Starch Reserve Testing of Woody Tissues

The Use of Lugol's Stain

Woody plants, especially trees, create reserves of stored energy to be used at later times for metabolism and respiration. This reserve of energy allows the plant to be a perennial, that is, it can maintain its form and build a larger plant from one year to the next. Since the plant cells do not stop functioning during the "dormant" season, but continue to metabolize, energy is required. Without funcitioning foliage during part of the year, the energy must come from stored reserves.

The tree requires energy for all of its functions, including respiration and reproduction of cells, growing new tissues, and maintaining defences against stress factors and invading insects and disease. Some of this energy comes directly from the leaves, as it is made. The rest comes from the stored reserves.

During the growing season, the tree manufactures sugars and carbohydrates in the foliage. These compounds contain the energy needed for metabolism. Whatever is not used for current needs in the plant is transported to the woody tissues and converted to starches for storage in the living cells of the branch, trunk, and root wood. The amount of available reserve energy stored within the plant is an indication of the vigor of that plant, and its general health.

Staining of Tissues

By staing the tree parts with an iodine stain, the stored starch granules can be readily seen, and the amount of starch can be observed. The process involves the use of Lugol's Stain, a compound of iodine and potassium iodide. When mixed in water, this stain produces a dark blue or black color where starch is present. To make the solution, combine 3 grams of iodine (I2) crystals, and 15 grams of potassium iodide (KI) crystals. Dissolve in 1000 milliliters of distilled water.

To apply the stain, simply place a few drops on a cut surface of the woody tissue. Since the tree stores starches in the woody roots last, this gives the best indication of tree vigor and total stored starch content. Twigs, branches, and even trunk wood can also be used.

The most accurate way to check the sample for relative comparison is to make a very thin sliced section of the root or wood sample. Use a scalpel or single-blade razor to cut a paper thin section. Uniformity of sections will provide the best comparisons. Place the section on a glass microscope slide, then add two or three drops of the stain. Allow a minute or two to soak in, then rinse with distilled water. The sample is now ready for inspection. View with a hand lens or Macroscope®, held against white paper.

The starch granules stain nearly black, so the relative amount of starch is higher for darker stained samples. By using wood or root samples from several trees, some of which are in a healthy condition, comparisons can be made. It is also necessary to make a seasonal adjustment, since the stored reserves vary over the course of a year. Starch levels are highest at the end of the growing season, at the onset of dormancy, and lowest during leaf expansion in the spring. Maintianing a file or collection of samples will allow more accurate comparisons over time. If you have the equipment and expertise to make photographs, the collection can be maintained on film.

A quick field procedure to check starch levels is to simly cut a twig or root cleanly, and apply a few drops of the Lugol's Stain directly to the end of the twig or root. The degree of discoloration will indicate starch reserves. One caution: since this is not a thin slice, many layers of cells will darken, and make interpretation of results more difficult.

This technique takes a little practice to become proficient at preparing and analyzing the samples. The results are not empirical, but can give you an additional indication of tree vigor. It is one more tool in the arborist's kit for evaluating tree condition.

Note: The prepared stain should be kept refrigerated if possible. The shelf life of small volumes, not refrigerated, is about 4 to 6 months. Refrigerated in larger volumes, it will last a year or more, and still give accurate results. The stain must be kept in darkened bottles, since light will cause it to break down and be inneffective.

© Copyright 1996, Russell E. Carlson

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This page was last updated on June 12, 2003.