Refuge Notebook • Vol. 19, No. 50 • December 15, 2017

Allelopathy: plant chemical warfare by Kyra Clark

Our native Lupine wages chemical warfare on other plants by releasing alkaloids that inhibit germination or growth.

How does one plant out compete another? How positively or negatively influences the germination, can invasive species so quickly take over well- growth, survival, or reproduction of a neighboring or- established areas? Some plants have specialized phys- ganism. Although allelopathy has a broad definition, iological adaptations that allow them to better access it is generally used when one plant negatively affects resources, some have unique germination cycles that nearby plants. This process is different from resource allow them to sprout earlier than others, while many competition, but it does lead to reduced competition nonnative invasives take advantage of not having nat- from nearby plants. ural predators in their new environment. There are Allelopathic plants vary in the chemical com- countless ways plants compete with one another, but pounds they use, how these compounds are formed, one of the most interesting ways plants can compete where they are stored, how they are released, and their in this battle for survival is with chemical warfare. modes of action in target plants. All allelochemicals, Several native plants and numerous invasive however, are secondary metabolites (not required for plants are allelopathic. Allelopathy describes the growth, development, or reproduction) and once re- process whereby plants, algae, bacteria, coral, or leased affect the survival or health of neighboring or- fungi produce and release an “allelochemical”, which ganisms. Allelochemicals are rarely auto-toxic, mean-

USFWS Kenai National Wildlife Refuge 101 Refuge Notebook • Vol. 19, No. 50 • December 15, 2017 ing plants do not release enough toxic compounds to compounds inhibit seed germination and growth by negatively affect its own growth. blocking H+-ATPase enzyme and root growth. Hy- Plants store allelochemicals in a single location, droxamic acids often work with phenolic compounds, multiple locations, or throughout the entire plant. another group containing allelochemicals. However, They are often found in the roots, leaves, pollen or phenolic compounds may not be true allelochemicals buds. Allelochemicals are introduced into the environ- as they have a generalized cytotoxicity when in mix- ment by being exuded (released) from roots, released tures, but individual phenolic compounds are poorly from decaying plant matter, leached from the plant, or cytotoxic when isolated. volatilized (become a gas). The last three common groups of compounds that Allelochemicals can be classified under several contain allelochemicals are quinones (e.g., Black Wal- groups of compounds. These groups describe an infi- nut), terpenoids (e.g., Rosemary) and polyacetylenes nite number of compounds found in plants but only a (e.g., Russian Knapweed). All of these groups use dif- select few within each group are actually allelopathic. ferent modes of action to inhibit seed germination or Over the years there has been a lot of research on alle- growth. lochemicals, but because they can be difficult to isolate One invasive species that has notoriety in Alaska and may have several functions, it is hard to determine is Orange Hawkweed (Hieracium aurantiacum). This if a compound is truly allelopathic. hawkweed is pollen-allelopathic, meaning that allelo- Here in Alaska, there are both native and invasive chemicals are carried in wind-dispersed pollen and fer- allelopathic plants. Lupines (Lupinus sp.) are a famil- tilized seeds, a strategy that allows it to rapidly estab- iar plant to most Alaskans and all lupine species re- lish in new areas. Meadow Hawkweed (Hieracium cae- lease an alkaloid allelochemical. The alkaloid group spitosum) may also be pollen-allelopathic and/or may includes recognizable compounds like morphine, nico- release allelochemicals in their decaying leaves. Our tine, and caffeine. Allelopathic alkaloids can target native Cottonwood is also allelopathic, releasing alle- multiple sites that inhibit seed germination or growth lochemicals in decaying leaves that inhibit herbaceous by impeding certain enzyme functions, photosynthe- seed growth nearby. sis, respiration, or DNA/RNA processing enzymes. The highly invasive Spotted Knapweed (Centautea Both native and nonnative invasive species use al- stoebe) uses an allelochemical within the flavonoid lelopathy to their advantage. Because native plants group of compounds. Allelopathic flavonoids gener- co-evolve, the effects of native allelopathy seem tobe ally inhibit seed germination and limit root or shoot more limited, perhaps because immunity or resistance length by blocking the H+-ATPase enzyme, the elec- becomes a rapidly-selected trait. On the other hand, tron transport chain, or oxygen uptake by mitochon- when nonnative invasive allelopathic species enter a dria. Spotted Knapweed exudes this flavonoid from its new environment, they quickly spread as native plants roots, which then leads to death of the root systems in have not adapted to counteract these new allelochem- nearby plants. icals. Another invasive plant that exudes allelochemi- The fact that certain plants produce specific com- cals from its roots is Garlic Mustard (Allaria petiolafa). pounds with the sole intention of harming neighbor- Garlic Mustard releases a type of glucosinolate com- ing plants for their own gain is both ruthless and fas- pound that, once released, becomes volatile and fills cinating. Occasionally the plants that appear the most soil pores as a gas where it then acts on neighboring robust and best suited for an environment are not the seeds. Glucosinolates can either be volatile or non- ones that survive—sometimes survival of the fittest volatile but, either way, they inhibit seed germination come down to the production and release of tiny com- by targeting the enzymes necessary for glycolysis and pounds. That’s right, plants just got a little more in- respiration. teresting. Quackgrass (Elymus repens), a common grass, re- Kyra Clark is a seasonal biological technician at leases a hydroxamic acid. Hydroxamic acids are usu- Kenai National Wildlife Refuge. Find more information ally exuded from roots (which is what quackgrass about the Refuge at http://www.fws.gov/refuge/kenai/ or does) or released from degrading plant parts. These http://www.facebook.com/kenainationalwildliferefuge.

102 USFWS Kenai National Wildlife Refuge