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| Frequently Asked Questions About Salinity |
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Plant Cell and Root Growth, Water and Sodium Chloride
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How does salinity affect root growth? Does it increase or decrease root growth? |
| Unfortunately, we don't know all the answers! Salt in the root zone decreases root growth. In some plants we call halophytes (literally salt plants) a little bit of salt seems to improve overall growth, in both roots and shoots (if you measure total biomass, i.e. the weight of the plant material). Examples of this are seen in barley and atriplex. Why this is the case is not known, but it is speculated that these plants require Sodium ion (Na+) or Chloride ion (Cl-) for growth. |
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| In most cases, however, salinity decreases both root and shoot growth in plants, especially in glycophytes. Glycophytes are plants adversely affected by salts, or literally sweet plants, as opposed to salt plants. Shoot growth is usually decreased more than root growth and as a result the root/shoot ratio changes (the total weight of the roots and divided it by the total weight of the shoot). |
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References:
- Braun, Y., Hassidim, M., Lerner, H.R., and Reinhold, L. 1986. Studies on H+-translocating ATPases in young plants of varying resistance to salinity. PLANT PHYSIOLOGY. vol. 81, pp. 1057-1061.
- Munns, R. and Termatt 1986. Whole-plant responses to salinity. AUSTRALIAN JOURNAL OF PLANT PHYSIOLOGY, vol. 13, pp. 143-160.
- Gilbert, G. A., Wilson, C., Madore, M. A. 1997. Root-zone salinity alters raffinose oligosaccaride metabolism and transport in Coleus. 1997. PLANT PHYSIOLOGY. vol 115, pp. 1267-1276.
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Does salinity affect cell division or cell enlargement? |
| Both. In Halophytes, growth is stimulated by low amounts of salt (equivalent to about 3000 ppm). If you had a lot of time on your hands and bothered to count each and every cell, you would find more cells. Also, you would find that the cells are, on average, larger. Mostly though, plants increase in size by cell enlargement. |
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| When Glycophytes are affected by salinity, cells in the roots are smaller and there are fewer of them. Under severe stress, there just isn't a whole lot of root there - fewer cells and smaller cells. Same with the shoot. |
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Do plants actually take up salt while the water diffuses in? |
| Ions get into the roots via several mechanisms. Ions that are at lower concentrations outside the plant that inside are taken up by a processes called active transport which requires energy and is mediated by a protein. Ions that exist at higher concentrations outside the plant than inside can diffuse in, but again, a protein is probably involved. These proteins are called transporters, pores or channels depending on their exact nature and how they operate. Both roots and shoots of plants grown in saline environments will have higher salts levels. |
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| Some plants exclude toxic ions like Na+ and Cl-. By exclude, it is really meant that they limit the influx of ions. This is accomplished by limiting ion uptake at the level of the roots or by compartmentalizing ions in areas of the plant, even in cells that are away from important metabolic sites and actively growing tissues. In some cases, it appears that salts are sequestered in older leaves that are eventually shed (abscised). Some halophytes have specialized leaf cells called salt glands that excrete salt. |
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Does water diffuse out of plants? |
| Water does indeed diffuse out of the leaves by the process called transpiration. Because water molecules cohere to each other via chemical bonds, called hydrogen bonds, water molecules at the top of the plants are connected to water molecules in the soil much like the cars of a train. When water transpires (a diffusion process) from the leaves, other water molecules are brought closer to the root surface. This waterway is actually called the transpiration stream. Ions move in the transpiration stream much like a non-powered boat floats along a river stream. Thus, the transpiration stream brings ions from the soil water, first to the root where they must cross the plasma membrane barrier, and eventually to the leaf. At the leaf the water molecules can escape back into the atmosphere through another specialized leaf cell called a stomata. Ions, however, will be left behind. |
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