Plants and salt stress

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Plants and salt stress

The research project that I am doing at the moment is focussing on identifying lipid protein interactions under abiotic stress conditions like salt and heat stress. Hence I am reading about salt stress to get an idea about how my study fits in the bigger picture that we have about how plants react towards salt stress. Therefore from time to time I will talk about  salt stress, probably getting slowly getting into more details.

First a few facts about salt stress and its effect on plants. More than 6% of the total land area is affected by salt. This is for example land close to the seashore, or land in river delta’s were due to the tides salt water and fresh water mixes. But also land with ancient marine deposits whose natural salt seepage can wash into nearby area’s.  Another way for land to become affected by salt is trough irrigation water, whereby traces of salt remain in the soil after evaporation of the water. Over time these traces of salt can accumulate to high concentrations. The salt NaCl is the major contributor to salinity, although there are other salts like Ca²⁺, Mg²⁺ and SO₄^2- that can contribute as well.

Plants can be roughly divided into halophytes (salt plants), which grow well on highly saline soils, and glycophytes (sweet plants) which are more salt sensitive.  Although it has to be said that salt is toxic for all plants, it is just that halophytes are better able to exclude salts up to higher concentrations. Examples of plants that are sensitive to salt are rice, maize and beans, while bread wheat, barley are moderately tolerant, and date palms, mangroves and sugar beets are highly salt tolerant.

The effects of salt stress on plants can be initially seen as a reduction in growth, with slower expansion of young leaves and slower emergence of lateral buds, resulting in fewer branches and lateral shoots. Later on the senescence of mature leaves can be observed. This later response is due to the toxic effect of Na⁺ ions. The earlier response is due to the osmotic stress caused by the high concentration of salt around the roots. Although, it would be expected for the roots to be more sensitive to high salt concentrations, the roots actually appear to be less affected. The initial response to salt stress in the roots is a complete stop in growth, but they recover quickly, within an hour for moderate stress to within a day for severe salt stress. While root growth recovers, it does not reaches the same rate as before being affected by salt stress. Furthermore, the root will adjust its growth into the direction where the concentration of salt is the lowest.

As salinity is a common occurrence in soils plants have evolved mechanisms to deal with its consequences. Plants can become more tolerant in three ways, firstly by becoming more tolerant to osmotic stress. When plants have a higher tolerance to osmotic stress they can keep up the rate of leaf growth. However, as a larger leaf area would mean more water loss via the leaves, this would only be beneficial when there is enough water in the soil. A second way to become more tolerant to salt stress is to exclude Na⁺ from the leaves. In this strategy Na⁺ would be excluded by the roots so it would not be able to accumulate in toxic concentrations the leaves. A third way to become more tolerant is tissue tolerance. Applying this strategy, Na⁺ and/or Cl^– is compartmentalised on cellular or intracellular level. By doing this Na⁺ or Cl^– would not be able to build up to toxic concentrations in the cytoplasm. Of course a plant does apply all these strategies to various degrees depending on the species. The effectiveness of these strategies also depend on the level of salt stress, the developmental stage of the plant and other environmental conditions like soil water level and air humidity.

A good example of a halophyte are mangrove trees which grow in coastal saline or brackish water. They are more tolerant to osmotic stress by actively limiting the amount of water that  they lose through their leaves. Either by restricting the opening of stomata, or by adjusting the orientation of their leaves so they are not exposed to the midday sun. Both strategies limit the water loss through the leaves.  To avoid Na+ building up in the leaves mangroves actively filter salt from the roots, in Red mangroves this results in 90-97% of salt being excluded.  While mangroves filter out a large proportion of the salt at the roots, the salt that does make it up the shoot is compartmentalised. Either in old ‘sacrificial’ leaves which are shed when the concentration becomes to high. Stored in vacuoles as done by red mangroves. Or secreted through salt glands at the base of the leave, which is done by white or grey mangroves.

References

• Munns and Tester, 2008,  Mechanisms of Salinity Tolerance, Annual Review of Plant Biology 59:651-681

• Julkowaska and Testerink, 2015, Tuning plant signalling and growth to survive salt, Trends in Plant Science 20:219-229

• Galvan-Ampudia et al., 2013, Halotropism is a response of plant roots to avoid a saline environment, Current Biology 23:2044-205

• “Morphological and Physiological Adaptations: Florida mangrove website”. Nhmi.org.