36.7:
Responses to Salt Stress
Salinity affects the growth and development of plants by impacting water and nutrient uptake as well as photosynthesis.
Plant cells have high solute concentrations that drive water from the soil into the plant. However, the presence of excess salt in the soil raises the soil solute concentration, impeding a plant’s ability to take up water even when there is sufficient water in the soil.
Another issue with excessive salt levels in soil is that sodium and other ions in salts are toxic to plants at high concentrations.
Many plants can respond to moderate levels of salt in the soil by producing solutes that they tolerate well at high concentrations; this raises the solute concentration in plant cells, enabling roots to absorb water from the soil without taking in toxic levels of sodium.
Excessive sodium at the root surface also reduces potassium uptake, which inhibits plant growth.
Calcium can protect plants from sodium stress by regulating ion transporters, enzymes, and gene transcription.
An excess of sodium and other salt-derived ions in the soil can also change the hormonal content in plants. For example, abscisic acid is produced in response to excessive salt levels, which closes stomata.
Not all plants are sensitive to salt in their environment. Salt-tolerant plants—called halophytes—have adapted to high-salt environments.
Halophytes have evolved features that confer resilience to salt stress, such as specialized epidermal glands—called salt glands—on their stems and leaves. The salt glands take up excess salt from neighboring tissues and excrete the salt onto the leaf surface, where it can be removed by rain or wind.
36.7:
Responses to Salt Stress
Salt stress—which can be triggered by high salt concentrations in a plant’s environment—can significantly affect plant growth and crop production by influencing photosynthesis and the absorption of water and nutrients.
Plant cell cytoplasm has a high solute concentration, which causes water to flow from the soil into the plant due to osmosis. However, excess salt in the surrounding soil increases the soil solute concentration, reducing the plant’s ability to take up water.
High levels of sodium are toxic to plants, so increasing their sodium content to compensate is not a viable option. However, many plants can respond to moderate salt stress by increasing internal levels of solutes that are well-tolerated at high concentrations—like proline and glycine. The resulting increased solute concentration within the cell cytoplasm allows the roots to increase water uptake from the soil without taking in toxic levels of sodium.
Sodium is not essential for most plants, and excess sodium affects the absorption of essential nutrients. For example, the uptake of potassium—which regulates photosynthesis, protein synthesis, and other essential plant functions—is impeded by sodium in highly saline conditions. Calcium can ameliorate some effects of salt stress by facilitating potassium uptake through the regulation of ion transporters.
Not all plants are sensitive to salt. Plants can be classified as halophytes or glycophytes based on their salinity tolerance. While halophytes are salt-resilient, glycophytes are not. In order to tolerate high salt concentrations, halophytes may reduce sodium uptake, compartmentalize sodium, or excrete sodium. A small group of halophytes called recretohalophytes have specialized epidermal glands—called salt glands—in their stems and leaves. Salt glands take up excess salt from neighboring tissues and excrete it onto the plant surface. By studying halophytes, scientists can uncover the mechanisms of salt tolerance in plants and potentially use this knowledge to improve crop production in regions affected by salinity.
Suggested Reading
Meng, X., Zhou, J., & Sui, N. (2018). Mechanisms of salt tolerance in halophytes: current understanding and recent advances. Open life sciences. 13 (1), 149-154. [Source]
Torabi, M., Halim, R. A., Mokhtarzadeh, A., & Miri, Y. (2013). Physiological and Biochemical Responses of Plants in Saline Environment. Roychowdhury, R. Crop Biology and Agriculture in Harsh Environments. LAP LAMBERT Academic Publishing. [Source]