Plants produce and export many different molecules out of their cellular and organismal confines. Some of those chemicals become so abundant that we can see or smell them. The most visible materials oozed by many plants are called “exudates.”
What are plant exudates? Generally, exudates are carbon-rich materials that many plants produce and release externally. When exudates are produced, they are often sticky to human touch. Such plant chemicals can be the visible expression of attack by bacteria, fungi, herbivores, or some other plant pathology. In other instances, such as in typical underground roots, exudate production appears to be part of the typical metabolism of healthy plants that helps stabilize the soil and foster interactions with other organisms around the roots.
Different plant tissue types and organs can produce exudates. We have collected resins and gums from the above ground portions of plants, or shoots, as well as from the generally below ground portion of plants, or roots. Root exudation has been known for decades and is responsible for many of the fascinating relationships in the interface of plant roots and soil microorganisms known as the rhizosphere.
Collecting and Analyzing Plant Exudates
After receiving collecting permission (if needed), we spend days walking the grounds of botanical gardens and arboreta, or do field work elsewhere. Exudates are easily collected directly from the trees with no harm to the plant and leaving no doubt about their botanical identity. Occasionally we use more forceful methods, such as carefully microwaving wood slabs to extract the exudates, then letting them resolidify. Once the material is collected, we place it in a small plastic zip-top bag. An additional, external bag is used to hold a paper label containing the collection data. If needed, we let the exudate dry slowly in an oven and, once dried, the materials are ready for subsequent analyses. In other instances, generous collaborators send us materials for chemical analyses.
Carbon-13 solid state Nuclear Magnetic Resonance spectroscopy (ssNMR) is a state-of-the-art research tool that generates spectra (or chemical signatures) of materials, including plant exudates and amber or greatly fossilized plant resin. The analyses, which use a tiny amount (as little as 50 to 100 milligrams, approximately the volume of a new eraser on a school pencil) of the exudate, are non-destructive. They are performed at Northwestern University (in Evanston, Illinois), one of a few research laboratories in the world with carbon-13 ssNMR capabilities. At times, we observe plants that evidently have produced exudates but the amounts are insufficient for our analyses.
Solid exudates are pulverized manually and undergo two sets of carbon-13 ssNMR analyses: normal decoupling, which gathers signals for all carbon atoms, and interrupted decoupling, which, among others, obtains signals from carbons lacking the attached hydrogens. Just like in spectra used in the health-allied sciences, different regions of the spectra provide valuable information (see Figure 1 on page 4). In the case of NMR, the peaks represent different atoms and reflect their molecular environment. The height of the peaks largely represents relative abundance of those atoms. The position of the peak along the horizontal axis (parts per million [ppm]) is the resonance frequency characteristic of the atom and its molecular neighborhood. This position is an indication of the chemical identity of the peak as compared to an external molecular reference. In carbon-13 ssNMR, peaks in the 0–80 ppm region are singly bonded carbon atoms (-C-C-), or alkanes; signals within the 80–100 ppm region are single bonded carbon atoms with electron-withdrawing neighbors, in particular, oxygen (C-O), as found in carbohydrates, such as sugars. Currently, we have analyzed over 1,800 exudates of all types, including amber, representing most of the major plant groups worldwide. However, a lot more samples still need to be acquired and analyzed.
Types of Plant Exudates
Using NMR, we have determined that there are three major types of plant exudates: resins, gums, and phenolics. Resins are made from terpene molecules. The basic molecular unit of terpenes is a five-carbon molecule, known as isoprene (see Figure 2 on page 6).
When freshly produced, many resins are sticky and smell like Christmas trees or incense. Resins are insoluble in water and thus do not dissolve during rains. As time passes and the resins begin to “mature,” many of their original chemical constituents evaporate. The materials remaining behind in the resin blob form chemical bonds, a process known as polymerization, and the blob begins to harden. With the passage of millennia, the resinous material becomes greatly polymerized and evolves into the robust gemstone called amber, produced only by specific plant species. Conifers such as pines (Pinus), firs (Abies), spruces (Picea), larches (Larix), and some other familiar cone-bearing trees in northern latitudes tend to produce resinous exudates. Many angiosperms (flowering plants) also produce resins.
The term “latex” refers to milky-looking exudates produced by numerous flowering plants, including those in the euphorbia or spurge family (Euphorbiaceae). Latexes can be dangerous to touch, causing dermatitis or other damage, especially to the eyes. Interestingly, all latexes we have examined thus far are resins in suspension.
A second type of exudates is known as gums. Gums are large carbohydrates consisting of myriad sugar molecules linked together chemically (see Figure 3 above). Gums do not tend to smell because of their low volatility stemming from their high molecular weight. When freshly produced, many gums are spongy to touch because of their high water content. Thus, freshly produced gums dissolve easily during rains. If somehow gums manage to survive and dry out, they can then be very hard to dissolve. However, as far as we are aware, gums are not known to survive millions of years as amber does. Gum exudates tend to be produced by flowering plants; fruit trees in the genus Prunus, including cherries, plums, peaches, and almonds, commonly produce gums.
The third major type of exudates is known as phenolics. Phenolics are chemically related to terpenes but form unsaturated ring compounds known as aromatics because of their often-pleasant odor. When freshly produced, phenolics tend to be watery and reddish brown, and lack the strong smell of resins. If they survive dissolution, phenolics tend to form brittle solids. As with gums, we are not aware of phenolics that have survived deep time. Phenolics tend to be common in Eucalyptus and related plants. Combinations of these major types of exudates, such as gum resins, as well as several other minor kinds of exudates are also known.
Use of Plant Exudates
In addition to their generally beautiful colors, pleasant aroma, and light weight, resins are water insoluble. These properties make resins, including amber, coveted natural products. Some uses of resins, including amber, include: ceremonial and artistic, as construction materials, ingestive, and, of course, as objects of science because they provide windows into past worlds.
Ceremonial and artistic uses
Amber, that is, greatly polymerized resin, has been used for ceremonial purposes as well as for objects of trade, jewelry, sculptures, and many other items. Although highly valued in the market, amber varies greatly in color and translucency, from white to black and from translucent to opaque. Because of this variability, color and translucency on their own are generally not good diagnostic traits for identifying amber.
On the other hand, copal (less polymerized resin) and modern resins are still used in some areas of Mexico and Central America for artistic and ceremonial purposes, prized because they smell of incense. Next time you encounter a pine, fir, or spruce tree, look carefully at its bark and you may be able to see some exudate blobs or “teardrops.” Pick one of them up and smell it! Pine resin has been used in the preparation of rosin, which is applied to the hairs of bows used to play string instruments such as the violin (rosin makes the hairs just sticky enough to grip the strings and create sound).
The metallic transatlantic cable that connected the Old and New Worlds telegraphically during the second half of the nineteenth century was insulated by gutta percha, the resinous exudate of Palaquium gutta, a tropical Southeast Asian tree. The modern aviation and aerospace industry uses human-made, lightweight and strong, synthetic resins and phenolics in building airplanes.
An old and interesting use of resins is in the preparation of retsina, a Greek wine that is flavored with a little bit of pine resin (typically from Aleppo pine, Pinus halapensis). Gums are also sometimes eaten; in places where the leguminous Acacia trees produce copious quantities of gums, these exudates are used as survival foods when other food is scarce. Although it has been alleged that amber has healing and other medicinal properties, we are not aware of scientific studies using a double-blind protocol that demonstrate any medicinal properties of amber.
For reasons that are not known, some forests in the past appear to have produced copious amounts of resins. Although these exudates may have attracted some organisms and repelled others, once small organisms such as insects landed on the sticky material it was difficult to detach from it. When subsequent resin flows covered the specimen it was protected from the action of decomposing organisms and the environment, allowing it to be preserved for a longer time. Subsequent polymerization of the resin preserved a fraction of the resin-entombed organisms, which, when found, now have great value to scientists. Amber encased plant and animal specimens have contributed insights in a number of scientific fields.
Amber specimens that contain larger, rarely found organisms (e.g., scorpions, amphibians, lizards, birds) are of great interest and may command great sums of money. However, buyer beware, as there are unscrupulous sellers willing to make money from objects that are not genuine amber.
Ongoing Research Goals
Ultimately, we seek answers to questions because we are curious about nature. Sometimes, our results can help answer a question. For example, along with several other colleagues, including Dr. Lisa Niziolek from the Field Museum of Natural History in Chicago, we answered the question: In what plant family was the tree that produced the blocks of resin found in a thirteenth century shipwreck excavated from the Java Sea? Our studies of many plant exudates have generated a large database of their NMR profiles. When we study a sample of unknown botanical provenance, that database allows us to compare the samples of unknown botanical origin, like the resin from the Java Sea wreck, with those in our database. With that information, we were able to suggest that the plant whose resins were harvested back in the thirteenth century was from the botanical family Dipterocarpaceae, and perhaps specifically the genus Shorea. Having an idea of the botanical provenance of archeological artifacts enriches our knowledge of how our predecessors used plants. In this case, research tells us that aromatic resins were an important commodity at the time and were often imported into China for use in Buddhist rituals as well as medicines, lacquers, and perfumes. We will continue to collect and analyze plant exudates from around the world, including amber and copal, as well as materials associated with anthropological artifacts, adding knowledge for future researchers to use.
Author Jorge A. Santiago-Blay dedicates this paper to his mother, Ángeles Blay Sálomons, who in the early 1980s suggested to him that he pursue the study of “las resinitas” (the little resins) as she used to call exudates. Her memory always lives with him.
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Lambert, J. B., E. W. Donnelly, E. A. Heckenbach, C. L. Johnson, M. A. Kozminski, Y. Wu, and J. A. Santiago-Blay. 2013. Molecular classification of the natural exudates of the rosids. Phytochemistry 94: 171–183.
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Santiago-Blay, J. A., R. L. Hoffman, J. B. Lambert, and Y. Wu. 2003. Cylindroiulus truncorum (Silvestri): a new milliped for Virginia (USA), with natural history observations (Julida: Julidae). Banisteria 20: 62–66.
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Jorge A. Santiago-Blay is a Resident Research Associate in the Department of Paleobiology at the Smithsonian Institution’s National Museum of Natural History in Washington, D.C. (firstname.lastname@example.org). Joseph B. Lambert is Research Professor of Chemistry at Trinity University in San Antonio, Texas , and Clare Hamilton Hall Professor of Chemistry Emeritus, Northwestern University, Evanston, Illinois (email@example.com).