions
- With over 50 cytoplasmic enzymes being activated by K+, the disruption to metabolism is severe, both in root and leaf tissues.
- Salinity also affects a plant’s ability to acquire and metabolize other essential nutrients such as Ca, N, P and Mg.
Several major factors contribute to this process
- High concentrations of Na+ in the soil substantially reduce the activity of many essential nutrients, making them less available for plants.
As an example, the presence of 100 mM NaCl in the soil solution results in nearly three-fold drop in Ca2+ activity.
- Na+ may directly compete at uptake sites with many essential cations such as K+ , Mg2+ or NH4+. This may affect both low- (e.g. NSCC) and high- (e.g. HKT) affinity transporters.
- A significant membrane depolarization occurs when positively charged Na+ crosses the plasma membrane. Such depolarization makes passive uptake of many essential cations impossible and, at the same time, dramatically increases efflux of some of them (e.g. K+ leak through depolarization-activated outward-rectifying K+ channels; KOR in Fig. ).
- Increased synthesis of various compatible solutes used for osmoprotection under saline conditions severely reduces the available ATP, making high-affinity cation uptake even more problematic. This also significantly reduces uptake of anions such as NO3− or PO4−3, as their transport is energized by the H+-ATPase activity.
3. Oxidative stress (Secondary Effect)
- Oxidative stress is defined as the toxic effect of chemically reactive oxygen species (ROS) on biological structures.
- Both osmotically induced stomatal closure and accumulation of high levels of toxic Na+ species in the cytosol under saline conditions impair photosynthetic machinery and reduce plant’s capacity to fully utilize light that was absorbed by photosynthetic pigments. This leads to formation of ROS in green tissues.
- The detrimental effects of ROS are a result of their ability to cause lipid peroxidation in cellular membranes, DNA damage, protein denaturation, carbohydrate oxidation, pigment breakdown and an impairment of enzymatic activity.
- The major sources of ROS production are
- cell wall peroxidase
- amine oxidase
- plasma membrane NADPH oxidase
- intracellular oxidases and peroxidase
- in mitochondria, chloroplasts and peroxisomes.
- Importantly, ROS production under saline conditions occurs not only in leaf but also in root tissues
ROS accumulation in roots occurs very rapidly (within minutes) and has an immediate and very significant impact on cell metabolism.
- First, ROS directly activate Ca2+-permeable plasma membrane channels triggering a rapid Ca2+ uptake.
- The resultant elevation in cytosolic Ca2+ activates NADPH oxidase and causes further increase in (Ca2+)cyt via positive feedback mechanisms (Fig. ).
- ROS are also known to activate a certain class of K+-permeable NSCC channels , resulting in a massive K+ leak from the cytosol and a rapid decline in the cytosolic K+ pool.
- Taken together, sustained elevation in cytosolic Ca2+ and depletion of the K+ pool activate caspase-like proteases and trigger programmed cell death (PCD) (Fig.).
- Importantly, salinity-induced PCD in plant roots is observed within 1 h after stress onset. This is comparable with the impact of the osmotic component of salinity stress.