The Magazine of the Arnold Arboretum VOLUME 75 • NUMBER 1 The Magazine of the Arnold Arboretum VOLUME 75 • NUMBER 1 • 2017 CONTENTS Arnoldia (ISSN 0004–2633; USPS 866–100) 2 Plant Exudates and Amber: is published quarterly by the Arnold Arboretum Their Origin and Uses of Harvard University. Periodicals postage paid Jorge A. Santiago-Blay and Joseph B. Lambert at Boston, Massachusetts. Subscriptions are $20.00 per calendar year 14 Other Order: Sound Walk for an Urban Wild domestic, $25.00 foreign, payable in advance. Peter Del Tredici and Teri Rueb Remittances may be made in U.S. dollars, by check drawn on a U.S. bank; by international 26 Floral Clocks, Carpet Beds, and the money order; or by Visa, Mastercard, or American Ornamentation of Public Parks Express. Send orders, remittances, requests to purchase back issues, change-of-address notices, Phyllis Andersen and all other subscription-related communica- tions to Circulation Manager, Arnoldia, Arnold 36 Uncommon By Any Name: Arboretum, 125 Arborway, Boston, MA 02130- Acer pensylvanicum 3500. Telephone 617.524.1718; fax 617.524.1418; Jon Hetman e-mail [email protected] Arnold Arboretum members receive a subscrip- Front cover: Plant parts, such as this flower, and tion to Arnoldia as a membership benefit. To animals trapped in amber can provide important sci- become a member or receive more information, entific insights to the distant past. Photo courtesy of please call Wendy Krauss at 617.384.5766 or Patrick R. Craig. email [email protected] Inside front cover: Many public parks, including Postmaster: Send address changes to Boston’s Public Garden, seen here, still create extensive Arnoldia Circulation Manager annual bedding out displays, a practice first popularized The Arnold Arboretum in the Victorian era. Photo by Norman Eggert/Alamy 125 Arborway Stock Photo. Boston, MA 02130–3500 Inside back cover: Whether you call it striped maple, Nancy Rose, Editor snakebark maple, or moosewood, Acer pensylvanicum Andy Winther, Designer is an admirable native tree. Photo of Arnold Arboretum accession 137-2003-F by Jon Hetman. Editorial Committee Back cover: A carpet of non-native lesser celandine Anthony S. Aiello (Ranunculus ficaria) blooms amid old stems of Helian- Peter Del Tredici thus. Photo by Peter Del Tredici. Michael S. Dosmann William (Ned) Friedman Jon Hetman Julie Moir Messervy Copyright © 2017. The President and Fellows of Harvard College Plant Exudates and Amber: Their Origin and Uses Jorge A. Santiago-Blay and Joseph B. Lambert lants produce and export many different some other plant pathology. In other instances, molecules out of their cellular and organ- such as in typical underground roots, exudate Pismal confines. Some of those chemicals production appears to be part of the typical become so abundant that we can see or smell metabolism of healthy plants that helps stabi- them. The most visible materials oozed by lize the soil and foster interactions with other many plants are called “exudates.” organisms around the roots. What are plant exudates? Generally, exudates Different plant tissue types and organs can are carbon-rich materials that many plants pro- produce exudates. We have collected resins and duce and release externally. When exudates are gums from the above ground portions of plants, produced, they are often sticky to human touch. or shoots, as well as from the generally below Such plant chemicals can be the visible expres- ground portion of plants, or roots. Root exuda- sion of attack by bacteria, fungi, herbivores, or tion has been known for decades and is respon- REPRODUCED WITH PERMISSION OF AMERICAN SCIENTIST Resinous exudates on a conifer. ALL PHOTOGRAPHS BY JORGE A. SANTIAGO-BLAY UNLESS OTHERWISE NOTED UNLESS OTHERWISE ALL PHOTOGRAPHS BY JORGE A. SANTIAGO-BLAY Prolific white, resinous exudation is seen on a tumor- Blobs of white resin on a relatively young shoot of a like growth on the trunk of a white pine (Pinus strobus) Japanese black pine (Pinus thunbergii, AA accession at the Arnold Arboretum. 11371-O). Plant Exudates and Amber 3 A slab of Great Basin bristlecone pine (Pinus longaeva) right out of the microwave oven showing extruded (and very hot!) resinous exudates. Microwave heating experiments were performed at the Laboratory of Tree-Ring Research, University of Arizona, Tucson. sible for many of the fascinating relationships (or chemical signatures) of materials, includ- in the interface of plant roots and soil microor- ing plant exudates and amber or greatly ganisms known as the rhizosphere. fossilized plant resin. The analyses, which use a tiny amount (as little as 50 to 100 mil- Collecting and Analyzing Plant Exudates ligrams, approximately the volume of a new After receiving collecting permission (if needed), eraser on a school pencil) of the exudate, are we spend days walking the grounds of botani- non-destructive. They are performed at North- cal gardens and arboreta, or do field work else- western University (in Evanston, Illinois), one where. Exudates are easily collected directly of a few research laboratories in the world from the trees with no harm to the plant and with carbon-13 ssNMR capabilities. At times, leaving no doubt about their botanical identity. we observe plants that evidently have produced Occasionally we use more forceful methods, exudates but the amounts are insufficient such as carefully microwaving wood slabs to for our analyses. extract the exudates, then letting them reso- Solid exudates are pulverized manually and lidify. Once the material is collected, we place undergo two sets of carbon-13 ssNMR analyses: it in a small plastic zip-top bag. An additional, normal decoupling, which gathers signals for external bag is used to hold a paper label con- all carbon atoms, and interrupted decoupling, taining the collection data. If needed, we let the which, among others, obtains signals from car- exudate dry slowly in an oven and, once dried, bons lacking the attached hydrogens. Just like the materials are ready for subsequent analyses. in spectra used in the health-allied sciences, In other instances, generous collaborators send different regions of the spectra provide valu- us materials for chemical analyses. able information (see Figure 1 on page 4). In Carbon-13 solid state Nuclear Magnetic the case of NMR, the peaks represent different Resonance spectroscopy (ssNMR) is a state- atoms and reflect their molecular environment. of-the-art research tool that generates spectra The height of the peaks largely represents rela- 4 Arnoldia 75/1 • August 2017 (A) tive abundance of those atoms. Interrupted Decoupling – only carbons with strong C-H interactions analyzed The position of the peak along the horizontal axis (parts per mil- lion [ppm]) is the resonance fre- quency characteristic of the atom and its molecular neighborhood. Normal Decoupling – all carbons analyzed This position is an indication of the chemical identity of the peak as compared to an external molecular reference. In carbon-13 C-O bonds ssNMR, peaks in the 0–80 ppm C=O bonds as in carboxyls C=C bonds as in alkenes as in C-C bonds as in alkanes sugars region are singly bonded carbon 220 200 180 160 140 120 100 80 60 40 20 0 atoms (-C-C-), or alkanes; signals f1 (ppm) within the 80–100 ppm region are single bonded carbon atoms with (B) electron-withdrawing neighbors, Interrupted Decoupling – only carbons with strong C-H interactions analyzed in particular, oxygen (C-O), as found in carbohydrates, such as sugars. Currently, we have ana- lyzed over 1,800 exudates of all types, including amber, represent- ing most of the major plant groups Normal Decoupling – all carbons analyzed worldwide. However, a lot more samples still need to be acquired and analyzed. C-O bonds C=O bonds as in carboxyls C=C bonds as in alkenes as in C-C bonds as in alkanes sugars Types of Plant Exudates 220 200 180 160 140 120 100 80 60 40 20 0 Using NMR, we have determined (ppm) that there are three major types of plant exudates: resins, gums, and (C) phenolics. Resins are made from Interrupted Decoupling – only carbons with strong C-H interactions analyzed 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 Normal Decoupling – all carbons analyzed Christmas trees or incense. Resins are insoluble in water and thus do not dissolve during rains. As time passes and the resins begin to C-O bonds “mature,” many of their original C=O bonds as in C=C bonds as in alkenes as in C-C bonds as in alkanes carboxyls sugars chemical constituents evaporate. 220 200 180 160 140 120 100 80 60 40 20 0 The materials remaining behind (ppm) in the resin blob form chemical bonds, a process known as polym- Figure 1. Chemical identity of peaks on a C-13 ssNMR spectra. Panel (A) is a erization, and the blob begins to resin, panel (B) is a gum, and panel (C) is a kino (a type of phenolic, often found in Eucalyptus). In all panels, the upper result uses interrupted decoupling, harden. With the passage of mil- which eliminates peaks representing C-H single bonds. The lower result uses lennia, the resinous material normal decoupling in which all carbon-to-atom bonds are represented. becomes greatly polymerized and Plant Exudates and Amber 5 Not On the Collection List Not everything that looks like an exudate is an exudate. Some living organisms, particularly fungi, can resemble the kinds of plant exudates we collect. In other instances, the watery—and often foul smelling—material that decomposing portions of plants produce can also resemble exudates. As you may guess, we do not collect those! JOSEPH O’BRIEN, USDA FOREST SERVICE, BUGWOOD.