Title
Monterey Bay Aquarium Research Institute
Marine Botany
Distichlis spicata

Salt Extrusion

Distichlis blade with salt crystals, Hopkins Marine Station (site A)

Salt Gland Mechanics:

As should be clear by this point, Distichlis owes its amazing halophytic characteristics to the activity of specially evolved salt glands. These appear as spherical protrusions along all surfaces of its blades and stem. Current research indicates that these glands extrude an extremely briny solution of mostly sodium and potassium ions dissolved in water [3].

Regarding design, through the use of transmission electron microscopy, the salt gland has been found to contain two cells, a basal cell and cap cell. Though the mechanics of salt extrusion in Distichlis have yet to be definitively determined, the hypothesis of Hansen and Dayanandan et. al. provide a reasonable explanation (see Hansen, D.J. et. al. [3] for a full explanation) - this hypothesis is summarized in diagrams and text below:

  1. Above the cap cell there exists a cuticular cavity, a space between the cuticle of the blade surface and the cap cell. The cap cell initially contains a high concentration of sodium ions in water.
  2. Salt ions and water are actively transported (involving use of ATP) [14] out of the cap cell and into the cuticular cavity.
  3. As the pressure in this space increases as a result of this ion pumping, the turgor pressure causes the cuticle to stretch to the point that the salts in solution are able to flow out of the cuticular cavity through microscopic pores in the cuticle.
  4. As the cuticular cavity drains, the pressure decreases, returning the cuticle to its previous, unstretched state. The briny solution left on the blade surface dries to form the classic large salt crystals found on the plant.

Photographs:

Salt glands can be identified in electron microscope photographs by their spherical shape and their encrustation with salt, as contrasted with the much mor common papillae which generally display a longer cylindrical shape. Two scanning electron microscope photographs, shown below, were taken of the outside of a Distichlis blade collected at site E at Hopkins Marine Station. Unfortunately, due to the drying and coating process used to prepare the sample, along with the relatively high accelerating voltage used (15 KeV), many of the salt glands in this sample appeared to have collapsed:

Scanning electron microscope photos, 15KeV, 1200x magnification

The photograph on the left shows a large, cubic salt crystal likely formed by the briny extrusions of a number of salt glands clustered closely together. Smaller cubic crystals can be seen towards the top of the photo as well.

The second photograph shows what appears to be a large collapsed salt gland towards the center of the photo - the salt "flakes" which once covered the spherical surface can still be seen clustered above the collapsed top of the gland. A smaller, uncollapsed gland, with its bulbous surface still covered in a solid layer of salt, can be seen in the upper left corner of the photo.

Information about the interactions between salt glands, salt extrusion rates, soil salinity, and growth can be found on the salinity and morphology page.

Last updated March 2003, Justin Kitzes.

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